The Chemical Senses Page 1 of 31 PRINTED FROM OXFORD HANDBOOKS ONLINE (www.oxfordhandbooks.com). (c) Oxford University Press, 2015. All Rights Reserved. Under the terms of the l icence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Handbooks Online for personal use (for details see Privacy Policy). Subscriber: OUP-Reference Gratis Access; date: 11 August 2015 Subject: Philosophy,	Philosophy	of	Mind Online	Publication	Date: May 2015 DOI: 10.1093/oxfordhb/9780199600472.013.045 The	Chemical	Senses Barry	C.	Smith The	Oxford	Handbook	of	Philosophy	of	Perception	(Forthcoming) Edited	by	Mohan	Matthen Oxford	Handbooks	Online Abstract	and	Keywords Long-standing	neglect	of	the	chemical	senses	in	the	philosophy	of	perception	is	due,	mostly,	to	their	being regarded	as	'lower'	senses.	Smell,	taste,	and	chemically	irritated	touch	are	thought	to	produce	mere	bodily sensations.	However,	empirically	informed	theories	of	perception	can	show	how	these	senses	lead	to	perception	of objective	properties,	and	why	they	cannot	be	treated	as	special	cases	of	perception	modelled	on	vision.	The senses	of	taste,	touch,	and	smell	also	combine	to	create	unified	perceptions	of	flavour.	The	nature	of	these multimodal	experiences	and	the	character	of	our	awareness	of	them	puts	pressure	on	the	traditional	idea	that	each episode	of	perception	goes	one	or	other	of	the	five	senses.	Thus,	the	chemical	senses,	far	from	being	peripheral	to the	concerns	of	the	philosophy	of	perception,	may	hold	important	clues	to	the	multisensory	nature	of	perception	in general. Keywords:	Taste,	smell, flavour,	multisensory,	trigeminal,	olfaction,	odour 1	Introduction The	long-standing	neglect	of	the	chemical	senses	in	the	philosophy	of	perception	is	due,	no	doubt,	to	their	being regarded	as	'lower'	senses,	suggesting	something	simple	and	uninteresting	in	comparison	with	the	reputedly 'higher'	intellectual	senses	of	vision	and	audition. This	flawed	view	of	the	senses	has	obscured	the	relevance	of the	chemical	senses	and	their	objects	in	the	philosophy	of	perception,	but	we	are	now	beginning	to	understand their	significance	in	the	light	of	empirical	results	in	neuroscience	and	psychology	about	the	chemical	senses,	and the	way	these	results	have	contributed	to	the	radical	overhaul	of	the	traditional	conception	of	the	senses. The	distinction	between	higher	and	lower	senses	was	largely	based	on	whether	the	associated	stimuli	were proximal	or	distal.	Aquinas	and	Kant	both	thought	that	the	lower,	or	bodily,	senses	could	only	provide	us	with information	about	ourselves;	they	produce	mere	bodily	sensations,	rather	than	enabling	us	to	perceive	the	world around	us.	By	contrast	with	the	lower	senses	(touch,	taste,	and	smell),	vision	and	audition	put	us	'in	touch',	so	to speak,	with	distal	objects,	revealing	their	perceiver-independent	properties	and	so	potentially	supplying	us	with objective	knowledge	of	the	environment.	This	hierarchy	of	the	senses,	with	its	view	of	the	lower	senses	as	giving rise	to	mere	bodily	sensations,	persists	in	contemporary	discussions	of	perception	(Lycan,	2000;	Smith,	2002).	Yet despite	that	persistence,	I	shall	offer	reasons	for	thinking	chemosensory	perception	puts	us	in	touch	with	objective features	of	our	environment	just	as	much	as	the	other	senses	do. 2	The	Scope	of	the	Chemical	Senses Like	all	other	creatures,	humans	have	receptors	that	respond	to	chemicals	in	their	environment,	though	as	humans we	have	just	three	chemical	senses:	taste,	smell,	and	a	certain	kind	of	touch.	Taste	and	smell	are	clearly	chemical 1 The Chemical Senses Page 2 of 31 PRINTED FROM OXFORD HANDBOOKS ONLINE (www.oxfordhandbooks.com). (c) Oxford University Press, 2015. All Rights Reserved. Under the terms of the l icence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Handbooks Online for personal use (for details see Privacy Policy). Subscriber: OUP-Reference Gratis Access; date: 11 August 2015 senses,	involving	contact	between	chemical	stimuli	and	chemosensory	receptors	in	the	nose	and	in	the	mouth. Touch	too	is	chemical	when	it	involves	chemesthesis,	a	process	due	to	chemical	irritation	of	the	free	nerve endings	in	soft	tissue	giving	rise	to	characteristic	sensations	of	tingling,	stinging,	burning,	and	cooling.	Of	course, not	everything	classified	as	touch	involves	chemesthesis	(de	Vignemont	and	Massin,	this	volume,	III.3).	Though	as we	shall	see,	flavour	experience	is	due	to	a	complex	interaction	of	the	chemical	and	other	somatosensory	senses. It	is	sometimes	claimed	that	the	chemical	senses	in	humans	include	signalling	between	people	by	means	of pheromones.	The	term	'pheromone'	was	defined	by	Karlson	and	Luscher	in	1959	to	cover	'substances	which	are secreted	to	the	outside	by	an	individual	and	received	by	a	second	individual	of	the	same	species,	in	which	they release	a	specific	reaction'. The	definition	makes	reference	to	the	type	of	information	transmitted	and	the	specific set	of	responses	elicited.	The	analogy	is	with	hormones,	which	carry	signals	between	organs	of	the	body.	In vertebrates,	processing	is	typically	carried	out	by	a	dedicated	set	of	receptors	in	the	vomeronasal	organ	leading	to a	fixed	behavioural	reaction. Do	humans	signal	by	means	of	pheromones?	There	is	little	empirical	evidence	that	they	do.	They	certainly	secrete chemicals	that	influence	their	own	and	other	animals'	behaviour	but	these	stimuli	are	processed	by	the	olfactory system.	Humans	do	have	vestigial	ducts	of	a	vomeronasal	organ	in	the	nose,	but	the	receptors	there	do	not	project to	the	olfactory	bulb	or	any	other	brain	region,	so	no	known	mechanism	exists	for	our	responding	to	pheromones, or	for	them	shaping	our	behaviour.	A	single	research	group	produced	the	evidence	of	pheromone	detection	in humans	but	their	findings	have	been	widely	challenged	and	other	explanations	have	been	offered	of	the	data.	So perhaps	the	most	charitable	conclusion	is	that: Proponents	of	a	human	vomeronasal	organ	and	of	human	pheromones	...	have	stimulated	much	debate and	raised	interesting	issues,	but	[they]	do	not	at	present	have	convincing	evidence	for	the	existence	of pheromones	in	humans,	functional	vomeronasal	organs,	or	an	accessory	olfactory	system	in	the	brain. (Johnston,	2000:	120) Humans	are,	nevertheless,	capable	of	chemical	signalling	by	means	of	odours	and	olfaction	operating	below	our threshold	of	conscious	awareness	(Pause,	2012;	Chen,	2006).	Such	chemical	signals	can	have	strong	behavioural effects	on	mate	selection	and	sexual	behaviour,	but	the	precise	behavioural	response	is	seldom	fixed	for	all individuals	of	the	species.	Unconscious	cues	and	responses,	including	attraction	and	aversion,	provide	good examples	of	how	the	neural	processing	of	sensory	signals	outside	the	sphere	of	consciousness	can	have	a	large impact	on	what	takes	place	in	consciousness,	but	all	of	these	effects	can	be	discussed	without	positing	human pheromones.	In	what	follows,	we	shall	be	concerned	exclusively	with	the	chemical	senses	of	taste,	touch,	and smell-and	their	interactions. 3	Taste,	Smell,	and	Touch Taste,	smell,	and	touch	are	among	the	most	basic	senses	with	which	we	explore	the	world.	We	rely	on	them	to assess	our	environment	and	to	guide	successful	food	choice.	These	responses	are	active	in	us	at	birth,	and perhaps	even	earlier.	Taste	and	smell	serve	as	the	gatekeepers	to	the	environmental	odours	and	substances	that enter	our	bodies,	and,	as	we	shall	see,	the	chemical	senses	play	a	key	role	in	mating	and	feeding,	avoiding danger,	laying	down	memories,	regulating	mood,	and	maintaining	quality	of	life.	As	such,	they	play	a	continuous role	in	everyday	conscious	experience. Deliverances	of	the	chemical	senses	typically	have	hedonic	value,	giving	us	sensations	we	find	appealing	or aversive,	and	a	key	question	for	us	will	be	whether	their	hedonic	values	are	intrinsic	parts	of	the	chemical	senses or	just	accompaniments.	For	example,	sweet	tasting	foods	are	innately	desirable,	while	bitter	tasting	ones	are initially	unpleasant.	This	helps	to	identify	energy-rich	nutritional	sources	and	to	protect	us	from	toxins,	many	of which	are	bitter.	In	this	way,	taste	promotes	homeostasis.	In	addition	to	the	above,	saltiness	is	used	in	maintaining electrolyte	balance,	sourness	to	guard	pH	levels,	and	savouriness	(umami)	to	motivate	protein	intake	(Small	et	al., 2007). Smell	functions	in	a	similar	hedonic	way	though	there	is	no	evidence	that	we	have	innate	preferences	for	particular odours	in	the	way	we	have	innate	preferences	for	tastes.	Although	we	are	not	usually	aware	of	it,	the	sense	of 2 The Chemical Senses Page 3 of 31 PRINTED FROM OXFORD HANDBOOKS ONLINE (www.oxfordhandbooks.com). (c) Oxford University Press, 2015. All Rights Reserved. Under the terms of the l icence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Handbooks Online for personal use (for details see Privacy Policy). Subscriber: OUP-Reference Gratis Access; date: 11 August 2015 smell	is	constantly	at	work.	As	J.	J.	Gibson	pointed	out,	we	smell	because	we	breathe.	The	sense	of	smell	guards the	quality	of	the	air	we	breathe	and	is	connected	to	limbic	areas	of	the	brain	responsible	for	memory	and	emotion. Thus	it	plays	an	important,	though	often	unrecognized,	role	in	our	affective	lives. Touch	too	has	its	hedonic	side.	Mild	tingling	sensations	are	pleasant	when	consuming	spices;	they	work	by stimulating	the	trigeminal	nerve:	the	fifth	cranial	nerve	that	serves	the	eyes,	the	nose,	and	mouth.	(This	makes mustard	feel	'hot'	and	peppermint	feel	'cool'	in	the	mouth,	even	though	there	is	no	change	of	temperature.)	Such sensations	enhance	our	enjoyment	of	food,	but	can	become	aversive	when	they	sting	or	burn.	Chemically mediated	touch	also	plays	a	role	in	safeguarding	us.	Specific	irritants	to	the	trigeminal	nerve	endings	in	the	nose can	be	dangerous,	and	need	to	be	avoided,	while	others,	such	as	the	mild	prickle	on	the	tongue	from	CO in	fizzy drinks	can	boost	aroma	perception	and	increase	the	pleasure	of	drinking. So	far,	we	have	been	speaking	about	smell,	taste,	and	touch,	but	care	is	needed	when	talking	about	the	chemical senses	because	these	terms	are	not	being	used	in	their	colloquial	sense.	In	colloquial	use,	terms	like	'taste'	and 'smell'	usually	pick	out	experiences	that,	as	we	shall	see,	combine	sensory	inputs	from	more	than	one	chemical sense.	So,	from	time	to	time,	it	will	be	necessary	to	use	the	more	precise,	scientific	terms	olfaction,	gustation,	and chemesthesis,	especially	when	speaking	about	the	interaction	of	the	chemical	senses. 4	The	Interaction	of	the	Chemical	Senses Of	particular	interest	in	what	follows,	and	distinctive	of	the	chemical	senses	in	humans,	is	the	way	they	collectively conspire	with	other	senses	to	give	us	experiences	of	tasting	something.	For	what	we	ordinarily	call	'taste'	involves input	not	just	from	the	tongue,	but	from	touch	and	smell.	Our	failure	to	recognize	the	complexity	in	our	tasting experiences	has	prevented	progress	not	just	in	our	understanding	of	what	we	call	taste,	but	in	our	understanding of	the	senses	more	generally.	For: Although	the	experience	of	the	sensory	qualities	of	a	food	are	often	described	in	terms	of	how	it	'tastes';	in practice,	this	experience	of	flavour	is	a	complex	interaction. (Yeomans	et	al.,	2008) The	experience	often	described	in	unisensory	terms	as	'taste'	depends	on	the	multi-modal	combining	of	inputs from	different	sense	modalities	into	a	unified	flavour	percept:	a	percept	produced	by	acts	of	tasting.	Volatiles	rising from	the	mouth	pass	over	the	olfactory	receptors;	this	accounts	for	our	ability	to	distinguish	between	the equivalently	sweet	essences	of	strawberry	and	cherry.	Touch	gives	us	information	about	the	texture	of	food- whether	something	is	creamy,	oily,	chewy,	sticky,	or	crunchy;	smell	can	affect	what	we	taste	on	the	tongue	and fuse	with	those	tastes	to	produce	complex	experiences	of	flavour.	The	term	flavour	picks	out	something	perceived conjointly	by	taste,	touch,	and	smell,	and	is	used	to	avoid	confusion	with	what	is	detected	by	taste	proper (gustation).	The	resultant	unity	of	our	experience	of	a	food	or	liquid's	flavour	provides	little	clue	that	it	is	a	complex interaction	effect.	This	may	be	why	people	think	of	it	in	simple	terms	as	an	experience	of	'taste'.	It	may	also	have	to do	with	the	crucial	component	of	smell	going	missing	in	the	phenomenology	of	taste	experiences.	It	takes experimental	or	clinical	findings	to	reveal	the	indispensable	role	that	smell	plays	in	what	we	call	'taste'. The	real	nature	of	these	multimodal	experiences	and	the	character	of	our	awareness	of	them	(or	lack	of	it)	is	where the	chemical	senses	hold	greatest	interest	for	philosophers	of	perception.	The	example	of	flavour	perception	as	a kind	of	multi-modal,	yet	unified	perceptual	experience	puts	pressure	on	the	traditional	idea	that	perceiving	is always	done	by	means	of	either	seeing,	or	hearing,	touching,	tasting,	or	smelling.	Moreover,	the	study	of	flavour perception	in	sensory	science	has	yielded	significant	insights	into	multisensory	integration-something	increasingly seen	as	the	rule	and	not	the	exception	in	perceptual	processing.	Thus,	the	chemical	senses	far	from	being peripheral	to	the	concerns	of	the	philosophy	of	perception,	may	hold	important	clues	to	the	multisensory	nature	of perception	in	general.	But	before	we	look	at	the	mechanisms	of	multisensory	integration	underlying	flavour perceptions	let	us	review	the	traditional	assumptions	about	perception	that	they	challenge. 5	The	Senses	Working	in	Isolation From	the	time	of	Aristotle	to	the	present	day,	people	have	thought	of	themselves	as	having	five	distinct	senses	that 2 The Chemical Senses Page 4 of 31 PRINTED FROM OXFORD HANDBOOKS ONLINE (www.oxfordhandbooks.com). (c) Oxford University Press, 2015. All Rights Reserved. Under the terms of the l icence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Handbooks Online for personal use (for details see Privacy Policy). Subscriber: OUP-Reference Gratis Access; date: 11 August 2015 enable	them	to	see,	hear,	taste,	touch,	and	smell;	a	classification	that	became	further	entrenched	in	the	nineteenth and	twentieth	centuries	(Matthen,	this	volume,	V.1.)	Aristotle	thought	that	each	sense	had	a	proper	object,	a	kind	of quality	that	could	only	be	perceived	with	that	sense:	e.g.,	colour	for	seeing,	sound	for	hearing,	taste	for	tasting. There	were,	in	addition,	qualities	or	properties	that	could	be	perceived	by	more	than	one	sense.	For	example,	size and	shape	could	be	perceived	by	both	sight	and	touch;	these	were	called	common	sensibles.	Some	philosophers are	tempted	to	think	of	common	sensibles	as	having	more	objectivity	than	qualities	which	could	be	detected	by	one sense	alone.	These	are	the	classic	primary	qualities	of	John	Locke,	whose	natures,	unlike	colours,	tastes,	and smells,	are	independent	of	perceivers	(Ross,	this	volume,	chapter	IV.2).	This	picture	of	the	senses	and	their	proper objects	encourages	us	to	think	of	the	senses	as	working	in	isolation.	The	experience	of	sounds	comes	to	us	via hearing,	the	experience	of	colour	via	seeing,	and	the	experience	of	tastes	via	our	sense	of	taste. In	the	science	of	perception,	the	largest	focus	was	on	seeing.	The	occipital	cortex	occupies	about	a	third	of	the processing	capacity	and	this	may	have	led	to	the	dominance	of	vision	science.	But	it	may	not	only	be	science	that explains	this	dominance.	Seeing	occupies	so	much	of	our	attention	that	it	is	not	surprising,	perhaps,	that	it dominates	discussions	of	perception.	This	may	be	why	so	many	philosophers	treat	problems	of	perception	as	if they	were	exclusively	questions	about	visual	perception,	assuming,	not	always	explicitly,	that	the	correct	account of	visual	perception	will	just	apply	with	minor	modifications	to	perceiving	by	means	of	other	senses. The	prominence	of	the	occipital	cortex	also	encouraged	a	misplaced	generalization.	Surely,	so	the	argument	goes, if	we	could	point	to	activity	in	specific	brain	regions	as	being	responsible	for	processing	information	from	other sensory	receptors,	then	we	would	be	entitled	to	study	the	senses	in	isolation.	This	strategy	for	studying	the	senses in	isolation	seemed	at	first	to	have	anatomical	support	from	the	brain	sciences.	The	advent	of	neural	imaging	led	to better	identification	of	separate	cortical	and	sub-cortical	regions	responsible	for	different	functions,	and	particular areas	were	identified	as	the	visual	cortex,	the	auditory	cortex,	the	primary	smell	and	taste	cortices,	and	the somatosensory	cortex.	