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1 Hearing

George: I hate ears!
me: You sound just like Hate-Smurf. You did not mind the semi-circular canals, so why hate ears?
George: I don't mind the canals, in fact I love them. They're really fun. Any time you move your head I get to whoosh from one pool to the other!
me: And riding the basilar membrane, that's not fun? That's just like a trampoline, isn't it?
George: Yeah, I guess it is. Or at least it would be, if I could play the drums instead of having them bellowing in my head!
me: I don't understand.
George: Of course you don't. You're not supposed to. It's a homunculus thing, and you're a scientist.
me: Hey! I'm not a scientist!
George: yeah, well, don't say it too loud. Not if you want them to take you seriously!

Some References:
- Mersenne: " L'Harmonie Universelle", 1636;
- von Helmholz: "On the Sensations of Tone as a Physiological Basis for the Theory of Music", 1862/1954;
- von Bekesy: "Experiments in Hearing", 1960;
- Bauert: "Spatial hearing", 1997;
- Beament: "How We Hear Music : The Relationship Between Music and the Hearing Mechanism", 2003;
- Gelfland; "Hearing: An Introduction To Psychological And Physiological Acoustics", 2004;
- Schnupp et al: " Auditory Neuroscience: Making Sense of Sound", 2011;
- Heller: "Why You Hear What You Hear", 2013;
- Moore: "An Introduction to the Psychology of Hearing", 2013;
- Langner: "The neural Code of Pitch and Harmony", 2015. The Physics of Hearing

Textbooks on Hearing traditionally start with the physics of sound, which is as a matter of fact quite different from the physics of hearing. Bauert (1997) seems to be one of the rare exceptions. He makes the distinction between a sound source and an auditory event, and refuses the easy amalgam of the two. The first one is a physical phenomenon that can be studied scientifically and can be the object of (dis)agreement between different individuals; the second is a personal sensory experience that can not be shared directly: what you hear and what I hear are not necessarily the same thing.
There must be of course a link between both, and the question then is which. The distinction Bauert makes does not stop him from following in the footsteps of his predecessors. He bases his analyses on the same facts as they do, even if the nuance he brings in his approach allows him to sometimes discern some discrepancies in the way certain experiments have been set. Some authors who do not make this distinction show results that are ambiguous because it is not clear whether they apply to one, the sound source, or the other, the auditory event. Ears and hearing can easily be compared with the way a __telephone __works. 
What I find particularly instructive is the fundamental difference in approach between acousticians and hearing aids engineers. While the first group concentrates on the mathematical description of waves, the second made it their main task to understand sound as a serial phenomenon. Cochlear implants are, just like the telephone, based on the translation/transduction of waves into electrical events. Those events cannot happen in a parallel way. After all, there is only one membrane to receive those impulses at any time.
The smallest bones in the body, the ones on which George would love to sit and pound on the tympanic membrane using hands and feet at the same time, seem to be relegated into obscurity while they are in fact the most important factor in the whole process. They are the first relevant phase in the sound production process.
Not that the__ Ear and Head Transfer Functions__ are not important. The way sound propagates from it source to humans is indispensable knowledge for sound engineers, be they designers or sonic pollution fighters. 
Nonetheless,_ it is knowledge that is inaccessible to the hearing brain_, and as such, irrelevant to the physics of hearing, this in contrast with the physics of sound.
Strictly speaking, even the role of the ossicles could be considered as irrelevant: all we need is the effect of their actions on the membrane, and its role in activating the auditory neurons. From serial to sensory:
Vision, as far as we know, is definitely a parallel process. We do not have the time to "look at" each element of the visual field one after the other. Hearing, even though it seems to us like we hear many sounds simultaneously is just the opposite. Sounds that reach the tympanic membrane make it move, and it makes in turn the ossicles pound on the oval window one hit a time. The number of hits per second, their frequency, will determine the reaction of the basilar membrane, the last stage before neurons take over.
How many hits does the basilar membrane need to know how to react? More than two? The frequency is set by the first two hits, all the others are only needed to indicate the duration of the stimulus, not its quality. But are two hits physically sufficient to make the membrane vibrate at the right frequency, or do we need many more than that? This is quite a serious question. After all, if the stapes are supposed to react fast enough to incoming sounds as to give us the illusion of simultaneity, the less the number of hits needed for each sound, the better. Puzzling phenomena like the missing fundamental, or the suppression of the second sound, might be more easily explained when taking into consideration the seriality of sound sensations.  There seems to be only__ two fundamental physical properties of sound: frequency and amplitude__. Everything else follows from those two. Likewise, those two properties seem to be the only properties that can be relayed to the brain. Which begs the question not only of pitch and timbre, after all, those are only sensations, and we are used to the idiosyncrasies of our brain. No, the mystery resides in the fact that these physical properties seem to hide a third one, as tangible as can be: the spatial location of sound. We very often know where a sound comes from. Even assuming imperfect  knowledge based on experience, it remains a mystery how we could know that. Where are the spatial codes hiding? The last puzzle (for which I have no solution) until now is the convergence of so many neurons (around 20) on the same inner hair cell.

