Elsevier

Cognition

Volume 89, Issue 3, October 2003, Pages 237-261
Cognition

Lexical-gustatory synaesthesia: linguistic and conceptual factors

https://doi.org/10.1016/S0010-0277(03)00122-7Get rights and content

Abstract

This study documents an unusual case of developmental synaesthesia, in which speech sounds induce an involuntary sensation of taste that is subjectively located in the mouth. JIW shows a highly structured, non-random relationship between particular combinations of phonemes (rather than graphemes) and the resultant taste, and this is influenced by a number of fine-grained phonemic properties (e.g. allophony, phoneme ordering). The synaesthesia is not found for environmental sounds. The synaesthesia, in its current form, is likely to have originated during vocabulary acquisition, since it is guided by learned linguistic and conceptual knowledge. The phonemes that trigger a given taste tend to also appear in the name of the corresponding foodstuff (e.g. /i/, /n/ and /s/ can trigger a taste of mince /mins/) and there is often a semantic association between the triggering word and taste (e.g. the word blue tastes “inky”). The results suggest that synaesthesia does not simply reflect innate connections from one perceptual system to another, but that it can be mediated and/or influenced by a symbolic/conceptual level of representation.

Introduction

People with synaesthesia involuntarily experience certain percepts (e.g. colours, tastes) when engaged in perceptual or cognitive activities that would not elicit such a response in non-synaesthetic individuals. For instance, colours may be experienced in response to music or spoken words (Marks, 1975) and shapes may be experienced in response to taste (Cytowic, 1993, Cytowic and Wood, 1982a). The stimulus that triggers the synaesthesia has been termed the inducer, and the modality in which the synaesthesia is experienced has been termed the concurrent (Grossenbacher, 1997, Grossenbacher and Lovelace, 2001). One way of deducing at which level in the cognitive system synaesthesia arises is to examine in detail the nature of the inducer→concurrent relationship, and it is this approach that we pursue here. An understanding of the mechanisms driving synaesthesia may help to elucidate the origin of perceptual awareness both on a developmental and an evolutionary scale.

Broadly speaking, there are two main accounts of the origins of synaesthetic experience: (1) that it reflects direct connections from one perceptual system to another; or (2) that it is mediated by symbolic levels of representation. Given that synaesthetic experiences are perceptual by definition it might seem reasonable, and indeed parsimonious, to explain synaesthesia in terms of direct, hard-wired neural connections from one perceptual system to another (Baron-Cohen et al., 1993, Harrison, 2001). Neonates in the first few months of life show evoked potentials in the primary visual areas of the brain in response to auditory stimulation, but this pattern soon drops out (Kennedy et al., 1997, Maurer, 1997). This type of programmed cell death (apoptosis) may be important for establishing segregated, modular sensory systems. One suggestion, however, is that such auditory-visual, or other perceptual, pathways are maintained by synaesthetes but lost by all other people (Harrison, 2001, Maurer, 1997). The fact that certain forms of synaesthesia have genetic inheritance may offer some indirect support for this (Bailey and Johnson, 1997, Baron-Cohen et al., 1996). Although it is not currently possible to observe such anatomical pathways directly to test the theory, the account may well hold true for certain types of synaesthesia (e.g. musical pitch→colour) in which the inducer is a relatively simple perceptual dimension.

However, an alternative explanation for synaesthesia also exists; namely, that links between perceptual systems occur indirectly via higher-order associative regions of the brain that are responsible for encoding more abstract properties of stimuli, such as conceptual and linguistic information (Cytowic and Wood, 1982b, Grossenbacher and Lovelace, 2001). This study will provide strong evidence in favour of this latter explanation. There are already a number of lines of evidence to suggest that synaesthesia may be something other than the result of direct perceptual links. Dixon, Smilek, Cudahy, and Merikle (2000) report a numeral-colour synaesthete, who was shown a sum (e.g. 5+2=) and then asked to name a coloured patch that appeared. If the coloured patch was incongruent with the subject's synaesthetic colour for that number then she was significantly slowed. Given that this number was never physically presented, Dixon et al. argue that the synaesthesia must have arisen at a conceptual level.1 Finally, it has been observed that synaesthetic inducers tend to be linguistic stimuli such as days of the week, months of the year, Arabic numerals and graphemes (Grossenbacher, 1997). Given that our knowledge of letters of the alphabet, the calendar, the base-10 number system and so on is learned, and not innate, this implies that there is some role for learned information in synaesthesia in at least some cases (see also Odgaard, Flowers, & Bradman, 1999). It suggests that the synaesthetic pattern is not necessarily pre-determined within the first few months of life.

The case study presented below offers support to the notion that the relationship between inducer and concurrent can be shaped by the linguistic environment. This case is also unusual in that it involves taste as opposed to colour as the concurrent. To preface this report then, we summarize the existing literature on taste-based synaesthesia.

The most detailed case study involving synaesthetic taste has been documented by Cytowic (Cytowic, 1993, Cytowic and Wood, 1982a). In response to the smell or taste of food, the synaesthete subject experiences geometric shapes, which he can feel with his hands as well as see. The shapes appear to morph over time (e.g. from pointed to round) as the taste develops on the tongue. The authenticity of the case is suggested by a non-random psychophysical stimulus-response mapping and by reduced cortical blood flow revealed by Xenon SPECT imaging.