This	encouraged	the	idea	that	significant	areas	of	the	brain	were	devoted	to	unimodal processing	by	analogy	with	early	vision 6	The	Merging	of	the	Senses The	picture	just	described	is	no	longer	dominant	in	the	sensory	neurosciences,	for	it	has	been	repeatedly demonstrated	that	perception	relies	on	combining	information	from	many	sources	(Bayne	and	Spence,	this	volume, chapter	IV.3): Our	brains	are	continuously	inundated	with	stimulation	arriving	through	our	various	sensory	pathways.	The processes	involved	in	synthesizing	and	organizing	this	multisensory	deluge	of	inputs	are	fundamental	to effective	perception	and	cognitive	functioning. (Talsma	et	al.	2010:	400) This	means	that focusing	solely	on	unisensory	processes	will	continue	to	provide	us	only	with	an	impoverished	view	of	both brain	and	behavior. (Ghazanfar	and	Schroeder,	2006:	278) The	field	has	been	transformed	through	a	recognition	that the	coordination	and	integration	of	information	derived	from	different	sensory	systems	is	essential	for providing	a	unified	perception	of	our	environment,	and	for	directing	attention	and	controlling	movement within	it.	The	capacity	of	the	central	nervous	system	to	combine	inputs	across	the	senses	can	lead	to marked	improvements	in	the	detection,	localization	and	discrimination	of	external	stimuli	and	to	faster reactions	to	those	stimuli. (King	and	Calvert,	2001:	322) Many	neuroscientists	believe	that	'Perception	is	multisensory'	because	'no	single	sensory	signal	can	provide reliable	information	about	the	three-dimensional	structure	of	the	environment	in	all	circumstances'	and	'	...	if	a The Chemical Senses Page 5 of 31 PRINTED FROM OXFORD HANDBOOKS ONLINE (www.oxfordhandbooks.com). (c) Oxford University Press, 2015. All Rights Reserved. Under the terms of the l icence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Handbooks Online for personal use (for details see Privacy Policy). Subscriber: OUP-Reference Gratis Access; date: 11 August 2015 single	modality	is	not	enough	to	come	up	with	a	robust	estimate,	information	from	several	modalities	can	be combined'	to	provide	a	better	fix	on	external	events	or	objects	(Ernst	and	Bulhoff,	2004:	162).	In	their	important review	paper	Ernst	and	Bulhoff	continue: To	perceive	the	external	environment	our	brain	uses	multiple	sources	of	sensory	information	derived	from several	different	modalities,	including	vision,	touch	and	audition.	All	these	different	sources	of	information have	to	be	efficiently	merged	to	form	a	coherent	and	robust	percept. And: The	key	to	robust	perception	is	the	combination	and	integration	of	multiple	sources	of	sensory	information. (Ernst	and	Bulhoff,	2004:	162) For	example,	we	sometimes	just	see,	and	sometimes	just	hear,	a	plastic	bottle	being	crushed,	but	when	we	both see	and	hear	it,	this	gives	more	neural	activation	than	the	sum	of	activation	involved	in	the	seeing	and	the	hearing. This	superadditivity	in	the	neural	pattern	of	responding	to	combined	information	is	one	sign	of	multisensory integration.	It	is	as	if	the	brain	has	learned	to	pay	attention	to,	and	mark	out,	complementary	multisensory information	to	help	it	track	environmental	events. Philosophers	have	been,	until	recently,	slow	to	recognize	this	important	shift	in	the	science	of	sensory	perception, but	one	who	has	stressed	its	importance	is	Casey	O'Callaghan: an	adequate,	complete	understanding	of	perception	requires	comprehending	the	ways	in	which	what	goes on	with	one	sense	modality	impacts	what	goes	on	with	another.	Theorizing	about	perception	is	not	just	a matter	of	assembling	independently	viable	stories	about	vision,	audition,	olfaction,	and	the	rest. Considering	the	relationships	and	interactions	among	perceptual	modalities	sheds	light	on	what	is	most striking	about	perception:	its	capacity	to	furnish	a	sense	of	awareness	of	a	world	of	things	and	happenings independent	from	oneself. (O'Callaghan,	2008:	316) The	prolonged	failure	to	appreciate	the	significance	of	multisensory	interactions	is	surprising.	As	DeGelder	and Bertelson	put	it: Research	on	perceptual	processing,	whether	behavioural	or	physiological,	has	generally	considered	one sense	modality	(sight,	hearing,	touch,	smell,	etc)	at	a	time.	Yet,	most	events	in	the	natural	environment generate	stimulation	to	several	modalities.	An	explosion	simultaneously	emits	light,	noise	and	heat,	and experiencing	all	of	these	together	make	for	a	richer	percept	than	each	individually;	a	speaker	produces facial	movements	in	a	predictable	temporal	relationship	to	corresponding	speech	sounds	and	experiencing both	together	can	provide	a	more	adequate	percept. (DeGelder	and	Bertleson,	2003:	460) The	two	cases	mentioned	here	are	somewhat	different,	but	the	contrast	between	them	provides	a	clue	as	to	why	it has	taken	so	long	to	discover	how	sensory	systems	interact	to	shape	our	perceptions,	not	least	our	perceptions	of flavour.	The	case	of	the	explosion	is	one	in	which	we	are	consciously	aware	of	the	light,	the	noise,	and	the	heat	of the	event,	though	perhaps	not	aware	of	how	the	interactions	of	the	sensory	inputs	intensify	overall	experience. The	second	case	is	one	in	which	we	are	entirely	unaware	of	how,	and	how	much,	visual	information	about	a speaker's	lip	movements	contributes	to	the	speech	sounds	we	hear,	and	in	particular	the	way	vision	dominates audition	when	there	is	sensory	conflict. It	is	clear,	from	the	case	of	speech	perception	that	the	workings	of	one	sense	can	affect	not	just	the	intensity	of another	sense,	but	the	contents	of	the	perceptions	that	rely	on	it.	(See	O'Callaghan,	this	volume,	chapter	IV.6)	In the	cinema,	for	example,	we	experience	the	voices	as	coming	from	the	mouths	of	the	actors	on	the	screen,	even though	the	sound	comes	from	loudspeakers	located	at	the	sides	of	the	movie	theatre,	under	the	seats,	or	even behind	the	viewer.	When	lip	movements	are	seen	in	synchrony	with	the	heard	speech	sounds,	we	get	visual capture	of	auditory	attention,	making	auditory	perception	present	things	as	if	the	mouths	on	the	screen	were	the sources	of	the	speech	sounds.	Known	as	the	ventriloquism	effect,	this	illusion	provides	a	clear	case	where	vision The Chemical Senses Page 6 of 31 PRINTED FROM OXFORD HANDBOOKS ONLINE (www.oxfordhandbooks.com). (c) Oxford University Press, 2015. All Rights Reserved. Under the terms of the l icence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Handbooks Online for personal use (for details see Privacy Policy). Subscriber: OUP-Reference Gratis Access; date: 11 August 2015 dominates	audition	in	specifying	the	(apparently)	common	source	of	sights	and	sounds	(Allais	and	Burr,	2004). Often	we	are	not	even	aware	of	sensory	conflicts	and	simply	assume	that	we	are	enjoying	complimentary	though distinct	sensory	experiences. A	different	case,	though	relevant	to	the	interactions	of	the	chemical	senses,	is	where	a	new	multisensory	quality results	from	the	joint	upshot	of	different	senses	working	together	to	produce	something	that	could	not	be	produced by	any	of	the	contributing	senses	alone.	The	McGurk	Effect	in	speech	perception	is	one	such	example,	where	one is	looking	at	a	film	of	a	face	making	the	speech	gesture	/ga/	while	hearing	a	synchronized	audio	feed	of	the	speech sound	/ba/,	resulting	in	the	percept	/da/,	which	is	acoustically	and	production-wise	intermediate	between	/ga/	and /ba/;	something	that	one	neither	saw	nor	heard.	The	significance	is	that	a	new	item	in	experience	has	been produced	by	cross-modal	interaction	between	the	senses,	although	it	is	not	recognized	as	such	by	subjects	of	the experience. Many	will	say	that	the	McGurk	Effect	is	a	carefully	arranged	illusion,	and	as	such,	it	is	not	a	natural	phenomenon. And	so	we	may	want	to	ask	if	there	are	any	natural	cases	of	such	new	multisensory	phenomena.	Experiences	of flavour	from	tasting	foods	or	drinks	provide	just	such	a	case	from	everyday	life.	The	objects	of	perception	in	tasting are	flavours,	though	we	classify	them	as	tastes.	How	should	we	understand	such	phenomena	given	how	are	they produced?	We	will	examine	that	question	in	some	detail.	But	at	the	outset	it	is	worth	noticing	the	considerable challenge	flavour	experiences	pose	to	the	traditional	picture	of	perception.	If	it	is	possible	to	experience	a	quality like	flavour	only	through	the	conjoint	exercise	of	several	senses,	then	we	have	a	category	of	perceptual	quality	for which	Aristotle's	classification	made	no	room.	Flavours	are	not	common	sensibles	accessible	by	more	than	one sense;	we	need	many	senses-chemical	and	contact-working	together	to	produce	flavour	perceptions.	The	only way	to	restore	Aristotle's	idea	of	there	being	proper	objects	of	the	senses	in	the	case	of	flavour	perception	is	to claim	that	we	have	a	single	sense	of	flavour,	albeit	one	that	draws	on	the	interactions	or	integration	of	other senses;	a	position	some	have	proposed	in	the	empirical	literature	(Auvray	and	Spence	2008;	also	Matthen,	this volume). Let	us	now	examine	the	senses	that	contribute	to	the	multi-modal	experience	of	flavour,	and	try	to	say	more	about why	we	fail	to	recognize	the	complexity	in	the	experiences	of	tasting	food	and	drink. 7	What	Do	We	Mean	by	Taste? It's	difficult	at	first	for	us	to	focus	on	what	happens	when	tasting	a	food	or	liquid.	We	pop	something	in	our	mouths; we	sip,	or	chew,	then	swallow,	and	the	sequence	of	experiences	we	undergo	is	fleeting	and	ephemeral;	gone	in	an instant	and	hard	to	focus	on.	The	transient	nature	of	these	experiences	often	means	the	temporal	dynamics	of tasting	go	unnoticed	until	they	are	pointed	out.	Mostly,	we	are	left	with	an	impression	of	liking	or	disliking,	and	often people	behave	as	if	the	whole	point	of	tasting	was	to	come	up	with	a	verdict	about	liking.	Though	as	we	shall	see liking	can	be	a	distraction. Like	seeing	and	listening,	tasting	is	an	activity	that	generates	experiences	of	a	distinctive	kind:	experiences	of	the flavours	of	the	foods	and	drinks	we	consume. What	is	it	for	something	to	taste	a	certain	way?	Begin	with	the	experiences	themselves.	They	occur	when	eating	or drinking,	but	we	also	think	of	tasting	as	the	having	of	those	experiences.	In	this	sense,	tasting	is	itself	an experience,	and	many	philosophers	will	say	that	it	is	a	subjective	experience	in	the	individual	taster.	In	one	sense, of	course,	this	is	correct.	Tasting	experiences	happen	to	individual	subjects	of	experience.	So	do	episodes	of seeing	and	hearing.	These,	too,	are	going	on	in	subjects	of	experience.	It	does	not	follow	that	what	one	sees	or what	one	hears	is	subjective;	nor	does	it	follow	that	tasting	is	all	about	the	subject.	What	we	taste-its	taste-need not	be	purely	subjective. For	it	to	be	purely	subjective	requires	treating	how	something	tastes	as	no	more	than	a	fact	about	the	taster,	about how	it	tastes	to	him	or	her.	So	there	would	be	as	many	ways	something	tastes	as	there	are	tasters,	or	ways	that tasters	taste	a	given	food.	And	yet,	ordinarily,	we	don't	talk	that	way.	We	speak	not	about	my	tastes	and	your tastes,	but	the	taste	of	a	dish	or	of	a	wine.	The	milk	tastes	sour,	the	anchovies	taste	salty,	the	wine	tastes	overly sweet.	These	are	not	ways	of	talking	about	me,	but	about	the	items	we	are	tasting. The Chemical Senses Page 7 of 31 PRINTED FROM OXFORD HANDBOOKS ONLINE (www.oxfordhandbooks.com). (c) Oxford University Press, 2015. All Rights Reserved. Under the terms of the l icence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Handbooks Online for personal use (for details see Privacy Policy). Subscriber: OUP-Reference Gratis Access; date: 11 August 2015 The	subjectivist	is	unmoved	by	the	linguistic	data,	and	will	rightly	point	out	how	promiscuous	our	use	of	the	word 'taste'	is.	It	is	variously	used	to	mean:	a	quality	of	a	food	or	liquid,	like	the	sourness	of	a	lemon;	the	characteristic experience	we	have	when	eating	a	lemon;	the	sense	by	which	we	detect	that	quality	or	generate	that	experience; and	even,	when	used	in	aesthetics,	a	refined	sensibility.	Subjectivists	treat	all	as	aspects	of	subjective	experience, deriving	from	mere	sensations	occurring	in	tasters.	According	to	the	subjectivist,	we	may	talk	of	the	taste	of	an apple	or	an	onion,	but	they	don't	really	have	tastes;	rather,	they	give	rise	to	tastes	in	us.	Tastes	are	just sensations	we	undergo	when	chewing	or	sipping	foods	or	drinks.	(A	more	objectivist	view	about	taste	would distinguish	between	tastes	as	properties	a	food	or	wine	has,	and	tasting	as	an	experience	that	a	subject	has.) 8	Tastes	as	Sensations? There	are	simple	and	more	sophisticated	views	of	tastes	and	tasting	as	subjective. The	simple	view	sees	tastes as	sensations	on	the	tongue,	inseparable	from	the	subject	who	has	those	sensations,	immediately	available	and knowable	through	and	through	for	what	they	are.	However,	to	think	of	the	tastes	of	a	wine	as	exhausted	by sensations	on	the	tongue	precludes	the	idea	that	not	every	tasting	experience	is	as	good	any	another.	When	we taste	a	wine	before	and	after	eating	a	lemon,	or	brushing	our	teeth,	it	doesn't	taste	the	same	way;	yet	we	know	that the	wine	hasn't	changed.	We	are	just	no	longer	able	to	taste	it	properly.	The	simple	subjectivist	has	to	say	there are	as	many	tastes	to	a	wine	as	there	are	tasters	and	moments	of	tasting.	But	a	wine's	taste	is	not	exhausted	by the	sensations	one	has	at	a	moment. Dispositionalists	might	say	that	what	matters	is	how	a	wine	or	a	dish	tastes	under	ideal	conditions.	But independently	of	matters	of	taste	how	are	we	to	spell	out	the	ideal	conditions,	and	why	should	they	be	the	same	for different	individuals?	As	we'll	see,	they	are	not.	Individuals	vary	considerably	in	their	responses	to	the	same stimuli. Should	we,	then,	regard	tastes	as	properties	of	foods	or	liquids	that	we	are	able	to	perceive	by	tasting?	This objectivist	stance	appears	closer	to	common	sense.	For	we	seem	to	rely	on	tasting	to	give	us	knowledge	of	the taste	of	things	we	eat	and	drink,	to	tell	us	whether	the	strawberry	is	sweet	and	ripe,	whether	the	coffee	has	sugar	in it	and	whether	the	soup	is	salty.	Taste	predicates	like	these	are	attributed	to	food	and	not	to	the	sensations	we have	when	eating	them.	The	subjectivist	is	unable	to	give	a	convincing	account	of	these	attributions.	Can	we	even be	sure	of	the	experiences	we	have	when	tasting	food	and	wines?	And	are	these	experiences	just	as	they	seem	to us? To	some	philosophers,	this	last	question	will	be	unintelligible	since	they	believe	that	how	things	appear	to	us	in experience	is	how	they	are.	There	is	no	appearance–reality	divide	for	conscious	experience;	experiences	just	are as	they	appear	to	be.	This	line	of	thought	is	part	of	a	residual	and	unquestioning	Cartesianism.	That	we	undergo	an experience	is	unmistakable,	but	that	doesn't	guarantee	that	we	know	that	experience	for	what	it	is.	Being	in	a mental	state	is	one	thing:	knowing	what	kind	of	mental	state	we	are	in	is	another.	As	we	shall	see,	subjectivists	who treat	tastes	as	simple	sensations	take	how	their	experiences	appear	to	them,	or	how	they	take	them	to	be,	as	the only	way	to	individuate	those	experiences.	However,	as	we	shall	see,	the	modal	signature	of	our	flavour experiences	is	far	from	clear,	nor	is	it	clear	what	gives	any	of	our	experiences	the	modal	signatures	we	take	them to	have. 9	The	Hidden	Complexity	of	Tasting	Experiences It	is	a	remarkable	fact	that	we	fail	to	recognize	the	complexity	of	our	own	tasting	experiences,	and	remarkable	that philosophers	still	readily	assume	that	we	have	transparent	access	to	those	experiences,	allowing	us	to	know	their natures	through	and	through.	It	is	this	assumption	of	self-intimating	experience	that	is	supposed	to	enable	us	to know	that	what	we	ordinarily	call	'taste'	is	a	matter	of	simple	sensations	on	the	tongue,	something	clearly unisensory.	And	yet,	it	has	taken	psychologists	and	neuroscientists	to	reveal	to	us	the	full	extent	of	the	complexity of	our	tasting	experiences. Brillat-Savarin	understood	this	point	early	on.	Writing	in	the	nineteenth	century,	he	was 'tempted	to	believe	that	smell	&	taste	are	in	fact	but	a	single	sense,	whose	laboratory	is	in	the	mouth	&	whose chimney	is	the	nose'	(Brillat-Savarin,	1825:	41).	These	comments	are	much	in	line	with	the	recent	findings	of neuroscience	and	psychology,	which	sees	the	experience	of	tasting	as	the	product	of	multisensory	integration:	a fusion	(confusion?)	of	inputs	from	different	sensory	modalities	that	gives	rise	to	a	unified	percept	(see	Auvray	and 3 4 5 The Chemical Senses Page 8 of 31 PRINTED FROM OXFORD HANDBOOKS ONLINE (www.oxfordhandbooks.com). (c) Oxford University Press, 2015. All Rights Reserved. Under the terms of the l icence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Handbooks Online for personal use (for details see Privacy Policy). Subscriber: OUP-Reference Gratis Access; date: 11 August 2015 Spence	2008).	