2  Sound Source Localization
The most striking aspect of all the experiments concerning localization of sound sources is their artificial character. Not only are subjects very often restrained in their searching (head and eye) movements, they are also generally expected to localize a single one-time sound of limited duration. The results after more than hundred years are very eloquent: nobody agrees with anybody, to put it somewhat dramatically.
There are of course some trends recognized by all, especially the fact that humans are much better at localizing sounds that happen in front of them. As soon as sounds are produced from above the horizon, above their head or behind them, results become quite erratic, depending on the way each experiment has been set up.
Also, the distinction between left and right seems to be made quite accurately within a reasonably wide range. 
Last but not least, results seem to differ from one individual to the other, making general conclusion even more hazardous. A very interesting article is that of the long winded title by Rankin et al "An Assessment of the Accuracy and Precision of Localization of a Stationary Sound Source Using a Two-Element Towed Hydrophone Array" (2013) in which they report of their attempts... Well, the title says it all. Let me just add that it happens on the open sea and that the sounds are produced by underwater transducers while the towers are towed by a ship.
I found the summary of their findings in the excellent abstract [something that happens alas very rarely] very insightful: 
"Bearings to the sound source were determined based on the time-delay of the signal arrival at two of the hydrophone elements. Localization was estimated by visually inspecting the convergence of bearing angles to the source. The accuracy of bearings and localization improved as the ship approached the sound source." Here are thekey words as I see them.
- different arrival times at the towers are the main acoustic source.
- The sounds are localized after a whole series of recordings and computations.
- visual analysis is used to implement acoustic findings.
- precise localization is quasi-impossible: only a closing approximation of the source is feasible. Except for the first one, all those points show how unrealistic most experiments in this field are. It is no wonder that it seems impossible to get any conclusive results. If hearing, for humans, is a secondary sense as far as spatial localization is concerned, if it is meant therefore to be supplemented by vision and experience, than the researchers' expectations may be just too high. The question is, would the same image remain plausible once we take into account the first point? Concepts like Interaural Time Difference (ITD or arrival time of the sound in respectively the left or right ear), and Interaural Level Difference (__ILD__ or difference in sound intensity as experienced by each ear, also called IID for Intensity. See Popper et al "Sound Source Localization", 2005) are fundamental concepts in this context. They are also typical homunculus concepts. They assume that the brain can compare two input streams and then take the appropriate action. Which would mean that the brain is able to "look at" both inputs in a neutral, analyzing way, just like we are able to in everyday life or in a scientific experiment. That is the difficulty with homunculus illusions. They seem so plausible. After all, we are capable of comparing two sounds, so our brain must somehow be able to do that also. Hearing sounds is a sensory process, analyzing them is an intellectual one. Some theories, starting with Helmholz, tend to consider the boundary between both stages as quasi inexistent. But it really becomes a problem when the intellectual process seems to precede the sensory one. That is if we somehow seem able to determine, even before we hear the sounds, what the difference is between them. Which seems the only way we could assign to each sound its place in our experience.
If the experience of the difference between two sounds does precede its analysis, then there is no problem with considering the intellectual process as fundamental in processing these inputs. It becomes a normal theoretical debate.