Downey (1911) reports a case of coloured taste. Colours were subjectively located in the mouth, although on a few occasions they were projected onto an external surface. The colours persisted for several minutes as an after-image, and were very consistent over time. The authenticity of the case is suggested by this consistency together with the fact that the subject could not recall or evoke the colour without a taste being present.

There are several other reported instances of coloured taste in the historical literature. Calkins (1895) documents six cases, and Bleuler and Lehman (1881) (cited in Krohn, 1892) document coloured taste in a polysensory synaesthete (with inducers in several sensory domains producing colour as a concurrent). Two other cases, if taken at face value, would appear to suggest that the symptom of coloured taste can have different underlying mechanisms. Myers (1911) notes a subject who experiences colours with tastes, for whom the synaesthetic colour was anecdotally described as being appropriate to the tasted object (presumably such as strawberry→red). Ginsberg (1923) documents a subject for whom a sugar solution elicited the colour of red-orange while a salt solution elicited blue. The fact that a mixture of the two liquids elicited a violet-red fusion provides some evidence of subtractive colour mechanisms.

It is interesting to note that in all the examples cited above, taste is the inducer rather than the concurrent. Nonetheless, the famous case of Luria (1968), S, did indeed have taste as a concurrent, albeit amongst a host of other concurrents and inducers. For instance a 50 Hz/100 dB tone tasted like sweet and sour borscht, and words, too, were experienced by S with concurrent tastes. Cytowic (1989) describes in passing two other cases with taste as a concurrent: CSc is a musician who experiences tastes and smells while playing music but not while listening to it (which suggests that the inducer could be kinaesthetic rather than auditory in nature) and MMo experiences flavours induced by words (e.g. Steve tastes of “poached eggs”). There are three other reports of words (or sounds) producing taste synaesthesia in the historical literature (Ferrari, 1907, Ferrari, 1910, Pierce, 1907). As with case MMo, the taste concurrent tended to consist of complex food experiences that were rich in texture, rather than representing primary taste properties (bitter, sweet, etc.). The only other published examples of taste acting as a concurrent that we are aware of are two case reports of patients with synaesthesia associated with major affective disorder (McKane & Hughes, 1988). EM had apparent voice→taste synaesthesia and MW had algesia (pain)→taste synaesthesia (caused by her cervical spondylosis). Although there is no reason to doubt these cases, they may differ in nature from those of a developmental origin.

In summary, the two most common forms of documented synaesthesia involving taste are taste→colour and word/sound→taste. The case study described below is of the latter type and is the first detailed case report of this type of synaesthesia in the contemporary literature.

Section snippets

Case history

JIW is a 43-year-old right-handed male who has enjoyed success in a business career. He has had synaesthesia all his life although he only became aware that other people did not share his sensations when he started school (aged 6). His mother does not have synaesthesia, but his sister reports associating words with concurrent smells and colours. The latter are projected onto the typeface. The status of other family members is unknown. There is no history of epilepsy or other neurological

Is the synaesthesia genuine?

Any purported test of genuineness for synaesthesia is potentially hindered by the fact that it is not possible to have access to another person's subjective experiences. One hypothesis that needs to be discounted is that synaesthetic experiences reflect memory or imagery associations. The genuineness of JIW's case is made here with three different types of evidence. Firstly, following Baron-Cohen, Wyke, and Binnie (1987), we show that JIW is more consistent over time than non-synaesthetes given

Are tastes reliably associated with phonemes?

This analysis was based on the corpus of specific word→taste mappings (N=524) described above, and investigated whether there is any systematic association between particular phonemes in the trigger word and the resultant taste. Those tastes that were reported only one time or twice were excluded from this initial analysis but will be considered again later.

There were 59 tastes that occurred with three or more different words, accounting for 83.8% (439 words) of the corpus. Some of these tastes

Fine-grained phonological factors affecting synaesthetic taste

JIW's pattern of synaesthetic tastes is influenced by a number of fine-grained linguistic properties of the inducing phonology, including allophony, and the specific ordering of phonemic triggers.

Language and conceptual factors affecting synaesthetic taste

The evidence above suggests a highly structured relationship between phonemic patterns and synaesthetic taste. However, we have yet to consider why certain triggers became associated with their corresponding concurrents. An analysis of the word–taste mappings reveals that many inducer words are semantically and/or phonologically related to the name of the word denoting the taste. Table 1 contains some examples and this is described in detail below.

Influence of dietary habit

It has previously been noted that some tastes occur more frequently than others (e.g. sweets and chocolate) while other tastes are conspicuous by their absence (e.g. alcohol). Some researchers have suggested that a non-random response space may distinguish between synaesthetes and controls (Cytowic & Wood, 1982a). Given our claim that the nature of the inducer has been shaped by language knowledge, it seems logical to ask whether the nature of the concurrent has been shaped by his dietary

General discussion

This study has documented an unusual form of synaesthesia in which speech sounds can trigger a subjective sensation of taste. The synaesthesia is very stable over time. This criterion has been used by other researchers as a test of genuineness (Baron-Cohen et al., 1987). The approach that we adopted was to analyze in detail the nature of the relationship between the trigger words and the resultant taste, with a view to understanding the levels of representation and cognitive mechanisms that are

Acknowledgements

This research was supported by the second author's British Academy Postdoctoral Fellowship (PDF/2001/345) and British Academy research grant (SG-33379).

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