Once	taste	and	smell	fuse	into	an	experience	of	flavour,	it	is	no	longer	possible	to	separate	out	the different	components	by	phenomenological	decomposition. Certain	things	go	missing	from	our	awareness	of	our	tasting	experiences	because,	at	first,	we	don't	recognize	that 'flavour	perception	is	not	a	single	event	but	a	dynamic	process	with	a	series	of	events'	(Piggott,	1994:	167). Tasting	has	a	dynamic	time	course	and	slowing	it	down	makes	a	difference	to	what	we	notice	and	what	we	can pick	out.	In	this	way,	how	we	taste	affects	what	we	taste;	and	attending	to	each	aspect	of	the	dynamic	time	course changes	the	temporal	scale	of	the	tasting	experiences	we	have.	Through	attention,	we	can	focus	on	particular qualities	of	the	taste,	texture,	and	aroma	of	the	foods	and	liquids	we	ingest.	(It	does	not,	however,	mean	that	we can	separate	taste	and	retronasal	olfaction.) Despite	such	careful	scrutiny,	we	tend	to	think	of	the	unified	experience	of	flavour	as	coming	just	from	the	tongue or	the	oral	cavity.	In	fact,	the	tongue	gives	us	very	little.	All	it	provides	is	awareness	of	the	basic	tastes:	salt,	sweet, sour,	bitter,	umami	(savoury),	and	metallic.	But	think	of	all	the	flavours	we	can	experience	when	tasting:	'ripe mangoes,	fresh	figs,	lemon,	canteloupe	melon,	raspberries,	coconut,	green	olives,	ripe	persimmon,	onion, caraway,	parsnip,	peppermint,	aniseed,	cinnamon,	fresh	salmon'	(Sibley,	2006:	216).	We	don't	have	taste receptors	on	the	tongue	for	mango	or	raspberries	or	chicken	or	lamb.	Nor	are	these	recognizable	flavours concocted	from	the	basic	tastes,	as	philosopher	of	aesthetics	Frank	Sibley	perspicuously	pointed	out: how	could	one	construct	a	blend	of	distinguishable	tastes	...	to	yield	that	of	coconut,	or	lemon,	or	mint?	Try to	imagine	a	recipe:	'To	make	the	flavour	of	onion	(or	pepper,	or	raspberries	or	olives)	add	the	following [basic	tastes]	in	the	following	proportions	... (Sibley,	2006:	216–17) Clearly,	there	is	no	such	procedure,	and	yet,	the	act	of	tasting	gives	us	knowledge	of	all	these	recognizable flavours.	The	objects	of	perception	in	tasting	are	not	tastes,	but	flavours,	and	as	Sibley	points	out	flavours	are single,	not	simple.	Tasting	flavours	involves	much	more	than	the	tongue,	and	tastes	proper	make	only	a	limited contribution	to	our	experience	of	flavours.	Properly	speaking,	taste	exclusively	concerns	the	gustatory	inputs	due to	firings	from	taste	receptors	in	the	oral	cavity,	the	soft	palate,	and	the	gastrointestinal	tract.	The	sensations	that the	tongue	produces-gustatory	sensations-are	hard	to	experience	alone	save	in	experimental	settings,	for example,	when	drops	are	put	on	parts	of	an	anaesthetized	tongue	and	we	prevent	any	other	sensations,	of	smell or	touch,	say,	from	contributing	to	the	subject's	experience.	What	we	call	'taste'	is	due	not	to	sensations	from	the tongue	alone	but	to	the	multisensory	interactions	that	produce	flavour	experience. Smell	plays	a	fundamental	role	in	forming	these	flavour	experiences.	But	not	smell,	as	we	ordinarily	think	of	it:	as	a matter	detecting	the	odours	we	inhale.	Rather	it	is	smelling	by	processing	of	odours	that	reach	the	nose	from	the mouth	when	they	are	released	from	the	foods	or	liquids	we	chew,	sip,	and	swallow.	This	is	due	to	retronasal olfaction,	mentioned	above,	where	odours	rise	from	the	oral	cavity	through	the	nasopharynx	to	the	olfactory epithelium	in	the	nasal	cleft.	Should	we	treat	this	other	route	to	the	olfactory	receptors	as	part	of	smell	or	a	second sense	of	smell? 10	The	Duality	of	Olfaction Olfaction	takes	place	when	odour	molecules	reach	the	olfactory	epithelium	in	the	nasal	cleft.	In	humans	(and	rats) there	are	two	different	pathways	by	which	molecules	reach	the	olfactory	epithelium:	via	the	nose	when	inhaling, and	via	the	mouth	when	eating	or	drinking.	The	orthonasal	route	is	when	chemical	stimuli	from	the	external environment	travel	to	the	nasal	mucosa	during	inhaling	or	sniffing.	This	is	what	we	commonly	think	of	as	'smell'.	In addition,	there	is	the	retronasal	route	by	which	odour	molecules	released	during	the	mastication	of	food	reach	the olfactory	epithelium	via	the	mouth.	The	act	of	swallowing	pulses	the	odours	up	to	the	olfactory	epithelium	via	the nasopharynx.	Should	we	take	these	two	routes	as	just	different	directions	in	which	stimuli	are	presented	in olfaction;	or	should	we	treat	them	as	giving	rise	to	different	modalities? Retronasal	olfaction	is	fundamental	to	the	formation	of	flavour	experiences.	Its	inputs	combine	with	inputs	from taste	to	create	unified	flavour	percepts.	And	yet	as	human	tasters	we	don't	recognize	this	olfactory	component. Although	we	smell	or	detect	odours	via	retronasal	olfaction,	it	seems	us	as	if	we	taste	them.	That	is,	the	qualities	of The Chemical Senses Page 9 of 31 PRINTED FROM OXFORD HANDBOOKS ONLINE (www.oxfordhandbooks.com). (c) Oxford University Press, 2015. All Rights Reserved. Under the terms of the l icence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Handbooks Online for personal use (for details see Privacy Policy). Subscriber: OUP-Reference Gratis Access; date: 11 August 2015 fruitiness,	or	more	precisely,	pineapple	or	cherry,	that	olfaction	contributes	to	the	flavour	we	are	tasting	appear	to occur	in	the	mouth.	The	event	of	swallowing	creates	the	most	intense	moment	of	flavour	release	due	to	odours being	pulsed	to	receptors	in	the	olfactory	epithelium.	In	fact,	when	food-related	odours	reach	the	olfactory epithelium	from	the	mouth,	the	qualities	detected	and	the	sensations	generated	are	typically	felt	to	be	experiences of	tasting	in	the	mouth.	This	location	illusion	is	due	to	oral	referral,	a	phenomenon	analogous	to	the	case	of	pain referral	or	phantom	limb,	where	a	sensation	is	felt	in	a	part	of	the	body	other	than	where	the	stimulus	is	being applied	(Soler	et	al.,	2010).	Because	the	resultant	flavour	experiences	are	classed	as	tastes	the	dimension	of	smell goes	missing	(or	unnoticed)	phenomenologically.	That	smell	doesn't	appear	as	a	separable	part	of	the	complex experiences	generated	by	the	interaction	of	taste	(and	touch)	and	retronasal	olfaction	doesn't	mean	that	we	can't focus	our	attention	on	the	dimension	it	contributes	(e.g.,	fruitiness).	Perhaps	the	fact	that	we	don't	recognize	it	as due	to	olfaction	provides	a	prima-facie	reason	for	thinking	that	retronasal	olfaction	should	not	be	counted	as	part	of our	sense	of	smell.	But	this	is	too	quick. We	can	experience	the	products	of	retronasal	olfaction	as	smells	under	certain	carefully	controlled	experimental conditions.	In	2004,	Heilman	and	Hummel	developed	a	device	for	directly	delivering	odours	above	the	soft	palate	to the	olfactory	epithelium	by	inserting	nasal	cannula	endoscopically	into	their	subjects'	noses	with	one	outlet	at	the nares	and	the	other	at	the	epipharynx.	This	allows	experimenters	to	pulse	odours	to	the	nasal	cleft	without interference	from	gustatory,	trigeminal,	or	thermal	mechanisms,	thus	allowing	the	study	of	retronasal	olfaction	in isolation	(Bender	et	al.,	2009a).	Non-food	odours	such	as	lavender	delivered	via	this	retronasal	route	will	be experienced	as	smells	not	tastes	(Small	2005). With	this	technique	it	is	possible	to	show	that	despite	having	the	same	volume	of	odour	in	the	olfactory	cleft, thresholds	for	detection	and	intensity	are	different	for	orthonasally	and	retronasally	delivered	odour	stimuli.	Less invasively,	participants	can	inhale	the	vapour	state	of	an	odorous	liquid	from	a	straw	inserted	into	the	headspace of	an	otherwise	sealed	glass	jar.	By	means	of	the	straw,	participants	inhale	the	odour	in	the	headspace	above	the liquid	and	exhale	the	odour	through	the	nose.	In	one	study	vanilla	odour	was	presented	alone	and	54.7	per	cent	of participants	reported	the	location	of	the	vanilla	experience	as	in	the	nose	(Lim	and	Johnson,	2011).	When	there were	incongruent	tastants	such	as	NaCl	or	caffeine	in	the	mouth,	this	made	little	difference	to	the	location	of	the vanilla	odour.	However,	when	the	inhaled	vanilla	was	presented	with	sucrose	in	the	mouth,	69.4	per	cent	of participants	localized	the	vanilla	to	the	tongue	or	oral	cavity.	This	indicates	that	if	retronasally	presented	odours are	combined	with	congruent	tastes	(i.e.,	taste-odour	combinations	they	have	encountered	before),	then	most people	experience	these	stimuli	not	as	separate	smells	and	tastes	but	as	unified	flavours. The	separately	experienced	smells	from	retronasally	delivered	odours	are	laboratory	created,	in	contrast	to	the everyday	cases	where	the	phenomenologically	individuated	experiences	of	flavour	give	us	no	clue	as	to	the influence	of	olfaction	in	generating	these	experiences,	so	we	may	want	to	draw	a	distinction	between	the	sense	of smell	and	olfactory	processes	in	general,	restricting	the	sense	of	smell	to	orthonasal	olfaction	alone. Another	option	is	to	treat	olfaction	with	its	orthonasal	and	retronasal	pathways	as	a	dual	modality,	and	on	that	basis try	to	identify	two	separate	senses	of	smell. 11	Two	Senses	of	Smell? The	first	person	to	propose	olfaction	as	a	dual	system	was	Paul	Rozin	(1982), who	spoke	of	different	evolutionary functions	for	orthonasal	and	retronasal	olfaction:	to	sense	objects	in	the	world	and	sense	(or	assess)	objects	in	the mouth,	respectively.	He	saw	these	in	Gibsonian	terms	as	two	perceptual	systems	subserved	by	olfactory processes.	One	system	is	to	perceive	things	in	the	environment	such	as	food	sources,	predators,	smoke,	and potential	mates.	The	other	system	is	to	perceive	and	evaluate	the	quality	of	what	we	are	eating	or	drinking	in	order to	know	whether	we	should	continue	to	eat	it	or	expunge	it.	Rozin	hints	at	a	reason	why	'the	olfactory	[retronasal] component	loses	its	separate	identity	[when]	combined	with	available	oral	inputs	into	an	emergent	percept'	(Rozin, 1982:	400).	This,	he	thinks,	is	due	to	the	fact	that	the olfactory	stimulus	may	differ	qualitatively	in	the	in-mouth	and	out-there	cases	...	It	seems	very	likely	that the	olfactory	component	of	flavour	differs	markedly	from	the	olfactory	consequences	of	the	same substance	in	the	external	world. 6 7 The Chemical Senses Page 10 of 31 PRINTED FROM OXFORD HANDBOOKS ONLINE (www.oxfordhandbooks.com). (c) Oxford University Press, 2015. All Rights Reserved. Under the terms of the l icence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Handbooks Online for personal use (for details see Privacy Policy). Subscriber: OUP-Reference Gratis Access; date: 11 August 2015 (Rozin,	1982:	400) Neuroanatomy	lends	support	to	this	idea	and	explanations	have	been	offered	as	to	why	the	same	molecules, reaching	the	same	olfactory	receptor	cells	in	the	epithelium,	are	experienced	differently	depending	on	the	route	by which	they	got	there.	One	explanation	is	that	the	brain	distinguishes	the	direction	of	airflow	and	projects	the	pattern of	receptor	firings	to	slightly	different	cortical	areas	of	the	piriform,	insular,	and	orbito-frontal	cortices	depending	on whether	the	molecules	come	from	the	nose	or	from	the	mouth	(see	Small	et	al.,	2005). A	more	psychologically	convincing	way	to	confirm	Rozin's	idea	is	by	pointing	to	our	experiences	of	matches	or mismatches	between	smells	and	flavours.	We	often	have	expectations	about	how	something	will	taste	from	how	it smells;	at	other	times,	we	may	be	surprised	by	the	mismatch	between	how	something	smells	and	'tastes',	as	when a	wine's	aroma	gives	no	guide	to	its	performance	on	the	palate.	There	are	two	considerations	here.	Why	should smells	provide	any	expectations	about	how	things	taste?	Smells	are	one	thing,	tastes	are	another.	We	can't	smell saltiness	or	bitterness;	so	smelling	a	dish	or	a	wine	may	not	give	us	enough	information	to	discover	its	flavour.	This would	argue	that	whatever	olfaction	may	contribute	to	flavour,	it	is	sufficiently	unlike	things	we	sniff,	to	see	these as	different	senses	(and	as	the	sceptic	would	say,	to	accept	only	one	of	these	senses	as	involving	smell). However,	there	are	other	cases	too-we	talk	about	sweet	smells,	although	sweet	is	a	taste.	Compare	this	with	our talk	of	an	acidic	wine	as	tasting	sharp,	although	sharp	is	a	feel.	These	cross-modal	attributions	indicate	how	easy	it is	for	us	to	confuse	taste	and	smell,	and	perhaps	to	see	aspects	of	flavour	as	resembling	what	we	can	smell orthonasally.	Smells	tend	to	acquire	the	characteristic	properties	that	belong	to	tastes-sweet,	sour,	etc.-because they	are	so	closely	associated	with	things	which	when	put	in	the	mouth	have	a	sweet	or	sour	taste.	Why	should	a smell	be	so	closely	associated	with	something	that	tastes	sweet	or	sour?	It	is	because	the	smell	is	so	closely associated	with	the	retronasal	smell	that	a	sweet	or	sour	smelling	food	or	liquid	has	in	the	mouth	and	that contributes	to	its	flavour.	Smell	is	a	part	of	the	flavours	we	'taste':	the	wonderful	perfume	a	ripe	strawberry	gives	off is	a	pretty	reliable	guide	to	the	experience	we'll	have	when	we	eat	it.	How	is	this	to	be	explained	except	by supposing	that	the	aroma	detected	by	orthonasal	olfaction	is	similar	in	quality	to,	though	less	intense	than,	some part	of	the	flavour	(detected	by	retronasal	olfaction)	we	get	when	we	put	the	strawberry	in	our	mouths.	The	idea that	the	heady	strawberry	smell	is	sweet-smelling	is	intricately	connected	to	the	olfactory	dimension	of	its	flavour. What	is	unpredictable	on	occasion	about	flavour	from	orthonasal	olfaction	shows	the	difference	between	the orthonasal	and	retronasal	olfactory	contributions,	and	what	is	predictable	and	shared	suggests	that	they	are	both senses	of	smell.	We	can	reconcile	these	claims	by	claiming	that	there	are	two	senses	of	smell.	The	idea	that	the heady	strawberry	smell	is	sweet-smelling	is	a	projection	back	onto	the	inhaled	smell	of	some	retronasally	detected olfactory	property	intricately	bound	up	with	the	gustatory	element	in	its	flavour. This	idea	of	matching,	or	sometimes	mismatching,	in	terms	of	odours	and	smells,	experienced	by	the	orthonasal	or retronasal	route,	helps	to	explain	asymmetries	between	smell	and	'taste'	(flavour)	recognizable	in	our	ordinary experience.	When	you	smell	a	ripe	Epoisse	cheese,	for	instance,	it	can	be	pretty	off-putting.	(That's	the	smell	of isovaleric	acid.)	Yet,	when	you	put	it	in	your	mouth,	it	can	be	surprisingly	good.	The	cheese's	odour	when processed	externally	and	internally	via	the	mouth	lead	to	very	different	experiences.	The	reverse	case	is	coffee, where	the	aroma	of	freshly	brewed	coffee	always	smells	so	wonderful	though	it	is	slightly	disappointing	in	the mouth.	Many	of	the	volatile	compounds	are	stripped	off	by	saliva,	and	many	are	not	experienced	the	same	way	as they	are	when	inhaled	through	the	nose. The	connections	between	the	experiences	of	odours	and	the	experiences	of	food	flavours	play	a	key	role	in considering	retronasal	olfaction	to	be	a	part	of	smell.	For	instance,	loss	or	deterioration	of	our	ability	to	smell- identified	phenomenologically	by	changes	in	our	experience	of	odour-coincide	with	the	loss	of	flavour	perception. At	first,	the	reduced	experiences	of	anosmia	sufferers	are	regarded	as	loss	of	taste	not	smell. For	a	clinical	example,	where	patients	lose	olfaction,	they	often	report	that	they	cannot	taste	or	smell. However,	when	questioned,	patients	acknowledge	that	they	taste	salty,	sweet,	sour	and	bitter,	but	'nothing else'.	The	'nothing	else'	is	the	contribution	of	retronasal	olfaction	to	flavour. (Bartoshuk	and	Duffy,	1998:	284) More	evidence	of	smell's	contribution	to	flavour	experience	comes	from	normally	functioning	subjects.	Just	about all	normonosmics	have	specific	anosmias-that	is,	despite	having	a	fully	functioning	sense	of	smell,	there	is	some small	set	of	odours	which	they	cannot	detect	at	all.	When	this	is	the	case	they	will	be	unable	to	detect	certain 8 9 The Chemical Senses Page 11 of 31 PRINTED FROM OXFORD HANDBOOKS ONLINE (www.oxfordhandbooks.com). (c) Oxford University Press, 2015. All Rights Reserved. Under the terms of the l icence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Handbooks Online for personal use (for details see Privacy Policy). Subscriber: OUP-Reference Gratis Access; date: 11 August 2015 elements	in	airborne	molecules,	and	also	be	unable	to	detect	the	contribution	these	volatile	compounds	make	to flavour.	For	example,	several	normally	functioning	subjects	are	anosmic	as	far	as	Trichloranisole	(TCA),	or	cork taint	goes.	If	it	is	undetectable	by	the	nose,	drinkers	will	also	fail	to	find	any	fault	with	a	corked	wine	on	the	palate. The	selective	loss	of	smell	is	responsible	for	a	corresponding	deficit	in	flavour	perception. Another	similarity	between	everyday	smell	and	retronasal	olfaction	as	it	contributes	to	the	perceptions	of	a	food's flavour	involves	the	Proust	Phenomenon	where	the	sudden	whiff	of	a	long	forgotten	scent	seems	to	take	us	right back	to	a	past	time	or	a	scene	we	didn't	know	we	had	stored	in	our	memories.	