It remains however very problematic to assign such intellectual processes to all animals or even humans (babies, children, illiterates, mentally challenged...). ILD and ITD concepts are very similar to that of binocular disparity. As I have tried to show, it is very unlikely that parts of the brain are able to compare what each hemisphere is receiving. This is something only the whole brain or organism can do. Still, whatever intellectual processes we believe are involved in the process, they have to work with whatever our sensory organs provide us with, and the corresponding sensations. _Once we accept the idea that we are somehow "aware" of a difference in arrival time or intensity in one ear relative to the other, we can ask ourselves whether the brain really needs anything else to determine the direction of the sound source. _Would the sensation that a sound has reached our left ear before our right ear not make all further computations redundant?__ ILD and ITD seem to me as the logical consequences of a mathematical/computational approach that denies the existence of sensations and must therefore seek refuge in complex calculations to explain what the brain can do so naturally. What does Tastee Wheat taste like?
The people in "The Matrix" had no way of knowing that, just like congenially deaf people have no way of knowing how voices or sounds in general are supposed to sound. Still, they are able to learn with the help of cochlear implants. A Youtube clip of a young child, under seven, shows him as having a conversation with the interviewer and other people present while continuing to play with what looks to be Lego-like blocks. Quite a technological and human achievement that would have been unthinkable a few decades ago.
There is no doubt that these implants will keep being improved with each year passing, a very positive development for people all over the world. A Fundamental Flaw?
I hope to convince the reader that I wish in no way to belittle the progress made in this area, but I am myself convinced that the approach to cochlear implants, however successful it might seem, is fundamentally wrong.
The following quote from Clark "Cochlear Implants: Fundamentals and Application ", 2003, is representative of the current approach:
"Cochlear implants should aim to reproduce the coding of sound in the auditory system as closely as possible, for best sound perception." (ch.5). (my emphasis) There was once a documentary on TV concerning a tribe that honored their elders by chewing their food for them when they lacked the tooth to do it themselves. This is the impression I get when I read this quote. The implants are doing the work that the patients auditory system is supposed to be able to do unaided . 
The fact that with only a very limited number of electrodes (about 22 according to the 2005 edition) relative to the 20.000 inner hair cells patients are able to learn so much is truly astounding. I will come back to the theoretical significance of this ratio in a little while. First I want to stand still by the question of pre-coding and its disadvantages.
The makers of cochlear implants have to remove the basilar membrane in its entirety if it is still present. Unluckily, instead of trying to emulate its effects, the technicians have followed the erroneous views of theoreticians of brain processes in general, and those of the auditory system in particular.
Researchers concentrate their effort in ameliorating the speech-processing abilities of their implants while they should be directing all their effort towards a better delivery of sound in the form of electrical impulses. In plain words,they should be trying to replace the effects of the basilar membrane instead of creating an auxiliary auditory system outside of the brain. Number of electrodes
The ratio of electrodes and inner hair cells necessary for a working hearing system would seem to indicate that just like with vision, each auditory fiber is capable of relaying all, or at least many sound sensations. Also, technical results show that applying serial instead of simultaneous sounds give better hearing results. This is in accordance with the serial nature of the stapes and therefore of the acoustic data that impinges on the basilar membrane, and from there on the auditory fibers.
Put very simply, implant technicians need "merely" to transduce the sounds received by the microphone into electrical impulses and relay them through as many electrodes as they can to as many locations in the cochlea as possible.
 I am not even sure whether they should bother with the same spatial distribution frequency-wise as seen in the natural basilar membrane. It could well be that the distinction between high (at the base) and high frequencies (at the apex) is related to the mechanical properties of the membrane, more than to the processing abilities of the corresponding neurons.