Smell	has	a	special	capacity	to trigger	episodic	memories-especially	emotionally	charged	memories-because	of	the	direct	connections	olfaction has	with	parts	of	the	limbic	system	involved	in	emotion	and	memory.	Olfaction	is	unique	in	being	the	only	sensory system	that	projects	directly	to	the	amygdala	without	going	via	the	thalamus.	As	a	result,	odour	memories	are	longlasting	and	less	susceptible	to	interference.	This	may	account	for	the	Proust	Phenomenon,	which	takes	its	name from	the	famous	episode	where	Proust's	narrator,	Marcel,	tastes	a	madeleine	soaked	in	lime-blossom	tea	and	is suddenly	jolted	by	the	pleasure,	which	gradually	evokes	a	memory	of	his	aunt	on	Sunday	mornings	dipping	crumbs of	madeleine	into	tisane,	which	she	presented	to	him.	Experimental	attempts	have	been	made	to	confirm	the phenomenon	(Chu	and	Downes,	2000;	Hertz	and	Schooler,	2002). Are	there	any	reasons	to	think	the	duality	of	olfaction	just	discussed	gives	us	two	senses	of	smell?	There	are number	of	things	to	be	said	here. a.	The	cases	mentioned	above	involving	cheese,	coffee,	and	chocolate	show	how	orthonasally	and retronasally	presented	odour	stimuli	lead	to	different	experiential	effects.	Anecdotally,	people	often	prefer	the smell	of	cigars	smoked	by	others	to	the	experience	of	smoking	them	themselves.	This	is	possibly	due	to	the different	qualities	of	the	smoke	when	experienced	orthonasally	or	retronasally.	Retronasal	olfaction	also	plays a	crucial	role	in	the	perception	of	food	flavours	and	when	these	are	compared	to	the	smell	of	foods,	the comparisons	are	between,	not	smell	and	taste,	but	smell	and	a	different	olfactory	dimension	that	contributes to	flavour.	Moreover,	neurophysiologically,	orthonasal	and	retronasal	stimulation	of	the	olfactory	receptors project	to	slightly	different	cortical	areas,	and	detection	thresholds	and	intensity	ratings	are	also	different. b.	There	is	also	a	non-clinical	way	to	demonstrate	how	olfaction	contributes	to	flavour,	not	by phenomenological	decomposition	but	by	subtraction.	It	is	to	hold	one's	nose	pinched	tightly	shut	while popping	a	jelly	bean	into	one's	mouth	to	chew.	Keeping	the	nose	and	mouth	tightly	closed,	one	has	no	clue	at this	stage	as	to	the	fruit	flavour	of	the	jelly	bean.	All	one	experiences	at	first	is	some	sweetness	and	perhaps some	sourness.	However,	when	one	releases	the	nose	there	is	a	rush	of	odours	to	the	nose	from	the	mouth, and	immediately	one	can	tell	which	flavour-cherry,	strawberry,	or	orange-the	jelly	bean	has;	and	although the	nose	is	evidently	involved	in	supplying	the	additional	flavour	component,	the	fruity	flavour	now	seems	to be	coming	from	the	tongue.	One	knows	this	can't	be	the	case	since	just	a	moment	ago	when	the	nose	was closed,	one	learned	that	all	the	tongue	provided	was	sweet	or	sour.	Only	when	the	nose	is	unblocked	can	one have	an	experience	of	flavour.	What	we	have	here	is	a	clash	of	appearances.	Normally,	it	seems	as	if	the	fruit flavour	is	experienced	in	the	mouth,	on	the	tongue;	and	yet,	with	the	nose	blocked	it	appears	that	the	tongue only	provides	awareness	of	sweetness,	sourness,	saltiness,	etc.	The	way	to	resolve	the	tension	is	to recognize	the	contribution	that	smell	makes	to	the	experiences	that	seem	to	occur	on	the	tongue.	When people	discover	this	for	themselves	they	express	surprise	that	the	nose	is	required	to	have	full	flavour experiences,	but	they	have	no	difficulty	in	updating	their	commonsense	conception	of	tasting	to	incorporate the	part	smell	plays	in	the	experience	of	flavour.	They	do	not-as	those	who	resist	the	idea	that	tasting involves	smell	would	have	to	suppose-conclude	that	these	flavour	experiences	of	tasting	indicate	that	smell is	not	involved	but	that	the	single	sense	of	taste	is	implemented	at	the	neural	level	by	among	other	things receptors	that	also	serve	in	olfaction. c.	Although	retronasal	smell	is	usually	only	experienced	through	its	contribution	to	flavour,	we	can	sometimes engage	in	phenomenological	decomposition	as	well.	The	key	case	involves	the	flavour	of	menthol,	normally experienced	as	a	single	percept.	On	reflection,	we	recognize	that	an	experience	of	menthol	comprises	(i)	a slightly	bitter	taste,	(ii)	a	minty	aroma,	and	(iii)	a	cool	sensation	in	the	mouth,	and	we	can	attend	to	each	of these	features	in	turn.	Remove	any	one	of	them	and	you	no	are	longer	tasting	menthol.	The	three	chemical senses	work	together	to	produce	a	single	unified	flavour	experience,	and	yet	the	reason	why	we	can separately	attend	to	the	contributing	sensory	components	in	this	case	but	not	normally	may	be	due	to	Imenthol's	exceptionally	strong	trigeminal	stimultaion	whose	cooling	sensations	may	keep	one	keenly	aware	of 10 The Chemical Senses Page 12 of 31 PRINTED FROM OXFORD HANDBOOKS ONLINE (www.oxfordhandbooks.com). (c) Oxford University Press, 2015. All Rights Reserved. Under the terms of the l icence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Handbooks Online for personal use (for details see Privacy Policy). Subscriber: OUP-Reference Gratis Access; date: 11 August 2015 a	minty	aroma	in	one's	upper	airways.	But	even	in	this	case,	one	has	to	be	prompted	to	attend	to	the	parts separately	and	recognize	the	complexity	of	the	experience. d.	In	a	food	context,	we	can	think	of	the	two	senses	of	smell	as	associated	with	two	different	sorts	of	pleasure: the	pleasure	of	anticipation	when	we	smell	foods	orthonasally;	and	the	pleasure	of	reward	when	we	assess the	flavour	of	the	food	as	we	chew	and	swallow.	This	hedonic	distinction	between	the	two	senses	of	smell	is closely	related	to	Kent	Berridge's	distinction	between	wanting	and	liking	(Berridge,	1996).	This	distinction supports	the	idea	that	olfactory	cues	given	orthonasally	can	trigger	appetite	and	a	salivary	response, anticipating	the	reward	of	the	food	we	can	smell.	If	one	continues	to	receive	stimulation	without	having	the food	the	salivary	response	diminishes	as	a	result	of	habituation.	But	if	the	same	stimulus	odour	is	presented retronasally-controlling	for	the	same	intensity	judgement-the	salivary	response	returns,	suggesting	that these	different	presentations	of	the	same	odour	are	perceived	differently,	just	as	different	odours	are,	where repeated	presentation	of	the	first	leads	to	habituation	that	is	reversed	with	the	new	stimulus	(Bender	et	al., 2009b).	A	distinction	between	the	two	pleasures	associated	with	each	sense	of	smell	fits	well	with	Rozin's claim	about	the	different	evolutionary	functions	of	the	two	senses	of	smell:	to	sense	food	sources	(or	danger) in	the	environment;	and	to	assess	the	quality	of	what	we're	consuming	and	are	about	to	ingest.	It	also contributes	to	the	explanation	of	the	match	and	mismatch	between	orthonasally	and	retronasally	experienced odours. e.	There	are	other	experiential	differences	between	orthonasal	and	retronasal	olfaction	when	we	look	at cross-modal	interactions	with	further	senses.	A	congruent	colour	cue	enhances	the	intensity	of	our experience	of	odours	orthonasally,	but	suppresses	their	intensity	retronasally	(Korza	et	al.,	2005).	Textureodour	interactions	also	showed	differences	in	experience	depending	on	route	of	odour	delivery.	Certain odours	can	increase	the	perception	of	thickness	and	creaminess	but	only	when	presented	retronasally	(Bult et	al.,	2007). f.	Finally,	there	are	dissociations	between	orthonasal	and	retronasal	olfaction	that	show	up	in	pathological smell	experiences.	There	can	be	deficits	in	retronasal	smelling	without	orthonasal	deficits	(Cowart	and Halpern,	2003).	Conversely,	there	can	be	intact	retronasal	smell	shown	by	preserved	flavour	perception,	but where	'ordinary'	orthonasally	derived	smell	is	impaired	because	of	nasal	polyps.	(Landis	et	al.	2003).	And even	without	evidence	of	polyps,	there	is	a	population	of	patients	who	show	event-related	potentials	for retronasally	but	not	orthonasally	presented	stimuli	who	experience	flavours	but	do	not	discriminate	smells	in the	environment	(Landis	et	al.,	2005). Together,	these	considerations	provide	behaviour,	neuroanatomical,	phenomenological,	hedonic,	and	pathological evidence	in	favour	of	counting	olfaction	from	either	route	as	belonging	to	the	sense	of	smell,	and	provide motivating	reasons	for	crediting	humans	with	two	senses	of	smell. It	is	worth	noting	that	we	are	not,	here,	distinguishing	between	a	unimodal	experience	of	smell	and	smell	as	a component	of	a	multisensory	experience	of	a	food's	flavour.	Our	ordinary	sense	of	smell	may	seem	intuitively	to	be unimodal;	however,	orthonasal	olfaction	produces	experiences	of	odour	mostly	in	concert	with	stimulations	of	the trigeminal	nerve	(Frasnelli	and	Hummel,	2005).	The	experiences	we	recognize	as	smelling	are	typically	the	joint upshot	of	olfactory	and	trigeminal	processing;	so	are	themselves	multi-modal	experiences.	Of	course,	we	can	smell pure	odorants	such	as	phenol	ethyl	alcohol	that	do	not	activate	the	trigeminal	nerve,	but	we	don't	consciously distinguish	experiences	of	these	odours	from	other	odour	experiences	although	they	can	be	distinguished behaviourally	because	we	are	less	good	at	localizing	odour	sources	when	odours	are	non-trigeminal	stimulants. 12	What	Does	the	Tongue	Contribute	to	Tasting	Flavours? So	far	we	have	acknowledged	the	hidden	complexity	of	our	tasting	experiences	and	the	fundamental	contribution smell	makes-especially	that	due	to	retronasal	olfaction-to	our	experiences	of	flavour.	Tastes	rarely	occur	alone. Putting	aside	contributions	from	smell,	there	is	still	touch.	We	speak	of	foods	as	creamy,	or	chewy,	crunchy,	sticky, or	oily.	To	experience	just	tastes	proper	without	any	other	sensory	additions	one	would	need	to	anaesthetize	the tongue	not	just	for	feel	but	from	movement	or	pressure:	something	that	cannot	be	achieved	save	in	clinical settings.	Nevertheless,	pure	taste	plays	a	vital	role	in	our	tasting	of	flavours.	So	at	this	stage	it	is	worth	asking	just what	the	tongue	does	contributes	to	the	experience	of	flavour,	and	whether	it	contributes	the	same	in	each	of	us. Remember,	taste	proper	concerns	gustation	alone	where	this	covers	a	range	of	responses	to	chemical	stimuli, The Chemical Senses Page 13 of 31 PRINTED FROM OXFORD HANDBOOKS ONLINE (www.oxfordhandbooks.com). (c) Oxford University Press, 2015. All Rights Reserved. Under the terms of the l icence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Handbooks Online for personal use (for details see Privacy Policy). Subscriber: OUP-Reference Gratis Access; date: 11 August 2015 including	salts,	sugars,	acids,	and	toxins	by	different	classes	of	taste	receptors	on	the	tongue,	in	the	oral	cavity, and	in	the	gut.	(Sweet	taste	receptors	in	the	gut	play	an	important	part	in	insulin	regulation.)	Receptor	firings	on	the tongue	code	for	the	so-called	'basic	tastes'	such	as	salt,	sweet,	sour,	bitter,	as	well	as	the	fifth	taste,	umami (savoury).	To	basic	tastes	we	can	add	a	sixth	taste,	metallic,	and	perhaps	a	fatty	acid	taste,	although	there	is	still controversy	about	fat	as	a	basic	taste	(see	Mattes,	2009).	The	list	is	determined	in	part	phenomenologically-by characteristic	experiences	of	human	tasters	associated	with	different	kinds	of	stimuli-and	in	part	neurobiologically -by	the	discovery	of	specific	receptors	for	detecting	each	of	these	tastes.	It	is	the	task	of	psychophysics	to	align these	two	ways	of	picking	out	qualities	like	sweetness,	bitterness,	or	saltiness. How	well	are	they	aligned?	It	took	discoveries	in	neurobiology	to	convince	many	Westerners	to	accept	the	positing of	a	fifth	'basic'	taste,	umami. Although	tasting	it	everyday,	the	fifth	taste	remained	unknown	for	some	time,	and even	now	it	is	still	not	easy	for	people	to	recognize	umami	as	a	common	element	in	the	flavour	of	tomatoes, mushrooms,	soy	sauce,	peas,	parmesan	cheese,	and	seafood,	though	it	is	a	natural	category	for	the	Japanese.	By contrast,	English-speaking	countries	often	confuse	sour	and	bitter;	leaving	open	the	question	of	whether	they cannot	tell	experiences	of	these	two	tastes	apart,	or	whether	they	use	the	term	'sour'	for	instances	of	either.	With training	it	is	easier	to	distinguish	sour	and	bitter	by	use	of	examples	like	lemon	juice	and	quinine	(or	caffeine).	Part of	the	problem	may	be	that	bitter-tasting	substances	like	coffee	can	also	exhibit	sourness. The	list	of	basic	tastes	has	also	changed	through	history.	In	medieval	England,	cooks	recognized	at	least	eight basic	tastes	with	two	categories	for	salt:	salty	and	salty	like	the	sea,	as	well	as	sour	and	vinegary	(Woolgar, 2006).	Even	now,	there	are	good	grounds	for	saying	there	is	no	single	taste	of	sour,	or	of	bitter.	We	have	several different	types	of	receptors	for	detecting	bitter	chemicals.	There	are	also	reasons	to	worry	about	identifying	the basic	tastes	by	a	subject's	experience	of	tasting,	since	gustatory	tastes	are	modified	by	other	influences.	Mojet and	her	colleagues	(Mojet	et	al.,	2005)	have	shown	that	the	basic	tastes	differ	in	perceived	intensity	when	subjects taste	them	with	and	without	a	nose	clip,	thus	showing	that	smell	makes	a	contribution	to	the	normal	experience even	of	the	so-called	basic	tastes. As	we	saw	above,	subjects	who	lose	their	smell	tend	to	believe,	falsely,	that	they	cannot	taste.	This	shows	how unusual	it	is	to	experience	salt,	sweet,	sour,	and	bitter	just	as	tastes	in	the	absence	of	smell,	and	how unrecognizable	such	experiences	are	at	first	to	those	who	lose	their	sense	of	smell.	The	ordinary	experience	of	a supposed	basic	taste	is	probably	in	fact	the	experience	of	a	flavour:	the	result	of	combining	taste	and	smell	(see Spence	et	al.,	2014). We	are	equipped	physiologically	to	respond	to	the	basic	tastes	and	some	researchers	have	suggested	that labelled	lines	of	neural	relay	from	these	distinct	receptors	carry	information	about	particular	groups	of	stimuli,	such as	salts,	acids,	toxins,	to	the	primary	taste	cortex	in	the	insula	and	from	there	to	the	orbitofrontal	cortex.	The separation	of	the	basic	tastes	would	seem	to	follow	from	the	labelled	line	hypothesis:	that	receptors	for	salt,	sweet, sour,	bitter,	etc.,	carry	discrete	information	about	each	of	these	tastes	to	the	brain.	But	how	are	they	separately experienced	phenomenologically?	Philosophers	often	speak	of	experiencing	a	taste,	such	as	the	taste	of	honey,	as if	it	were	an	irreducible	item	of	our	phenomenology.	In	fact	we	can	dissect	the	tasting	of	honey,	and	even	more basic	flavours,	into	(i)	the	quality	by	which	the	experience	is	identified	(here,	its	sweetness),	(ii)	the	intensity	of	the experience,	(iii)	its	temporal	duration,	including	its	onset	and	offset	times,	and	(iv)	its	spatial	location	on	the	tongue. Let's	explore	these	aspects	of	tasting	further. Just	what	are	we	perceiving	by	means	of	basic	taste	detection?	Unlike	some	other	component	of	flavour	mixtures, salt	seems	to	retain	a	recognizable	identity.	We	can	often	discover	whether	foods	contain	salt	just	by	tasting	them, and	on	the	basis	of	intensity	determine	whether	foods	are	too	salty.	Do	we	therefore	perceive	salt	when	the response	to	the	stimulus	is	above	our	individual	detection	threshold?	Not	necessarily:	low	levels	of	salt concentration	just	above	threshold	can	appear	sweet	to	a	taster,	and	'concentrations	of	salt	that	are	weaker	than levels	in	saliva	may	give	rise	to	bitter	tastes'	(Breslin,	2000:	435).	If	we	take	the	function	of	such	taste	sensations	to be	to	generate	perceptual	experiences	of	salt,	then	these	cases	count	as	misperceptions.	In	these	cases,	the quality	of	the	experience	is	modified	by	differences	in	the	intensity	of	the	stimulus.	Moreover,	saltiness	is	a response	not	only	to	sodium	chloride	but	also	to	other	salts,	like	potassium	chloride.	We	can	discriminate	these	by tasting,	because	potassium	chloride	has	not	just	a	salty	taste	but	also	a	slightly	bitter	taste.	Again,	with	sweetness, we	can	distinguish	between	sucrose	and	artificial	sweeteners,	because	Aspertame	has	a	later	onset	time	and longer	offset	time;	here	it	is	the	temporal	difference	in	the	onset	and	duration	of	receptor	firing	that	allows	us, 11 The Chemical Senses Page 14 of 31 PRINTED FROM OXFORD HANDBOOKS ONLINE (www.oxfordhandbooks.com). (c) Oxford University Press, 2015. All Rights Reserved. Under the terms of the l icence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Handbooks Online for personal use (for details see Privacy Policy). Subscriber: OUP-Reference Gratis Access; date: 11 August 2015 phenomenologically	and	perceptually,	to	distinguish	between	the	two.	Hence,	the	different	aspects	of	taste sensations	identified	above	play	a	crucial	role	in	determining	the	character	and	object	of	the	resultant	perceptual experience. Tastes	also	differ	in	their	perceived	location	on	the	tongue.	The	receptive	fields	of	the	different	taste	receptors	are not	uniform	in	their	sensitivity	on	all	parts	of	the	tongue	(Collings,	1974).	So	we	can	localize	our	greatest	sensitivity to	bitterness	at	the	back	of	the	tongue,	to	sourness	at	the	sides,	and	sweetness	at	the	front,	even	though	we	have greater	receptor	capacity	to	detect	bitterness	at	the	front.	Detecting	bitterness	at	the	back	of	the	tongue	gives	us	a last	resort	to	reject	before	swallowing	potentially	toxic,	bitter	tasting	compounds.	Other	changes	to	the	experience of	a	taste	result	from	increase	in	perceived	intensity	or	concentration	of	the	stimulus.	When	salt	concentration	is too	high	the	perceived	intensity	creates	a	burning	sensation,	which	is	more	accurately	described	as	feel	or	touch, rather	than	a	taste. There	are	other	cross-modal	effects	involving	taste.	Cruz	and	Green	(2000)	have	found	that	thermal	stimulation	of the	tongue	can	evoke	sensations	of	taste	in	about	a	quarter	of	the	population.	Warming	an	area	of	the	tongue	can give	rise	to	a	sweet	taste,	while	cooling	it	can	give	rise	to	a	salty	or	sour	taste.	Green	calls	these	subjects	thermal tasters	and	although	the	effects	among	this	population	are	quite	stable	and	affect	their	tasting	of	foods	and	drinks, we	can	think	of	these	as	illusions	of	taste. There	are	other	ways	of	producing	taste-like	sensations	through	other	modalities,	including	electrical	stimulation	of the	tongue,	odour-induced	tastes,	and	colour-induced	tastes.	The	latter	occurs	when	subjects	are	given	coloured drinks	to	rate	for	sweetness	or	sourness.	The	drinks	do	not	contain	any	tastants.	They	are	tasteless	and	odourless, and	yet	a	significant	proportion	of	subjects	rate	the	lime	green	drink	as	sour	and	the	cherry	red	drink	as	sweet (Spence	et	al.,	2010). The	case	of	odour-induced	tastes	is	crucial	for	our	understanding	of	the	extent	of	flavour	perception	(see	below): To	experience	odor-induced	tastes,	one	need	simply	sniff	odors	such	as	strawberry,	vanilla,	mint,	or chocolate,	all	of	which	are	routinely	described	by	Westerners	as	'sweet	smelling'. (Stevenson	and	Tomiczeck,	2007:	295) This	is	a	case	of	sweetness	enhancement,	where	the	orthonasal	presentation	of	a	congruent	odor	in	the	presence of	a	taste	can	increase	the	perceived	intensity	of	the	taste,	and	suppress	sourness,	thus	functioning	like	taste proper.	Some	researchers	have	even	reported	that	subjects	can,	under	certain	circumstances,	experience	sweet taste	in	a	water	solution	with	no	sucrose	when	it	is	accompanied	by	a	retronasally	administered	odour	of	isoamyl acetate	(banana)	(Hort	and	Hollowood,	2004).	This	phenomenon	of	sweetness	enhancement	is	a	cross-modal effect	where	the	workings	of	a	single	modality,	taste,	is	modified	in	that	modality	by	stimulating	another	modality, olfaction,	without	the	later	being	fused	with	taste	into	a	single	more	complex	flavour	percept. The	increase	in	perceived	sweetness	for	the	same	item	tasted	and	rated	for	sweetness	before	and	after accompanying	with	a	sweetness-enhancing	odour	(vanilla,	or	strawberry)	is	superadditive.	The	result	of	combining a	sucrose	solution	with	the	odour	is	greater	in	terms	of	neural	activation	and	experiential	effect	than	the	summation of	activation	of	the	two	components.	This	phenomenon	of	sweetness	enhancement	is	a	cross-modal	effect	but	in this	case	we	do	not	get	the	multisensory	integration	or	fusion	of	ordinary	(orthonasal)	smell	and	taste	into	a	single percept,	but	merely	the	influence	of	one	modality	on	another.	We	still	recognize	the	distinct	components	of orthonasal	olfaction	and	gustation.	Temperature	has	cross-modal	effects	on	taste	too:	when	coffee	gets	cold	this increases	its	perceived	bitterness. All	of	these	taste-altering	effects	can	be	induced	under	special	conditions,	but	in	normal	conditions	of	perception do	we	tasters	perceive	the	same	things?	Does	the	psychophysics	give	us	grounds	for	intersubjective	agreement about	the	basic	tastes?	As	we	saw	earlier,	there	is	cross-cultural	variation	in	taste	experience.	In	addition,	there are	complexities	introduced	by	blending	stimuli	with	the	basic	tastes	in	mixtures.	Sometimes,	combined	tastes	don't blend	but	keep	their	separate	identity,	as	in	sweet	and	sour	sauces.	At	other	time,	sweetness	suppresses sourness.	Umami	can	enhance	saltiness:	so	by	adding	monosodium	glutamate	to	a	dish	it	can	taste	saltier. Combining	different	types	of	umami	creates	synergistic	reactions	to	intensify	flavour.	These	produce	greater intensity	and	are	the	basis	for	popular	flavour	pairings	such	as	anchovy	and	tomato	in	pizza,	and	scallops	and	pea The Chemical Senses Page 15 of 31 PRINTED FROM OXFORD HANDBOOKS ONLINE (www.oxfordhandbooks.com). (c) Oxford University Press, 2015. All Rights Reserved. Under the terms of the l icence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Handbooks Online for personal use (for details see Privacy Policy). Subscriber: OUP-Reference Gratis Access; date: 11 August 2015 puree.	Combining	all	the	basic	tastes	usually	leads	to	an	overall	suppression	in	taste	intensity,	unless	one	includes umami.	This	is	maybe	why	tomato	ketchup	is	so	popular	since	it	combines	all	of	the	five	basic	tastes. Despite	being	able	to	recognize	salt,	sweet,	sour,	and	bitter,	people	often	disagree	about	the	intensity	of	these tastes	in	a	dish,	arguing	about	whether	the	food	has	enough	salt	or	too	much.	Does	this	show	that	tasting something	as	salty	is	a	wholly	subjective	matter?	Not	necessarily;	their	tongues	could	have	different	numbers	of fungiform	papillae	or	taste	buds.	Those	who	have	dense	clustering	of	papillae	on	the	tongue	often	show	a	quick and	intense	reaction	to	salt,	sour,	or	bitter	compounds.	The	explanation	is	clear	in	the	case	of	salt.	What	makes something	taste	salty	is	the	number	of	salt	receptors	that	are	simultaneously	touched	by	NaCl	molecules.	So	if	an individual's	taste	buds	are	densely	packed	they	will	need	little	salt,	whereas	those	whose	taste	buds	are	more widely	spread	out	will	need	lots	more	salt	to	get	the	same	effect.	Using	this	fact,	nanotechnologists	have	been working	on	a	salt	molecule	that	will	unwrap	itself	and	touch	more	receptors	simultaneously	leading	to	the	use	of less	salt	for	the	same	perceived	effect.	So,	while	individuals	have	different	sensitivities,	they	are	picking	up	on	the same	quality	and	perceiving	it	differently. Those	who	show	an	increased	sensitivity	to	basic	tastes	are	the	so-called	super-tasters.	The	term	was	coined	by Linda	Bartoshuk	(see	Bartoshuk	et	al.,	1994)	who	fashioned	a	quick	test	to	sort	people	into	three	groups:	supertasters,	tasters,	and	non-tasters.	The	test	is	based	on	their	sensitivity	to	tasting	strips	soaked	in	PROP (propthiouricil)	or	PTC	(phenylthiocarbamide).	When	the	strip	is	placed	on	the	tongue	people	show	one	of	three reactions-they	find	it	intensely	bitter,	mildly	bitter,	or	tasteless.	There	is	a	normal	distribution	with	more	people being	(mild)	tasters,	25	per	cent	being	super-tasters	and	25	per	cent	being	non-tasters.	General	conclusions	have been	drawn	from	the	test	about	how	sensitive	these	groups	will	be	to	other	basic	tastes,	but	the	conclusions	have been	overdrawn.	Juyum	Lim	and	Barry	Green	(2008)	have	shown	that	while	high	sensitivity	to	PROP	correlates	with sensitivity	to	other	tastes,	there	is	no	evidence	that	non-tasters	have	lower	sensitivity	to	other	tastes.	Some	PROP tasters	do	not	even	show	increased	sensitivity	to	bitter	tasting	quinine.	Lim	and	Green	suggest	that	we	may	need	to distinguish	between	PROP	tasters	and	general	tasters,	and	that	only	the	latter	category	is	useful	for	predicting	oral sensitivity	and	preferences.	General	super	tasters	are	often	conservative	eaters,	finding	some	foods	too	bitter	and white	wines	too	sour;	whereas	low	sensitivity	(non-)tasters	are	more	adventurous	eaters	and	enjoy	spicy	foods. Super-tasters	with	more	taste	buds	on	their	tongue	will	have	more	trigeminal	nerve	endings	exposed	to	irritation	by spices,	which	may	explain	why	they	tend	to	avoid	spicy	food	and	strong	alcohol.	Bartoshuk	is	right,	though,	that such	sensitivity	is	a	spectrum	and	at	the	lowest	end	we	find	people	for	whom	no	dish	is	too	sweet	or	too	salty.	So before	we	try	to	draw	conclusions	about	the	subjectivity	of	taste	from	disagreements	in	judgements	about	the saltiness	or	sweetness	of	a	dish	we	need	to	be	sure	we	are	comparing	like	with	like	and	that	the	same	phenomena are	in	dispute. Finally,	we	must	not	confuse	the	subjective	experience	of	tasting	that	depends	on	detecting	a	basic	taste,	with	the hedonic	reaction	to	it.	What	we	taste	is	one	thing,	and	what	we	feel	about	it	is	another.	The	liking	for	sweetness	and the	disliking	for	bitterness	are	innate	but	they	are	not	fixed	and	may	be	changed	by	experience	or	conditioning. Disagreements	about	taste	are	harder	to	establish	than	is	commonly	thought,	and	attempts	to	make	philosophical hay	from	then	have	to	be	treated	with	caution	(see	MacFarlane,	2007;	Smith,	2010). 13	Flavour	as	a	Psychological	Construct-the	Sophisticated	Sensation	View As	we	saw	earlier,	the	simple	sensation	view	of	tastes	as	the	experiences	we	have	when	we	eat	or	drink	is hopelessly	flawed.	The	items	we	consume	strike	us	as	fruity,	creamy,	spicy,	sour,	meaty,	oily:	and	these properties,	or	our	experiences	of	them,	can	only	be	had	through	the	combined	workings	of	the	chemical	senses and	touch.	So	the	simple	sensation	view	of	tasting	is	too	simplified	to	cover	the	range	of	phenomena	we	include under	experiences	of	tasting	flavours. The	sophisticated	sensation	view	denies	that	'taste'	(flavour)	experiences	are	simply	complex	bundles	of sensations.	Flavours	are	experienced	as	unified	wholes	without	a	part-whole	structure.	Despite	their	unified appearance,	flavour	experiences	arise	through	the	multi-modal	integration	of	taste	proper	with	retronasal	olfaction, to	which	we	have	to	add	somatosensory	sensations	of	texture	and	temperature,	mechanoreceptors	triggered	by chewing,	and	often	the	chemical	irritation	of	the	trigeminal	nerve.	The	unity	of	flavour	percepts	seems	to	be created	by	the	brain,	so,	on	this	view,	there	need	be	no	flavours	in	nature:	the	flavours	we	experience	only	come 12 The Chemical Senses Page 16 of 31 PRINTED FROM OXFORD HANDBOOKS ONLINE (www.oxfordhandbooks.com). (c) Oxford University Press, 2015. All Rights Reserved. Under the terms of the l icence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Handbooks Online for personal use (for details see Privacy Policy). Subscriber: OUP-Reference Gratis Access; date: 11 August 2015 into	existence	when	our	brains	combine	information	from	these	different	sensory	systems.	They	are	just psychological	constructs,	or	brain	products.	This	is	psychologist	John	Prescott's	position	(1999)	and	many	others in	psychology	and	neuroscience	have	followed	suit.	A	fairly	typical	statement	of	the	view	is	given	by	Dana	Small, echoing	remarks	made	by	sensory	scientist	Gordon	Sheppard: Flavour	is	in	the	brain,	not	the	food.	It	is	the	brain	that	unites	the	discrete	sensory	inputs	from	the	food	and drinks	we	ingest	to	create	flavour	perceptions. (Small,	2012:	540) In	what	follows,	I	will	resist	this	position	and	make	out	a	case	for	the	objectivity	of	flavours	and	flavour	perception. But	first	let	us	take	a	critical	look	at	the	view	of	flavours	as	constructs. There	is	a	slip	in	the	immediately	preceding	quote	between	the	first	and	second	sentence.	Small	is	right	that	the brain	binds	discrete	sensory	inputs	to	create	flavour	perceptions,	but	this	doesn't	support	the	claim	that	flavours are	made	by	brains.	Flavours	are	one	thing,	flavour	perceptions	are	another.	Many	empirical	scientists	simply	elide them,	but	without	a	distinction	between	the	perceptions	and	what	they	are	perceptions	of,	it	becomes	hard	to	draw a	line	between	veridical	and	non-veridical	perceptions,	and	harder	still	to	know	which	aspects	of	the	overall experience	of	eating	and	drinking	contribute	to	flavour	perception.	What	is	it	that	makes	an	experience	one	of flavour? When	it	comes	to	defining	flavour	experiences,	psychologists	tend	to	be	maximalists.	Here	is	Martin	Yeomans: arguably,	multi-sensory	integration	may	be	at	its	most	extreme	in	the	case	of	flavour	perception	since	few other	experiences	offer	the	opportunity 	for	concomitant	stimulation	of	all	the	major	senses:	gustation through	the	five	primary	tastes,	olfaction	through	both	orthoand	retronasal	stimulation	of	olfactory receptors	by	volatile	compounds	released	from	food,	mechanoreception	contributing	to	our	perception	of texture	and	providing	information	on	temperature,	pain	arising	from	oral	irritants	and	hearing	that	results from	sounds	and	vibrations	coming	from	the	mouth	contributing	to	our	perception	of	aspects	of	texture. (Yeomans	et	al.,	2008:	565) Notice	that	we	can	selectively	attend	to	some	of	the	multisensory	components	of	flavour	experience,	such	as	the contribution	of	touch:	i.e.,	when	a	food	or	liquid	is	described	as	being	creamy,	oily,	crunchy,	or	melting;	these seem	to	be	part	of	flavour	experiences.	But	what	of	the	look	of	a	dish,	or	the	sound	of	a	crunchy	food?	Are	they part	of	flavour?	They	can	affect	how	we	experience	foods	and	drinks.	For	example,	the	colour	of	liquids	can influence	our	perception	of	sweetness	or	sourness	(Spence	et	al.,	2010).	So	should	we	speak	of	a	visual component	of	flavour?	And	in	a	classic	experiment	by	Zampini	and	Spence	(2004),	participants	who	ate	stale potato	crisps	while	wearing	headphones	that	amplified	the	high	frequency	sound	of	their	own	crunching	found	the crisps	to	taste	fresh.	So	is	sound	part	of	flavour? Often	we	don't	know	that	these	visual	or	auditory	inputs	are	changing	our	perceptions	of	flavour.	We	cannot phenomenologically	separate	them	from	the	flavours	we	would	experience	anyway,	nor	can	we	attend	to	the contribution	they	make,	as	we	can	do	with	say	the	effect	of	the	touch	or	texture	of	foods.	Notice,	though,	that	the contribution	of	touch	is	not	always	so	easily	phenomenologically	separable.	We	say	biscuits	'taste'	stale,	but	these have	the	same	taste	and	smell	properties	as	fresh	biscuits;	but	crumble	differently	in	the	mouth.	It	is	this	texture clue	that	leads	to	us	say	they	'taste'	stale.	And	while,	without	input	from	sight	and	sound	we	still	taste	flavours,	it would	be	hard	to	experience	tastes	or	flavours	without	touch.	But	is	touch	part	of	flavour?	To	experience	tastes without	a	sense	of	touch,	pressure,	or	localization	in	the	mouth	requires	one	to	have	the	chorda	tympani	nerve anesthetized.	How	would	the	free-floating	tastes	be	experienced?	Too	little	is	known.	So	touch	usually accompanies	taste	and	smell,	but	is	it	part	of	flavour?	Calling	something	creamy	is	attributing	something	to	its flavour.	Also,	the	creaminess	of	a	food	can	affect	its	smell	and	therefore	it's	flavour,	and,	conversely,	certain odours	can	increase	the	perception	of	creaminess	and	richness	of	a	food	in	the	mouth	(Bult	and	Hummel,	2007). An	example	of	tactile	inputs	to	tasting,	though	not	recognized	as	such,	is	the	experience	of	black	pepper.	The tongue	doesn't	have	taste	receptors	for	black	pepper.	It	activates	nerve	endings	that	belong	not	to	taste	but	to touch,	though	they	make	a	genuine	contribution	to	our	experience	of	the	flavours	of	food	and	drinks.	The	chemical irritants	in	mustard	oil	and	peppermint	oil	(or	I-menthol)	elicit	different	responses	in	the	trigeminal	nerve.	Mustard The Chemical Senses Page 17 of 31 PRINTED FROM OXFORD HANDBOOKS ONLINE (www.oxfordhandbooks.com). (c) Oxford University Press, 2015. All Rights Reserved. Under the terms of the l icence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Handbooks Online for personal use (for details see Privacy Policy). Subscriber: OUP-Reference Gratis Access; date: 11 August 2015 feels	hot	and	peppermint	cool,	although	there	is	no	change	of	temperature	in	the	mouth.	These	effects	are	due	to chemesthesis.	The	extension	of	the	burning	and	stinging	to	other	places	on	the	skin	shows	that	chemesthesis	is not	a	single	sense	but	overlaps	with	nociception	and	thermoreception	and	is	part	of	somatosensation. The	trigeminal	nerve,	though,	is	crucial	to	flavour	experience	and	one	of	the	hidden	flavour	senses.	It	governs	our response	to	carbonation	in	fizzy	drinks	and	sparkling	wines.	CO is	a	trigeminal	stimulant	that	produces	the	prickle we	feel	on	the	tongue	and	around	the	mouth.	The	stimulation	of	trigeminal	nerves	in	the	nose	accompanies	most but	not	all	odorants	and	boosts	olfaction.	Thus	those	who	lose	smell	can	sometimes	continue	to	detect	strong odours	by	means	of	trigeminal	stimulation.	