In summary: get the sound (impulses) to the auditory system and let it do its work. 

3 The homunculus explained

me: George, would you mind?
George: You know you are asking me to reveal secrets that have been kept, well, secret, since the dawn of Man? Gosh! I have always wanted to say that. Just for allowing me this opportunity, I think I will help you out a little bit.
me: Much obliged.
George: Now you sound just like Elvis.
me: would you get on with it!
George: Okay! Okay! Imagine you are listening to a sound that is reaching both your ears, but not at the same time.
me: ILD or ITD?
George: Whatever. What I do is take what you hear, not me, since they are your ears, if you catch my drift. [George giggles!] Then I decide which sound arrived first, and what they look, I mean, what they sound like, and then I whisper these differences in your ear. I mean in your mind.
me: How can you do that?
George: Easy. Wait! Do what?
me: How can you know the difference if you don't use your own ears?
George: Helloo! Because I don't need to! Homunculus? Moi? Remember?
me: George, you don't think you have a little George in you, do you?
George: This is really sick, you know! That'll teach me to... Teach humans! 

4 Calculations and Sensations
I have repeatedly hammered on the necessity not to lose sensations out of sight because all our calculations not only are based on them but, eventually, point back to them. Ignoring them compels the researcher to look for artificial, biologically implausible artifacts that would explain those calculations. Sound is no exception, or rather, it is a beautiful illustration of the intimate relationship between calculations and sensations. Why the Basilar Membrane?
After all, the percussions of the stapes on the oval window are what it is all about.
The problem of course is for the brain to interpret serial, telegraph- or Morse-like streams of data, and turn them into a meaningful whole. Each series has to be distinguished from the next, and for that the brain would need a neural decoder which it does not have. This is where the basilar membrane comes into play. Sounds are distinguished through their respective frequency without any computation or decoding process. Physical and mechanical laws take care of that. The membrane reacts differently to different rhythms. Just like the jumping rope of little girls reacts differently to each wrist and arm movement. The rope does not need to compute all those variables before it can react accordingly. And neither does the basilar membrane when it reacts to a seemingly meaningless stream of percussions. The basilar membrane is seen by all as a biological frequency analyzer, or even a Fourier analyzer by those who need the mathematical security blanket.
One percussion moves the membrane in an indeterminate way, but the second one defines the final reaction, the one following either continuing the same pattern of breaking it.
The movements of the basilar membrane activate different neurons according to their specific patterns. The details are, for now, irrelevant. What counts is the following paradigm: series of percussions --> membrane movement --> neuronal activation Still, membrane movements can be considered as continuous, and even when they are not, the brain need some means of distinguishing one series of movements from the other.
This is where we realize that we cannot do without the sensations. A percussion or movement pattern evokes a definite sensation depending on its frequency. The brain does not need first to determine a frequency to feel the corresponding sensation. In fact, once the sensation, and its intensity, has been felt, there is no need anymore for the brain to record any of those two. And even if it wanted to, that is if we decided that brain must somehow code for frequency and amplitude, we know that it cannot. There is nowhere in the neuron where such codes could be hidden. A second series of percussions will evoke another sensation, and that is the way the brain can differentiate between different sounds. It does not need to know when a series ends and another one starts. It just feels it. The same way that the little girls' rope just knows when one of them is ready to jump. Or is it the other way around? Can little girls compute the frequency of the rope before deciding when to get under it and jump? Do our researchers have some equations hidden at the bottom of a drawer somewhere that would explain such wondrous abilities of our children?

5 Calculations and Sensations (2)
The cocktail party effect has created many insoluble problems for the researchers.. Bregman's  "Auditory Scene Analysis : The Perceptual Organization of Sound", 1994, is considered as a classic in the field. This is how the same author describes the problem of hearing in his foreword to a more recent work  by Divenyi, editor of  "Speech Separation by Humans and Machines", 2004: "There is a serious problem in the recognition of sounds. It derives from the fact that they do not usually occur in isolation but in an environment in which a number of sound sources (voices, traffic, footsteps, music on the
radio, and so on) are active at the same time. When these sounds arrive at the ear of the listener, the complex pressure waves coming from the separate sources add together to produce a single, more complex pressure wave that is the sum of the individual waves. The problem is how to form separate mental descriptions of the component sounds, despite the fact that the “mixture wave” does not directly reveal the waves that have been summed to form it." 
(my italics) Understood as a non-serial series the numerous sounds that reach us seem to be all part of a sonic soup we are supposed to drink with forks. 
[I agree with George. It really sounds terrible!] This of course does not mean that sounds cannot be scrambled beyond recognition. But to think that the brain has a way of filtering different frequencies beforehand is a homunculus illusion. 
Attention and concentration are processes which still await a plausible explanation as to how they are able to enhance certain sound streams above others,  it remains nonetheless the question whether they do that with the help of frequency filters, therefore at the source, or at the end of the acoustic process: after different sounds have been perceived. 

6 Calculations and Sensations (3)
The conundrum faced by Bregman and the different participants to Divenyi's (2004) is I think insoluble mathematically without assuming previous knowledge of the different sound sources contributing to form a specific spectrogram. In other words,__ The mathematical approach either relies implicitly on a homunculus, or it admits it is powerless to analyze sound events in real time.__
That certainly does not mean that a scientific analysis is therefore impossible. Taking example of the way biological hearing systems have solved the problem we must acknowledge the fact that a technical, or technological, solution is necessary instead of brute mathematical computations. What mathematics cannot do is quite elementary for an artificial membrane and corresponding stapes.
It would certainly not be the first time that physical manipulation took the place of computation. Just think of the way transistors are built. Computers would not exist if not the peculiar reaction of two physical surfaces put in close relationship to each other (Levinshtein&Simin "Transistors: From Crystals to Integrated Circuits", 1998). Man does not create electricity, he makes it possible by combining matter. We much too easily forget this fundamental phase in which electrical currents suddenly seem to surge from nowhere just because two different molecular configurations were allowed to interact with each other. Electricity is not a mathematical concept, even if all its manifestations can be expressed mathematically. It is a real physical phenomenon.
The same way, maybe frequency, and amplitude, are more than mathematical parameters, but also real physical attributes of physical events. If that is the case, physical manipulation might be the only way to distinguish them in a complex environment. Maybe we need the physical vibrations to further analyze sounds mathematically, and not only at the start of the analysis.
We most probably cannot, armed solely with the mathematical concepts, distinguish one frequency from the other in a noisy environment. But a physical device might do just that.
Too often have scientists thought that whatever a device can do could be reduced to a set of mathematical equations or algorithms. This is certainly the deep conviction of Rodney Brooks ("The relationship between matter and life", 2001) in his quest of creation of intelligent, embodied robots. He rejects categorically the idea that the way to better AI is through the discovery of "new stuff". I wholeheartedly agree with him. But not the way he would hope. What I am proposing is in fact very "old stuff": matter. I will leave the mind equation out for the moment. Suffice to say that not all brain or even computation processes can be solved mathematically of computationally. Sometimes the only way to understand them is to manipulate matter and take note of the changes.
In other words, maybe auditory scene analysis (ASA) or computational auditory scene analysis (CASA) are not mathematical or computational problems, but merely technical issues. [Bregman's approach is less mathematical than it is based on Gestalt principles. ]
It is ironic that the trend of inventing all kinds of mathematical devices for brain processes should have blinded most of the researchers to this elementary truth: the brain can solve complex problems with very simple means: before being mathematically reduced, Archimedes' lever had been used for thousands of years by non-scientists. No mathematical analysis a priori could have discovered it.
What is even more staggering is that the idea of an artificial membrane is far from new. Already Nobel laureate Bekesy in the 1930's had built models of a basilar membrane to study its reactions to sound stimulations. You would have thought that researchers after almost a century would have grown accustomed to the idea that a rudimentary physical device could do what mathematics could not (microphones for instance still need a membrane). Apparently, the ideological hold of mathematics and the computational view is just to strong to be ignored. Strangely enough, Brooks' approach is based on the idea that intelligence cannot be disembodied. The next step would be to acknowledge that manipulations of the physical world (beyond mere bodily actions in the world) are a fundamental way of gaining new knowledge. Just like the ancient Greek, we call that physics. [Brooks would of course not object to such a view.]
The question "Why a basilar membrane?" sounds hopefully even less theoretical than before: such a device was a physical necessity, and not only an evolutionary shortcut. [I apologize for speaking on Evolution's behalf. Nobody should be allowed to do that. The only thing that Evolution teaches unambiguously is that things change. The reasons how and why are of our own making.] This also shows the necessity for any scientific analysis to go beyond a mere mathematical description of brain processes and take the issue of their physical realization as even more fundamental than abstract equations than can be interpreted in so many ways. 