(Should	we	think	of	this	as	smelling	by	feel?) Does	the	sense	of	touch	play	a	role	in	flavour	and	flavour	perception,	and	is	it	just	a	single	sense	of	touch	that	is involved,	including	as	it	does	tactile	sensations,	haptic	explorations,	thermal	responses,	pain,	and	chemical irritation?	This	just	shows,	perhaps,	how	difficult	it	is	to	think	of	touch	as	single	sense,	let	alone	flavour	as	a	single sense. 14	The	Nature	of	Flavours	and	Flavour	Experiences Taking	all	these	factors	into	account,	how	should	we	understand	our	experiences	of	flavour?	How	should	we	react to	the	claim	that	there	are	non-phenomenologically	distinct	components	that	contribute	to	the	experience	of	tasting a	single	flavour?	Should	we	let	the	phenomenology	dictate	the	ultimate	nature	of	flavour	experiences,	or	should	we individuate	flavours	via	the	interacting	factors	that	give	rise	to	our	experiences	of	flavours?	Should	we	view	them as	the	complex	products	of	multisensory	integration?	Or,	as	conscious	experiences,	should	we	take	them	to	be	just as	they	seem	to	us:	whole,	unified,	unimodal	experiences	being	just	as	they	appear?	The	trouble	with	both	of	these ways	of	proceeding	is	that	they	seem	to	let	in	all	sorts	of	contributions	from	external	touch,	sound,	and	sight	to	the constitution	of	flavour. Let	us	consider	the	scientifically	informed	way	of	individuating	flavours	and	flavour	experiences	as	constructs	of the	brain	by	looking	at	the	following	definitions: (1)	Flavour	is	a	'complex	combination	of	the	olfactory,	gustatory	and	trigeminal	sensations	perceived	during tasting.	The	flavour	may	be	influenced	by	tactile,	thermal,	painful	and/or	kinaesthetic	effects'	(AFNOR,	1992). (2)	'Flavour	perception	should	be	used	as	the	term	for	the	combinations	of	taste,	smell,	the	trigeminal	system, touch,	and	so	on,	that	we	perceive	when	tasting	food'	(Auvray	and	Spence,	2008,	italics	mine). (3)	'Flavor	perception	arises	from	the	central	integration	of	peripherally	distinct	sensory	inputs	(taste,	smell, texture,	temperature,	sight,	and	even	sound	of	foods)'(	Small	et	al.,	2004). (4)	'Taste'	is	often	used	as	a	synonym	for	'flavour'.	This	usage	of	'taste'	probably	arose	because the	blend of	true	taste	and	retronasal	olfaction	is	perceptually	localized	to	the	mouth	via	touch	(Bartoshuk	and	Duffy, 2005:	27). Each	of	(1)	to	(4)	acknowledges	that	the	flavour	experiences	we	have	when	tasting	depend	on	sensory interactions,	but	they	vary	in	mentioning	different	elements	in	the	interactions.	And	despite	being	offered	as definitions	of	flavour,	what	they	actually	describe	are	flavour	experiences	or	flavour	perceptions.	(The	definition	of Auvray	and	Spence	in	(3)	is	more	careful	though	elsewhere	they	collapse	flavour	and	flavour	perception.)	If	this conflation	is	deliberate	then	flavours	should	be	inseparable	from	our	experiences	of	flavours.	However,	the objectivist	who	seeks	to	distinguish	flavours	from	flavour	perceptions	can	appeal	to	cases	where	something	can causally	affect	our	perception	of	a	food	or	liquid's	flavour	without	it	being	a	contributing	factor	to	flavour perception.	Certain	elements	can	influence,	without	being	part	of,	flavour	perception.	Here,	we	need	a	distinction between	constitutive	and	causally	affecting	factors	in	flavour	perception:	i.e.,	between	components	that contribute	to	what	a	flavour	perception	is	and	those	that	merely	have	a	causal	effect	on	flavour	perceptions.	This is	not	an	easy	distinction	to	draw,	but	before	we	try	to	do	so,	let	us	look	at	the	problems	that	arise	for	those	who take	flavours	to	be	exhausted	by	flavour	experiences.	What	can	they	tell	us	about	the	boundaries	to	flavour experiences?	What	does	and	doesn't	belong	in	the	list	of	contributing	factors? Researchers	in	psychology	and	neuroscience	agree	on	what	is	going	on	when	we	eat	and	drink	and	which	factors are	present.	What	they	disagree	about	is	which	factors	belong	to	flavours	and	which	don't.	For	Auvray	and Spence,	taste,	smell,	touch,	and	the	trigeminal	systems	are	involved	and	maybe	more;	while	for	Bartoshuk,	touch	is 2 13 The Chemical Senses Page 18 of 31 PRINTED FROM OXFORD HANDBOOKS ONLINE (www.oxfordhandbooks.com). (c) Oxford University Press, 2015. All Rights Reserved. Under the terms of the l icence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Handbooks Online for personal use (for details see Privacy Policy). Subscriber: OUP-Reference Gratis Access; date: 11 August 2015 not	part	of	flavour.	It	merely	plays	a	role	in	helping	to	combine	taste	and	retronasal	olfaction.	For	Yeomans,	and maybe	Small,	sight	and	sound	are	also	in	there. Why	stop	there?	Is	the	hedonic	value	of	the	experience	part	of	flavour?	After	all,	when	we	eat	something	we immediately	assess	it	for	pleasantness	or	unpleasantness.	Tasting	is	an	evaluative	activity,	letting	us	judge whether	something	is	delicious,	bland,	or	disgusting.	Some	people	take	the	whole	point	of	tasting	not	to	be	about discovering	the	flavours	of	the	food	or	drink,	but	about	arriving	at	a	hedonic	rating:	they	like	it	or	don't,	which	will decide	subsequent	behaviour.	It	is	also	difficult	to	separate	the	unpleasantness	of	what	we're	tasting-sea	urchin, say-from	the	food	itself,	which	is	why	we	find	it	so	hard	to	understand	how	others	can	overcome	this	flavour	and enjoy	it.	So	should	we	include	the	hedonic	dimension	of	eating	or	drinking	in	our	account	of	flavour	making	that notion	even	more	subject	to	the	varieties	of	subjectivities	among	tasters?	For	Yeomans,	the	issue	of	whether hedonic	value	is	part	of	flavour	is	indeterminate	since	they	go	together	and	are	tested	together.	Even	expectations of	the	same	food	generated	by	different	words	can	have	an	effect	on	its	acceptability.	Here	is	how	it	works. Yeomans	et	al.	(2008)	gave	two	groups	of	subjects	smoked	salmon	ice	cream	to	taste:	one	group	was	told	that	it was	ice	cream,	while	the	other	was	told	it	was	frozen	savoury	mousse.	The	first	group	liked	it	less	than	the	second and	reported	it	as	tasting	saltier	and	more	savoury	than	the	second	group.	So	according	to	Yeomans: flavour	perception	is	an	integration	of	sensory	information	with	past	memory	of	similar	stimuli	predicated	by the	visual	qualities,	and	accompanying	written	descriptor,	for	the	rated	food.	(2008:	569,	italics	mine) Verhagen	(2007)	argues	that	we	should	include	hedonics	in	flavour	since	the	processing	in	the	food	recognition network	is	paralleled	by	processing	in	a	different	but	overlapping	reward	value	system.	The	neural	evidence	is mixed,	however,	and	does	not	entirely	support	this	verdict,	as	we	shall	see	below.	But	why	stop	here?	The experience	of	eating	and	drinking	responds	to	the	posture	of	our	bodies,	the	ambient	temperature,	sounds	we	are listening	to,	as	well	as	other	affective	states.	Why	not	consider	the	total	sensory	input	to	the	overall	experience	of eating	or	drinking	as	part	of	flavour?	What	are	the	constraints	on	the	combination	of	co-occurring	sensory	inputs? We	know	some	of	the	rules	of	binding	across	sensory	modalities.	In	the	case	of	multi-modal	flavour	experiences, the	key	idea	is	congruence.	Lim	and	Johnson	(2011)	show	that	it	is	congruent	tastes	and	retronasally	presented odours	that	lead	to	the	referral	of	the	olfactory	component	of	flavour	perception	to	the	oral	cavity.	The	subjectivist who	thinks	flavours	are	in	the	brain	owes	us	an	account	in	purely	internal	terms	of	what	makes	inputs	from	different sensory	modalities	congruent	or	incongruent.	(This	will	be	a	more	or	less	notion	for	the	sophisticated	subjectivist.) Perhaps,	it	will	be	said	that	it	is	a	matter	of	their	occurring	together,	but	this	would	include	all	the	cases	of	potential illusions	we	worried	about	above.	The	objectivist	can	better	explain	congruence	in	terms	of	the	configurations	of odorous	and	sapid	properties	in	the	foods	that	make	up	the	naturally	occurring	flavours.	Banana	odour	and sweetness	are	combined	in	bananas,	and	they	are	integrated	when	one	is	retronasally	and	the	other	is	orally registered.	So	why	not	think	of	bananas	as	having	a	flavour	which	it	is	the	job	of	the	flavour	perceptions	produced by	the	brain	to	track? No	such	appeal	is	possible	for	the	'flavour	is	in	the	brain'	theorist.	She	still	hasn't	told	us	what	flavours	are.	Do	they include	sounds	and	colours?	As	pointed	out	above,	researchers	agree	about	the	nature	of	co-occurring	inputs. They	disagree	about	which	inputs	count	as	parts	of	flavour.	Empirical	inquiry	will	not	settle	this	matter.	So,	if	there	is a	line	to	be	drawn	between	what	constitutes	flavour	experience	and	what	merely	causally	affects	it,	this	will	be drawn	theoretically	or	philosophically	by	deciding	on	the	best	overall	account	to	make	sense	of	the	phenomena	in the	light	of	the	underlying	facts	about	neuroanatomy,	neural	connections,	behaviour,	experience	and	stimuli. One	might,	at	this	point	try	to	argue	for	a	single	flavour	sense,	whatever	the	underlying	processing	story,	and	try	to pick	out	experiences	generated	by	that	sense	on	the	basis	of	their	characteristic	phenomenology.	But	how	sure are	we	that	we	can	focus	on	our	experiences	for	the	purpose	of	picking	out	what	is	flavour	and	what	isn't,	and	how sure	are	we	about	what	belongs	exclusively	within	that	category?	Touch	contributes	to	how	things	taste	to	us	and certain	aspects	of	touch,	temperature,	stinging,	and	burning	sensations	from	trigeminal	irritations	can	be	attended to	in	isolation.	So	how	does	this	sense	relate	to	the	single	flavour	sense?	In	one	way,	it	seems	to	be	part	of	it,	and	in another	it	is	a	separate	but	overlapping	sense.	There	is	also	the	experience	of	a	menthol	flavour	that	we	can	break down	into	a	minty	aroma,	a	bitter	taste,	and	a	cool	sensation.	So	is	flavour	perception,	or	more	accurately,	the sense	that	generates	flavour	experiences,	unimodal	or	multi-modal? Consider	the	four	definitions	of	flavour	(perceptions)	just	given.	How	are	the	different	components	mentioned	in The Chemical Senses Page 19 of 31 PRINTED FROM OXFORD HANDBOOKS ONLINE (www.oxfordhandbooks.com). (c) Oxford University Press, 2015. All Rights Reserved. Under the terms of the l icence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Handbooks Online for personal use (for details see Privacy Policy). Subscriber: OUP-Reference Gratis Access; date: 11 August 2015 each	definition	bound	together,	integrated,	or	blended?	More	importantly,	what	are	the	components	that	get	bound together	in	flavour	perceptions?	The	different	definitions	of	flavour	quoted	above	go	different	ways	on	the	how	and what	questions.	Answers	given	to	the	how	and	what	questions	constrain	one	another:	what	is	the	ultimate	product of	combining	inputs	from	different	sensory	systems,	and	how	do	the	different	sensory	inputs	combine? We	do	not	yet	have	a	full	explanation	of	multisensory	integration	or	flavour	binding,	but	various	proposals, principles,	and	criteria	have	been	offered.	These	include: (i)	Spatio-temporal	unity (ii)	Superadditivity (iii)	Semantic	congruence (i)	Spatio-temporal	unity	hypothesis	treats	the	unity	of	flavour	as	arising	from	the	fact	that	sensory information	of	various	kinds,	from	various	sources,	are	put	together	when	presented	close	in	space	and time.	This	leaves	open	how	we	arrive	at	a	unified	percept.	Perhaps	it	is	no	more	than	that	what	fires together	wires	together.	But	this	is	not	sufficient.	Interactions	between	multi-modal	components	go	far beyond	mere	co-occurrence	in	consciousness. Is	the	hedonic	component	of	eating	and	drinking	a	constitutive	part	of	flavour	or	flavour	perception (Smith,	2007,	2010)?	It	would	be	hard	to	exclude	it	on	the	spatiotemporal	unity	hypothesis.	But	we	should exclude	it.	Even	if	the	main	purpose	of	eating	and	drinking	is	to	determine	whether	particular	foods	are pleasurable,	this	is	nonetheless	different	from	how	the	food	or	drink	tastes	and	what	its	qualities	are.	If force-fed	the	same	food	repetitively,	even	a	food	we	like	such	as	chocolate,	may	disliked	after	excess consumption.	The	identity	of	the	stimulus	stays	the	same	even	when	the	hedonics	vary,	for	if	you	were suddenly	offered	a	different	type	of	chocolate	you	would	notice	the	difference	(Kringelbach	and	Stein, 2010;	O'Docherty	et	al.,	2000). (ii)	Superadditivity	occurs	when	the	neural	activation	level	of	two	or	more	sensory	inputs	is	greater than	the	sum	of	their	individual	activation	levels,	as	we	saw	above	when	both	seeing	and	hearing	a water	bottle	being	crushed	leads	to	greater	neural	activation	in	the	perceiver	than	the	sum	of	the separate	activations	for	the	visual	and	the	auditory	stimuli.	Some	take	superadditivity	to	be	a	clue	to multisensory	integration,	signalling	how	significant	it	is	for	the	brain	to	link	these	pieces	of	information	to track	single	objects	or	events.	Perhaps,	the	integration	that	gives	rise	to	unity	of	experience	starts	from	a unity	assumption	about	the	different	sensory	inputs	coming	from	a	single	object	or	event.	The	problem	is that	superadditivity	can	also	occur	without	integration.	The	perceived	creaminess	of	a	food	or	liquid	in the	mouth	can	be	affected	by	aroma.	This	does	not	yet	create	the	kind	of	multisensory	experience	that results	from	the	integration	of	information	from	different	sense	modalities. What	of	the	perceived	weight	of	a	bottle	or	feel	of	a	food	in	the	hand?	These	factors	can	causally	affect our	perception	of	flavours	(Piqueras-Fiszman,	2011),	but	they	surely	are	not	constitutive	of	flavour.	This is	the	philosophical	point	that	needs	to	be	stressed.	Not	every	component	of	our	experience	of	flavour reveals	a	component	of	flavour	itself.	The	difficultly	is	that	although	there	may	be	agreement	about	all that	takes	place	during	eating	and	drinking,	there	is	no	empirical	agreement	about	which	parts	of	the overall	experience	constitute	flavour.	We	need	a	philosophical	account	sensitive	to	the	empirical	facts. (iii)	Semantic	congruence.	Does	semantic	congruency	reveal	a	necessary	feature	of	the	relations	that constitutes	flavour?	For	example,	strawberry	odour	+	(congruent)	sweet	taste	are	combined	into	a flavour,	but	strawberry	odour	+	(incongruent)	salty	taste	are	not,	or	less	so.	Congruency	could	also explain	the	role	of	expectation	in	flavour	experiences.	We	get	oral	referral-by	which	flavour	is experienced	as	located	in	the	mouth,	even	though	some	components	originate	from	the	olfactory	cleft and	elsewhere	(Lim	and	Johnson,	2011)-most	for	congruent	taste-odour	pairs.	So	congruency	could help	explain	localization. Can	the	congruency	hypothesis	explain	flavour	perception	by	explaining	the referral	of	retronasal	olfactory	components	to	the	oral	cavity? The	full	congruency	hypothesis	would	be	as	follows:	unified	flavours	are	constituted	by	congruently related	sensory	cues.	Congruency	is	not	to	be	thought	of	as	'all	or	nothing'-a	sensory	cue	S1	is	more or	less	congruent	with	sensory	cue	S2.	Flavours	would	be	constituted	by	these	various	'more	or	less' congruency	relations.	Their	role	in	determining	flavours	suggest	that	they	intervene	at	the	level	of processing	and	(somehow)	determine	categorization	resulting	in	determinate	flavours	(e.g.,	chicken);	or they	can	be	manifested	as	flavours	with	more	or	less	resolution	(e.g.,	chicken-like	flavours). If	the	congruency	of	different	sensory	stimuli	or	inputs	is	supposed	to	explain	the	unity	of	flavour The Chemical Senses Page 20 of 31 PRINTED FROM OXFORD HANDBOOKS ONLINE (www.oxfordhandbooks.com). (c) Oxford University Press, 2015. All Rights Reserved. Under the terms of the l icence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Handbooks Online for personal use (for details see Privacy Policy). Subscriber: OUP-Reference Gratis Access; date: 11 August 2015 experiences,	we	need	to	ask	what	explains	congruency?	How	is	the	notion	to	be	understood	by	the subjectivist	who	thinks	of	flavour	as	in	the	brain:	the	result	of	combinings? It	would	be	easy	to	explain	congruency	by	saying	that	congruent	features	are	considered	to	be attributes	of	the	same	kind	of	object.	This	is	the	idea	offered	by	Lim	and	Johnson	(2012),	who	argue	that one	needs	an	ecologically	appropriate	taste	in	the	mouth,	not	just	touch,	to	get	oral	referral	of retronasally	sensed	olfactory	components	like	fruitiness.	This	presupposes	a	unity	and	category	to which	these	features	belong	as	an	ecologically	valid	part	of	the	environment.	This	implies	that	flavour	is not	made	in	the	brain,	since	the	basis	for	congruence	is	ecological.	Subjectivists	can,	however,	fall	back on	sensory	congruencies.	They	can	say	that	two	features	'match'	when	the	estimation	of	the	one	affects the	estimation	of	the	other:	e.g.,	the	darker	the	colour,	the	more	intense	the	flavour	(ripeness),	the heavier,	the	thicker	the	yogurt	(density).	Without	resort	to	semantic	congruencies,	determined	by naturally	co-occurring	configurations	of	texture,	taste,	odour,	and	irritant	properties	of	foods	and	liquids, we	just	have	these	subjective,	internal,	and	unexplained	experienced	sensory	congruencies	to determine	which	bundles	of	inputs	are	flavours.	On	such	an	account,	flavours	would	reveal	nothing beyond	themselves. But	rather	than	settle	for	this	unexplanatory	stopping	point,	with	no	way	to	prescribe	precisely	the	extent of	flavour	experiences,	there	is	the	objectivity	strategy	of	positing	flavours	as	properties	of	foods	and liquids.	On	this	view,	we	could	see	flavours	as	affordances	which	our	capacities	for	multisensory	flavour perception	tracks	so	as	to	guide	successful	food	choice. By	recognizing	that	flavours	are	external	features	of	the	environment	we	need	not	see	flavours	and flavour	perceptions	as	always	coinciding.	The	term	flavour	does	not	describe	a	construct	of	the	brain, but	it	is	a	technical	term	used	to	describe	the	sapid	and	odorous	properties	of	a	solid	or	liquid,	including properties	of	its	temperature	and	texture,	as	well	as	the	power	to	irritate	the	trigeminal	nerve.	On	this view	we	can	distinguish	the	hedonics	of	eating	from	the	perceptual	experience	of	tasting. 15	Flavours	and	Flavour	Perception Flavour	is	a	configuration	of	the	sapid	and	odorous	properties	of	a	substance,	including	its	temperature	and texture,	as	well	as	its	power	to	irritate	the	trigeminal	nerve.	So	when	speaking	about	the	taste	of	a	food,	we	are actually	speaking	about	its	flavour.	This	point	is	often	missed	because	we	fail	to	notice	all	the	components	of	our tasting	experiences	and	because	we	are	unaware	of	the	large	role	smell	plays	in	sustaining	them. Flavours	are	perceived	when	retronasal	olfaction	and	gustation	jointly	give	rise	to	a	fused	percept	of	flavour	as	a result	of	multisensory	integration	at	the	sub-personal	level.	We	perceive	flavours	by	the	mechanisms	of	olfaction and	gustation,	under	the	right	conditions,	involving	touch	and	sometimes	irritation	of	the	trigeminal	nerve. What	is	flavour	perception	for?	The	flavour	perception	system	guides	successful	food	choice.	It	needs	to	pick	out and	track	perceptible	properties,	or	sensible	qualities	of	foods	and	liquids.	And	to	explain	why	we	bind	the	sensory elements	we	do	and	not	others	we	rely	on	inference	to	the	best	explanation.	Multisensory	integration	tends	to	take place	when	there	is	a	common	environmental	source	or	unity	responded	to	by	many	modalities.	These	are	flavours -configurations	of	properties-that	can	give	rise	to	multisensory	responses	in	creatures	like	us. Having	dwelt	a	great	deal	on	the	role	of	smell	in	flavour	perception,	the	next	section	will	deal	with	remaining	issues about	our	ordinary,	orthonasal	sense	of	smell. 16	The	Sense	of	Smell	and	its	Role	in	Experience The	role	of	the	sense	of	smell	in	perception	and	conscious	experience	is	subtle	and	elusive	and	this	creates	a puzzle	for	philosophy	of	perception	about	the	nature	of	olfactory	experience.	Many	say	that	they	hardly	notice their	sense	of	smell	and	imagine	it	would	be	easy	to	give	it	up	if	they	had	to	lose	one	of	their	senses.	But	smell	is always	with	us:	we	smell	because	we	breathe	and	we	live	in	a	world	full	of	odours	that	subtly	shapes	our	moods, influences	our	eating	habits,	our	choice	of	sexual	partner,	recognition	of	kin,	and	our	response	to	one	another's fear	or	aggression.	Odours	and	our	responses	to	them	play	an	important	part	in	generating	memories,	particularly early	memories;	and	these	responses	to	odours	help	to	create	feelings	of	familiarity	and	presence	in	certain surroundings.	Given	this	wide	role	that	our	olfactory	system	plays	in	the	conscious	experiences	of	daily	life,	it	is The Chemical Senses Page 21 of 31 PRINTED FROM OXFORD HANDBOOKS ONLINE (www.oxfordhandbooks.com). (c) Oxford University Press, 2015. All Rights Reserved. Under the terms of the l icence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Handbooks Online for personal use (for details see Privacy Policy). Subscriber: OUP-Reference Gratis Access; date: 11 August 2015 surprising	that	we	pay	so	little	attention	to	smell.	Often,	it	is	only	when	people	sniff	at	something	that	they	become aware	of	their	sense	of	smell.	All	the	other	ways	odours	contribute	to,	or	condition,	our	experience	are	perhaps mediated	by	the	unconscious	workings	of	the	olfactory	system?	Or	is	it	that	smell	is	so	much	part	of	the	fabric	of experience-so	much	a	background	to	our	conscious	states-that	we	are	unaware	of	it	until	we	attend	to	it?	There is	no	straightforward	answer	to	this	question,	which	is	why	smell	creates	a	puzzle	for	the	philosophy	of	perception. Consider,	the	connection	between	smell	and	memory.	The	highly	emotive	character	of	many	odour	memories	is thought	to	be	due	to	the	direct	connection	between	the	primary	olfactory	cortex,	the	amygdala,	and	the	entorhinal cortex	that	are	involved	in	emotional	and	memory	processing.	When	we	get	a	whiff	of	scent	that	suddenly	triggers a	memory	of	a	far	distant	time,	it	seems	to	take	us	right	back	there.	The	feeling	of	recognition	is	surely	a	way	of tapping	into	a	remembered	episode	of	which	the	triggering	odour	was	a	part,	whether	noticed	at	the	time	or	not.	It	is worth	noticing	that	the	odour-memory	does	not	simply	conjure	up	a	past	experience	of	smelling,	but	rather	a multisensory	scene	with	sights	and	sounds. Reasons	given	for	thinking	we	don't	experience	smells	all	the	time	include	our	supposedly	poor	sense	of	smell.	But how	good	or	poor	is	the	human	sense	of	smell?	It	can	be	trained	and	shows	improvement	(until	old	age),	unlike	our senses	of	sight	and	hearing.	The	increased	sensitivity	to	and	ability	to	discriminate	and	recognize	odours	shown by	perfumers	and	wine	tasters	attests	to	the	fact	that	humans	may	have	a	reasonably	good	sense	of	smell	but simply	fail	to	use	it.	As	Sela	and	Sobel	put	it:	'Paradoxically,	although	humans	have	a	superb	sense	of	smell,	they don't	trust	their	nose.'	Navigation	is	an	interesting	case.	As	upright	creatures,	we	rely	more	on	our	eyes	and	our ears.	Nevertheless,	we	retain	olfactory	navigational	capacities.	Noam	Sobel	and	colleges	have	demonstrated	that when	blindfolded	and	on	all	fours,	humans	can	navigate	by	using	odour	(chocolate)	trails	(Porter	et	al.,	2007). People	will	take	longer	than	dogs,	but	they	will	succeed.	Sobel's	research	group	also	demonstrated	a	limited	human ability	to	locate	odour	sources	in	space	(Porter	et	al.,	2005).	Participants	were	fitted	with	a	mask	that	had	an artificial	septum	to	exaggerate	the	separation	of	the	nostrils.	When	using	the	mask,	participants	showed	a	rate	of accuracy	for	the	spatial	location	of	odour	sources	of	around	70	per	cent,	suggesting	that	the	brain	was	using	a mechanism	akin	to	bi-aural	hearing	in	audition.	In	commenting	on	these	results	Jay	Gottfried	asks	the	relevant question:	whether	the	apparatus	endows	participants	with	a	new	ability	or	just	uncovers	an	old	one	(Gottfried 2007).	The	lack	of	training	and	the	immediate	improvement	in	olfactory	spatial	awareness,	lost	again	when	the mask	is	removed,	strongly	suggests	the	answer	is	the	latter. Smell	is	constantly	with	us,	and	unlike	other	animals	we	don't	need	continuous	sniffing	to	smell	odours	in	our environment	(pace	Richardson	2013,	though	see	Sobel,	2006).	Active	sniffing	may	be	useful	when	we	are	trying	to locate	the	source	of	an	odour,	or	making	special	efforts	to	take	in,	or	attend	to	a	particular	smell.	Normally,	odours simply	reach	the	nostrils	through	the	dispersal	of	volatile	molecules.	Pleasant	and	unpleasant	smells	aside,	abovethreshold	odours	often	go	unnoticed	because	of	sensory	adaptation.	You	no	longer	notice	the	smell	of	your	own home,	though	you	register	it	well	enough	to	notice	change	straightaway.	(Is	something	burning?)	Normally,	it	is	only when	you	return	after	a	period	of	time	that	you	become	consciously	aware	again	of	the	smell	of	your	home. How	much	of	olfactory	processing	results	in	the	conscious	experiences	of	odours?	Is	ordinary	olfactory experience	just	unattended	to,	or	is	there	less	olfactory	experience	than,	say,	visual	and	auditory	experience?	To answer	these	questions	let	us	look	at	the	role	smell	plays	in	perception	and	conscious	experience	more	generally. Jay	Gottfried	holds	that	the	function	of	the	olfactory	system	is	to	track	behaviourally	relevant	smells	in	the environment	and	extract	meaningful	information	from	them.	To	do	this,	higher-order	brain	regions,	such	as	the orbitofrontal	cortex,	are	recruited	to	assemble	patterns	of	odour	qualities	encoded	at	the	level	of	olfactory receptors.	The	receptors	activated	by	an	odorant	do	not	map	onto	the	odour	percept	directly;	the	olfactory receptor	neurons	will	code	for	thousands	of	different	volatile	compounds	that	get	synthesized	into	a	unified	whole. Coffee	has	over	800	volatile	compounds,	chocolate	over	600,	and	yet	they	are	perceived	as	a	whole.	The	route from	odorant	to	odour	quality	coding	and	categorization	to	conscious	percept	is	largely	unknown	for	a	variety	of reasons.	Over	1,000	different	receptor	types	are	attuned	to	aspects	of	particular	odorants,	but	these	do	not determine	the	resulting	percept:	'the	same	olfactory	input	may	generate	different	odor	percepts	depending	on	prior learning	and	experience',	and	perceptual	learning	will	continue	to	modify	odour	percepts:	'neural	representations of	odor	quality	can	be	rapidly	updated	through	mere	perceptual	experience'	and	'[l]earning	also	changes	odor quality.	For	instance,	cherry	odor	becomes	smokier	in	quality	after	being	experienced	together	with	a	smoky	odor' (Stevenson,	2001	quoted	by	Li	et.	al.	2006:	1097).	Experience	and	familiarity	significantly	enhance	odour	quality discrimination,	while	exposure	to	odour	mixtures	alters	the	perceived	quality	of	the	individual	components The Chemical Senses Page 22 of 31 PRINTED FROM OXFORD HANDBOOKS ONLINE (www.oxfordhandbooks.com). (c) Oxford University Press, 2015. All Rights Reserved. Under the terms of the l icence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Handbooks Online for personal use (for details see Privacy Policy). Subscriber: OUP-Reference Gratis Access; date: 11 August 2015 (Gottfried,	2007). Wilson	and	Stevenson	(2006)	also	subscribe	to	the	view	that	the	brain's	job	is	to	synthesize	olfactory	objects	that correspond	to	collections	of	volatile	molecules	in	the	perceiver's	environment.	They	term	this	the	Object Recognition	Model.	There	is	evidence	that	the	brain's	ability	to	represent	the	wholes	of	olfactory	processing	is configurational,	like	face	recognition,	rather	than	analytical,	in	terms	of	their	parts,	which	may	be	why	we	are	poor at	identifying	an	odour's	constituents.	We	can	still	recognize	mixtures,	though	we	seldom	identify	more	than	two	or three	constituents	in	a	mixture	of	six	or	more	elements	(Laing	et	al.,	2002).	We	can	also	recognize	the simultaneous	presence	of	many	odours.	For	example,	it	is	possible	to	smell	fresh	coffee	and	bacon	frying,	without these	blending	into	a	single	odour	percept. Should	we	think	of	these	as	distinguishable	odour	objects	or	as	representing	overlapping	properties	around	us	with no	definite	spatial	location?	The	latter	is	Clare	Batty's	(2010)	view.	The	lack	of	definite	objects	as	sources	is	due, she	thinks,	to	the	smudginess	of	the	odour	trails	we	perceive.	However,	there	are	clear	cases	where	we	are	aware of	a	source	object	for	an	odour,	be	it	a	flower	or	someone's	hair.	Many	sensory	inputs	may	be	involved	in identifying	the	odour	source,	just	as	there	is	integration	of	vision	and	audition	to	identify	the	sound	source	of	a human	voice.	Gottfried	and	Dolan	(2003)	showed,	moreover,	that	subjects	detected	an	odour	more	quickly	and accurately	when	paired	with	a	semantically	congruent	image:	e.g.,	a	picture	of	a	bus	with	diesel	odour,	and	of	a lemon	with	citrus	odour. Just	as	we	can	experience	mixtures,	and	co-occurring	odours,	we	can	tell	when	an	odour	is	complex	(or	one	is more	complex	than	another),	even	though	we	are	unable	to	identify	any	of	the	constituent	parts.	What	gives	us	this cue	as	to	an	odour's	complexity?	It	may	have	something	to	do	with	the	temporal	dynamic	of	smelling	and	the differential	rates	of	processing	different	odorants.	We	may	be	unaware	of	the	temporal	sequence	though	it	may leave	a	trace	indicating	its	complexity.	What	we	know,	however,	is	that	there	is	no	obvious	relation	between perceived	complexity	and	the	chemical	complexity	of	the	compounds	involved.	Single	molecules	can	be	perceived as	having	parts;	that	is,	a	single	compound	like	salicyladehyde	can	produce	multiple	smell	percepts,	appearing	to smell	both	like	aspirin	and	like	almonds	(Lawless,	1997). Perceptual	constancy	for	odours	is	an	important	feature.	Gottfried	points	out	that	odour	objects	are	constantly maintained	by	processing	in	the	piriform	cortex	despite	variations	in	intensity.	However,	the	perceived	quality	of	a compound	will	change	when	the	intensity	changes	greatly.	Hexanoate	at	low	intensity	in	wines	can	be	perceived as	pear,	then	at	slightly	higher	intensity	as	grapefruit,	while	at	higher	intensity	still	it	comes	to	be	perceived	as fecal.	Notice	that	odour	perception	is	maintained	throughout	gaps	in	perception.	As	Ophelia	Deroy	points	out	(p.c.), we	breathe	out	as	well	as	in,	but	we	nonetheless	recognize	an	ambient	odour	as	present	throughout. Batty's	view	of	the	representational	content	of	olfactory	experience	leads	her	to	deny	there	can	be	olfactory illusions.	On	her	view,	olfactory	experiences	merely	represent	odour	properties,	not	objects.	Thus	it	cannot represent	an	object	as	having	a	property	it	doesn't	have.	A	problem	with	Batty's	view	is	its	exclusive	focus	on unimodal	olfactory	perception.	However,	in	the	normal	case,	perceptual	experience	is	multi-modal.	We	see,	hear, feel,	smell,	and	sometimes	taste	the	objects	we	interact	with,	and	in	that	context	our	overall	perceptual	experience of	a	smell	can	be	illusory	if	it	represents	a	familiar	seen	or	felt	object	as	having	a	surprising	and	perhaps	disturbing smell.	One	patient	reported	(p.c)	finding	that	cartons	of	orange	juice	she	had	bought	smelled	of	fish.	The	smell	was attributed	to	this	multi-modal	object;	she	was	suffering	from	a	vivid	olfactory	illusion.	And	as	Thomas	Hummel	points out,	in	clinical	settings	there	is	a	difference	in	smell	disorders	where	patients	present	symptoms	of	parosmia	or phantosmia.	The	latter	is	the	vivid	experience	of	odours	in	the	absence	of	a	relevant	odour	source,	the	former	is where	patients	experience	the	wrong	odour	for	a	familiar	odour	source.	For	example,	patients	with	parosmia perceive	something	after	being	presented	with	an	odour	of	roses,	which	is	not	the	expected	odour	of roses,	but	rather	a	distorted	and	often	undefined	odorous	perception	...	these	'other'	odour	sensations	are experienced	as	unpleasant.	And	they	are	generally	only	described	in	vague	terms,	for	example	as 'chemical'. (Hummel	et	al.,	2011:	2) By	contrast	to	these	representational	views,	Noam	Sobel	sees	the	function	of	the	olfactory	system	as	enabling	the subject	to	arrive	at	hedonic	ratings.	The	hedonic	tone	of	an	odour	as	pleasant	or	unpleasant	can	predict	the The Chemical Senses Page 23 of 31 PRINTED FROM OXFORD HANDBOOKS ONLINE (www.oxfordhandbooks.com). (c) Oxford University Press, 2015. All Rights Reserved. Under the terms of the l icence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Handbooks Online for personal use (for details see Privacy Policy). Subscriber: OUP-Reference Gratis Access; date: 11 August 2015 reward	potential	of	food	as	a	result	of	learning. Humans	can	detect	and	discriminate	countless	odorants,	but	can	identify	few	by	name.	The	one	thing humans	can	and	do	invariably	say	about	an	odor	is	whether	it	is	pleasant	or	not.	We	argue	that	this hedonic	determination	is	the	key	function	of	olfaction.	Thus,	the	boundaries	of	an	odor	object	are determined	by	its	pleasantness,	which-unlike	something	material	and	more	like	an	emotion-remains poorly	delineated	with	words. (Yeshurun	and	Sobel,	2010:	219) Sobel	believes	sniffing	both	responds	to	and	affects	odorant	intensity	and	therefore	affects	pleasantness	and unpleasantness. Sobel	also	shows	that	although	olfactory	processing	has	significant	effects	on	behaviour-much	that	happens above	conscious	threshold	is	simply	unattended:	'although	human	odorant	detection	thresholds	are	very	low,	only unusually	high	odorant	concentrations	spontaneously	shift	our	attention	to	olfaction'.	So: whereas	vision	and	audition	consist	of	nearly	continuous	input,	olfactory	input	is	discreet,	made	of	sniffs widely	separated	in	time.	If	similar	temporal	breaks	are	artificially	introduced	to	vision	and	audition,	they induce	'change	blindness',	a	loss	of	attentional	capture	that	results	in	a	lack	of	awareness	to	change. Whereas	'change	blindness'	is	an	aberration	of	vision	and	audition,	the	long	inter-sniff-interval	renders 'change	anosmia'	the	norm	in	human	olfaction. (Sela	and	Sobel,	2010:	13) 'All	this,	however,	does	not	diminish	the	role	of	olfaction	through	sub-attentive	mechanisms	allowing	subliminal smells	a	profound	influence	on	human	behavior	and	perception.'	For	example,	there	are	odours	in	women's	tears that	lower	men's	libido	(Gelstein	et	al.,	2011).	A	great	deal	of	chemical	signalling	happens	without	explicit awareness.	Betina	Pause	has	shown	that	humans	process	body	odours	of	kin	differently	from	non-kin	and	are surprised	to	learn	that	they	can	tell	kinship	just	by	smelling	tee-shirts	that	have	been	worn	by	family	members.	The emotional	state	of	others	is	also	communicated	chemically,	with	effects	on	the	perceiver. The	role	of	chemical	communication	in	humans	might	have	been	strongly	underestimated	as	chemical communication	between	humans	usually	does	not	reach	the	level	of	conscious	processing. (Pause,	2012:	56–57). Pause	also	shows	that	emotional	states	of	others	are	communicated	chemically,	with	effects	on	the	perceiver. 'Besides	the	effects	on	motor	behavior,	the	[unconscious]	perception	of	stress-related	chemosignals	significantly alters	the	perception	of	visual	social	signals	in	humans'	(Pause,	2012:	58).	Such	important	environment	signals about	others'	emotions	are	processed	in	a	rapid	and	automatic	way	with	immediate	impact	on	the	perceiver. From	all	these	cases,	it	is	clear	that	a	great	deal	of	olfactory	processing	goes	on	unconsciously	and	with	a	very specific	function.	But	what	is	the	function	of	conscious	olfactory	experience? We	can	think	of	olfactory	experience	as	a	background	to	consciousness:	part	of	the	fabric	that	we	no	longer	pay attention	to	until	it	changes	dramatically.	In	that	background	role,	smells	can	modulate	our	moods,	and	condition our	responses.	This	idea	has	led	to	an	even	more	radical	suggestion	put	forward	by	Ep	Köster,	Per	Møller,	and Jozina	Mojet	(Köster	et	al.,	2014)	that	the	function	of	conscious	olfaction	is	simply	to	detect	change:	that	is	what smell	is	for.	Smelling	the	same	things	over	a	period	of	time	leads	to	sensory	adaption. Köster,	Møller,	and	Mojet	suggest	that	unlike	vision	there	is	no	need	for	the	brain	to	maintain	a	constant	olfactory	as well	as	visual	scene.	By	not	doing	so,	we	free	up	resources	for	other	cognitive	activity.	However,	should	something change	in	the	background	landscape-smoke,	rotten	food	smells,	garbage,	the	perfume	from	a	dress-this	will immediately	switch	on	our	conscious	attention	to	smell.	They	call	this	the	Misfit	View	of	conscious	odour perception.	It	has	a	number	of	things	going	for	it:	it	can	deal	with	the	effects	of	sensory	adaptation	where	smell	is no	longer	experienced;	but	it	can	also	account	for	why	we	can	smell	if	someone	has	been	in	our	room,	or	if	there	is a	smell	of	a	gas	leak.	It	also	suggest	that	our	greatest	sensitivity	is	to	subtle	changes,	which	may	be	just	what works	to	capture	our	attention	and	provide	us	with	pleasure	in	the	blends	created	by	perfumers	and	wine	makers. 14 The Chemical Senses Page 24 of 31 PRINTED FROM OXFORD HANDBOOKS ONLINE (www.oxfordhandbooks.com). (c) Oxford University Press, 2015. All Rights Reserved. Under the terms of the l icence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Handbooks Online for personal use (for details see Privacy Policy). Subscriber: OUP-Reference Gratis Access; date: 11 August 2015 Those	small	changes	from	a	predictable	to	an	unpredictable	aroma	may	enthral	us	(see	Smith,	2014). 17	The	Nature	of	Olfactory	Experience This	leads	us	to	the	vexed	question	of	how	we	get	at	the	nature	of	olfactory	experience.	Is	the	nature	of	olfactory experience	ultimately	answerable	to	how	it	appears	to	us:	i.e.,	is	the	phenomenology	of	smell	the	ultimate	guide	to the	nature	of	such	sensory	experiences?	Some	take	this	for	granted,	but	it	is	far	from	clear	either	that	how	things appear	in	experience	is	the	best	guide	to	how	they	are,	or	that	we	can	be	entirely	sure	of	bringing	the phenomenology	of	smell	sharply	into	view.	Philosophers	are	wont	to	ask	what	it's	like	to	smell	a	rose,	and	to	think that	by	this	simple	locution	they	have	isolated	a	paradigm	example	of	olfactory	experience.	But	is	there	such	a thing	as	a	pure	olfactory	experience	had	in	isolation	from	the	rest	of	conscious	experience?	Surely,	it	is	more	likely that	that	we	are	attempting	to	attend	to	the	olfactory	element	in	experience	rather	than	an	olfactory	experience alone. Smelling	is	more	likely	to	be	a	matter	of	what	we	take	it	to	be	like.	Most	odours	are	also	trigeminal	stimulants	and	the experiences	we	have	will	be	integrated	products	of	trigeminal	and	olfactory	stimulation	(Hummel	and	Livermore, 2002).	Anosmia	sufferers	may	still	be	able	to	distinguish	between	different	chemicals	on	the	basis	of	the	trigeminal sensations	they	produce,	so	may	think	they	retain	some	of	their	capacity	to	smell	(Bryant	and	Silver,	2000).	This can	be	demonstrated	by	testing	them	with	Cognac	in	a	black	glass.	When	smelling	they	react	as	if	to	a	foul	smell because	of	the	stinging	of	the	alcohol.	So,	what	normal	subjects	are	experiencing	is	not	a	unimodal	experience.	So what	exactly	is	given	in	an	olfactory	experience,	or	an	experience	with	an	olfactory	component? All	smells	can	be	considered	in	terms	of	their	particular	quality	('rose',	'mint'),	their	intensity	('mild',	'pungent'), and	their	hedonic	tone	('pleasant'	or	'unpleasant').	Each	of	these	dimensions	of	olfactory	experience	can	interact with	the	others.	An	odour	can	change	its	quality	and	hedonic	tone	with	a	change	in	intensity	due	to	an	increase	in concentration.	It	can	flip	from	unpleasant	to	pleasant	in	a	sexual	context,	say.	The	identification	of	the	odour's quality	and	hedonics	may	change	given	a	different	label	or	priming.	For	example,	isovaleric	acid	can	be experienced	as	Parmesan	cheese	or	as	vomit.	So	is	the	phenomenal	quality	or	character	the	same	when	we	flip from	one	to	the	other?	It	is	possible,	by	reflection,	to	recognize	something	unchanging?	Should	we	think	about	such ambiguous	experiences	as	one	or	two?	The	point	here	is	not	to	answer	these	questions	but	to	cast	doubt	on	the idea	that	we	have	a	clear	way	to	move	from	the	phenomenal	character	of	an	experience-the	supposed	'what	it's like'-to	any	precise	characterisation	of	such	experiences.	The	three	dimensions	of	smell	experiences	(quality, intensity,	and	hedonic	tone)	make	it	clear	that	we	are	not	dealing	with	a	simple,	ineffable,	and	unanalysable experience	of	the	sort	philosophers	usually	allude	to	with	simplistic	talk	of	'what	it's	like	to	smell	a	rose'. When	we	try	to	focus	our	attention	on	what	we	are	smelling,	we	seem	to	get	hold	of	something	about	which	we	can say	next	to	nothing.	Famously,	we	lack	odour-specific	terms	for	smells-except	for	aspects	of	smell,	like	acrid, pungent,	floral;	though	some	of	these	might	describe	effects	of	trigeminal	stimulation	rather	than	smell.	Instead,	we use	the	names	of	the	sources	of	those	smells:	lemon,	rose,	tar,	leather,	cloves,	mint;	and	this	may	explain	our difficulty	in	being	able	to	name	the	odorants	we	are	smelling.	There	may	be	no	special	problem	finding	words	for odours,	as	some	have	suggested. Rather,	the	problem	may	be	in	identifying	the	odour	source	just	by	smelling	the odour	itself.	Jonsson	et	al.	(2005)	suggest	that	the	difficulty	may	lie	not	with	naming	but	with	finding	the	object	that is	the	source	of	this	odour.	If	we	were	able	to	identify	the	odour	source	there	may	be	no	trouble	finding	a	name	for the	odour,	or	describing	it,	but	when	presented	with	a	colourless	odour	from	a	vial-a	highly	unnatural	way	to	be presented	with	an	odour	object	in	the	environment,	which	are	usually	encountered	in	combination	with	other	ways of	sensing	the	odour	source-we	may	struggle	to	identify	it.	Presentation	of	an	object	with	that	characteristic	odour, such	as	a	pineapple,	is	usually	a	multisensory	affair,	of	which	its	smell	is	an	integral	part.	Reconstructing	the	odour source	from	a	unimodal	clue	may	be	a	very	difficult	and	highly	unnatural	task	that	requires	training. Perhaps	the	best	way	to	understand	the	constant	role	that	smell	plays	in	our	daily	lives	is	to	learn	what	would happen	if	one	lost	the	sense	of	smell.	Acquired	anosmia	can	result	from	front-on	head	injury	in	which	delicate	fibres emanating	from	the	olfactory	receptor	sheet	are	severed.	Other	causes	include	viruses,	medication,	and	also neurodegenerative	diseases	like	Parkinson's	disease	and	dementia.	Patients	who	lose	the	sense	of	smell	suddenly complain	of	life's	having	lost	its	savour.	Affect	is	flat	and	the	quality	of	life	diminishes.	Food	loses	its	flavour	and	as we	saw	patients	often	think	they	have	lost	their	sense	of	taste.	None	of	the	places	or	people	these	patients	know 15 The Chemical Senses Page 25 of 31 PRINTED FROM OXFORD HANDBOOKS ONLINE (www.oxfordhandbooks.com). (c) Oxford University Press, 2015. All Rights Reserved. Under the terms of the l icence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Handbooks Online for personal use (for details see Privacy Policy). Subscriber: OUP-Reference Gratis Access; date: 11 August 2015 smell	the	same.	Sufferers	of	anosmia	often	describe	themselves	as	living	behind	glass,	cut	off	from	the	world	and feeling	alienated	from	familiar	settings.	This	frightening	loss	of	a	dimension	of	our	conscious	waking	lives	shows how	much	smell	was	contributing	to	consciousness	without	our	apparently	noticing	it.	The	dimension	that	smell contributes	to	experience	is	something	we	often	don't	know	about	until	it	is	gone.	Old	age	also	leads	to	reduction	or loss	of	smell	and	may	be	a	factor	in	the	widespread	depression	among	the	elderly.	In	fact,	people	who	lose	their sense	of	smell	remain	depressed	longer	than	those	who	lose	their	sight. Of	course,	for	some	people,	the	dimension	that	smell	contributes	to	conscious	experience	was	always	missing. Those	with	congenital	anosmia	may	not	realize,	at	first,	that	anything	is	missing	from	their	perception	of	the	world. They	may	think	their	experience	of	the	world	is	complete,	and	that	they	have	the	use	of	all	of	their	senses.	This	is the	position	Marta	Tafalla	was	in	as	a	child	who	was	not	diagnosed	with	anosmia	until	the	age	of	11.	As	a philosopher,	Tafalla	describes	her	surprise	at	learning	that	some	part	of	her	access	to	the	world	and	some perceptible	dimension	of	the	world	was	missing.	She	has	since	spent	time	and	attention	trying	to	find	out	what	this invisible	dimension	people	speak	about	consists	in	(Tafalla	2013).	One	of	her	interesting	speculations	is	that	the sense	of	smell	contributes	to	spatial	awareness	in	normonosmics.	She	considers	our	appreciation	of	gardens	and suggests	that	those	who	can	smell	the	flowers	and	plants	may	have	a	greater	sense	of	space	and	immersion.	In this	way	smell	contributes	to	experiences	created	by	the	other	senses. 18	Conclusions In	contrast	to	the	traditional	view	of	the	chemical	senses	as	peripheral	and	marginal	to	the	main	questions	in	the philosophy	of	perception,	we	have	seen	how	our	understanding	of	the	interactions	among	the	chemical	senses can	reveal	examples	of	cross-modal	interaction	and	multisensory	integration	that	we	are	coming	to	see	as	the	rule not	the	exception	in	perceptual	experience.	The	multisensory	perception	of	flavour	displays	examples	of:	smell's effect	on	taste	and	vice	versa;	touch's	effect	on	taste;	smell's	effect	on	touch	and	vice	versa,	temperature's	effect on	taste;	audition's	effect	on	taste;	sight's	effect	on	smell;	and	the	way	a	multitude	of	sensory	inputs	from	different modalities	are	integrated	into	a	single,	unified	perception	of	flavour.	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Zampini,	M.	and	Spence,	C.	(2004).	Multisensory	contribution	to	food	perception:	the	role	of	auditory	cues	in modulating	crispness	and	staleness	in	crisps.	Journal	of	Sensory	Studies,	19(5),	347–363. Notes: ( )	Many	people	have	helped	with	discussions	of	material	for	this	chapter	including	Dominic	Alford-Duguid,	Malika Auvray,	Marco	Azevedo,	Tim	Bayne,	Colin	Blakemore,	Paul	Boghossian,	Adriano	Naves	de	Britto,	Denise	Chen, Jonathan	Cohen,	Kevin	Connolly,	Ophelia	Deroy,	Chris	Frith,	Matthew	Fulkerson,	Jay	Gottfried,	Paul	Horwich, Thomas	Hummel,	Ep	Koster,	Ron	Kuypers,	Juyun	Lim,	Per	Moller,	Bence	Nanay,	Ann	Noble,	Matthew	Nudds,	Casey O'Callaghan,	John	MacFarlane,	Fiona	MacPherson,	Tony	Marcel,	Josef	Parvizi,	John	Prescott,	Maurice	Ptito,	Peter Railton,	Louise	Richardson,	Jon	Silas,	Charles	Spence,	Marta	Tafalla,	Dominique	Valentin,	Keith	Wilson,	Chris 1 The Chemical Senses Page 30 of 31 PRINTED FROM OXFORD HANDBOOKS ONLINE (www.oxfordhandbooks.com). (c) Oxford University Press, 2015. All Rights Reserved. Under the terms of the l icence agreement, an individual user may print out a PDF of a single chapter of a title in Oxford Handbooks Online for personal use (for details see Privacy Policy). Subscriber: OUP-Reference Gratis Access; date: 11 August 2015 Woolgar,	Hong-Yu	Wong,	Martin	Yeomans,	Benjamin	Young,	and	audiences	in	Aberdeen,	Abu	Dhabi,	Antwerp, Austin,	Barcelona,	Beaune,	Beijing,	Bergen,	Berlin,	Bordeaux,	Bogota,	Cardiff,	Copenhagen,	Crete,	Curitiba, Glasgow,	East	Anglia,	Edinburgh,	Hertfordshire,	Houston,	Istanbul,	Kent,	Lisbon,	London,	Milan,	Oxford,	Paris,	Porto Alegre,	Riga,	Rio	de	Janeiro,	San	Diego,	Toronto,	Tubingen,	Turin,	Venice.	Thanks	are	due	to	them	all.	But	the largest	debt	of	gratitude	is	to	Mohan	Matthen	for	his	patience,	persistence,	judicious	editing	and	critical	insights. ( )	Quoted	by	Johnston,	2000:	120. ( )	For	an	objectivist	view	of	tastes	and	tasting,	see	Smith,	2007. ( )	See	Jonathan	Cohen,	2013. ( )	Richardson	(2013)	tries	to	defend	a	non-naturalist	view	in	this	area	by	wisely	discarding	the	idea	that	tastes come	from	the	tongue	on	the	grounds	that	philosophers	don't	have	to	be	committed	to	this.	She	does	not,	however, tell	us	where	tastes	are	thought	to	occur,	or	what	makes	something	an	experience	of	taste. ( )	This	appears	to	be	the	strategy	in	Richardson,	2013. ( )	In	his	1982	article,	Rozin	thought	olfaction	was	the	only	dual	sensory	modality,	although	we	can	think	of	other candidates:	the	dual	pathways	of	vision	(Milner	and	Goodale)	and	active	and	passive	touch	(Marcel). ( )	fMRI	studies	reveal	that	retronasal	presentation	of	chocolate	odour	produces	preferential	activation	in	the medial	orbitofrontal	cortex,	the	perigenual	cingulate,	the	superior	temporal	gyrus,	and	the	posterior	cingulate cortex.	Orthonasal	presentation	produces	activation	in	the	thalamus,	right	caudolateral	orbitofrontal	cortex,	right hippocampus,	frontal	operculum	bilaterally,	temporal	operculum,	left	anterodorsal	insula	and	right	anterior	insula (Small,	2005;	Negoias,	2008). ( )	Coffee	is	all	about	the	aroma	and	advertisers	know	it.	That's	why	we	are	told	to	wake	up	and	smell	the	coffee, not	taste	it. ( )	See	Richardson	(2013)	for	this	philosophically	mandated	alternative	to	the	commonsense	response. ( )	First	posited	in	1908	by	Ikeda,	who	wrote,	'Those	who	pay	careful	attention	to	their	tastebuds	will	discover	in the	complex	flavour	of	asparagus,	tomatoes,	cheese	and	meat,	a	common	and	yet	absolutely	singular	taste	which cannot	be	called	sweet,	or	sour,	or	salty,	or	bitter	...	' Dr	Kikunae	Ikeda,	Eighth	International	Congress	of	Applied Chemistry,	Washington	1912.	It	was	confirmed	as	a	basic	taste	with	its	own	proprietary	receptors	in	Chaudhari	et al.,	2000. ( )	Things	initially	found	bitter,	such	as	coffee	and	alcohol,	may	come	to	be	liked	and	desired	as	a	result	of	the post-ingestive	reward	effects	of	alcohol	and	caffeine. ( )	Many	(though	not	all)	would	see	touch	as	encompassing	several	senses	(although	see	Fulkerson,	2014.) ( )	'Fertile	individuals	prefer	the	body	odors	of	partners	with	a	relatively	dissimilar	[immune	system]	to	their	own. This	preference,	in	turn,	seems	to	be	one	of	the	major	reasons	for	mate	selection	in	many	vertebrates	(Boehm	and Zufall	2006;	Restrepo	et	al.	2006)'	(Pause,	2012:	57). ( )	Tyler	Lorig	has	suggested	that	activating	the	olfactory	system	interferes	with	use	of	the	language	areas	of	the brain,	which	explains	our	difficulty	in	naming	or	describing	odours. Barry	C.	Smith Barry	C.	Smith,	Institute	of	Philosophy,	London 2 3 4 5 6 7 8 9 10 11 12 13 14 15 The Chemical Senses Page 31 of 31 PRINTED FROM OXFORD HANDBOOKS ONLINE (www.oxfordhandbooks.com). (c) Oxford University Press, 2015. All Rights Reserved. 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