7 An empirical test
The question whether the stapes and therefore the basilar membrane, react serially to many different sounds, some simultaneous, some not, and how this affects the frequency and amplitude analysis of each particular sound; or whether they react to the acoustic garbage formed by all the sounds received within a certain period, can be considered as an empirical matter. It should be possible to answer the question unambiguously with physical experiments. And if it turns out that there no easy answers, that would be a very important result as well. 
In fact, we already have such empirical tests. Telephones and microphones do exactly that: they convert different sounds, of which many are produced simultaneously, in a serial series of sounds that we can distinguish.
Why would then the brain re-scramble that which has been so dutifully separated by the basilar membrane? Spatial codes of hearing
It sounds so strange when we think that sound knows only two fundamental properties: frequency and amplitude. The Ear and Head Related Transfer Functions are supposed to explain the metamorphosis of ubiquitous sound into directional sound. The changes that the external ear and the head bing about must explain how the brain is thought to be able to compute equations like the Minimum Audible Angle (Mills "On the minimum audible angle", 1958; see also Hartmann " On the minimum audible angle - A decision theory approach",  1989) and other equations that help localizing a sound source in space. 
But directional sound is something that we Humans cannot perceive [unless we step into the path of an ultra-sound wave. A search on the web will provide a wealth of technical information on the subject, and a plethora of commercial products that are supposed to enhance our experience. Fish are supposed to have directional hearing, but that is something I really know nothing about. See Fay and Popper "Comparative Hearing: Fish and Amphibians, 1999; also the ch.3 of Popper et al mentioned below).
Let us not dwell on the homunculus aspect of these computations and accept them at face value. Let us even accept the idea that those equations do not allow a perfect navigation instrument and must be supplemented by vision, head and eye movement, and experience in general. 
[I would like to remind the reader though that whatever effects the external ear and the head can have on how the sounds reach our brains, this knowledge is only available to an external observer. As listeners we have no way of distinguishing between a sound that has impinged directly on our inner ear, and one that had to literally pass over our head.] The fact remains that we are apparently able to recognize a sound after it has changed its aspect by moving to another location. We can worry later about how we do that exactly.
So there must be something in the sound itself that not only keeps it recognizable, which does not "sound" extremely improbable, but also that indicates its possible location. And that is exactly the problem, isn't it?
How could previous experience explain the fact that we recognize the same sound as having now another location?
1) We must recognize the sound as such, even if it has now somehow changed. No problem.
2) We must have previous knowledge of the location of the same sound relative to our body. For instance, we have previously identified a sound source in front of us, and had experienced the sound from different angles by moving around it. The identification of the different sounds as being of the same nature or origin does not necessarily mean that those sounds could be linked to the same source objectively by a sound instrument. Maybe their spectrum, Fourier shapes or whatever it is that sound experts like to play with, would seem completely unrelated to each other. The association of all the sounds to relative locations and sources is a psychological one, not a scientific endeavor.
This is how Fay expresses it in ch.3 of Popper et al (2005): "it is also quite clear that binaural processing is subject to several essential ambiguities that must be solved using other strategies. These include the fact that all sources on the median sagittal plane result in equal (zero) interaural differences, and that, in general, “cones of confusion” exist bilaterally on the surfaces of which interaural cues are equal for many possible source locations. These ambiguities seem to be solved adequately among humans and many terrestrial species that use head or pinnae movements, information from other senses, judgments of the plausibility (...) of potential locations, and a processing of the head-related transfer function (HRTF)
through frequency analysis..."
(my italics) Another interesting article is " Infants' Localization of Sounds within Hemifields: Estimates of Minimum Audible Angle", by Morrongiello and Rocca of 1990 where they show that human babies and children learn with time how to localize sound more and more accurately. Already in 1940 Wallach had pointed at the role of extra auditory cues in the localization process of sound ("The role of Head Movements and Vestibular and Visual Cues in Sound Localization"). All in all, sound localization seems to me be very similar to how we know how hard or how high to throw the ball to score in a basket ball game even though our brain has no way of computing the different trajectories. Still, experience cannot explain the sensations we have that a sound seems to come from a certain direction, however vague those sensations might be. 
Experience does not create corresponding sensations, only corresponding behavior or conceptions.  Where do therefore our auditory spatial sensations come from? If we want to avoid turning around in circles, we will have to point either at the sound itself, or at our own body as the origin of those sensations. Our brain, once again, cannot produce spatial sensations out of nowhere. Or so I will assume.

8  Are our ears symmetrically located on our head?
Not according to scientific measurements (Abaza and Ross"Towards Understanding the Symmetry of Human Ears: A Biometric Perspective", 2010). The difference is nigh undetectable, and it is really doubtful whether this minimal asymmetry can account by itself for our spatial sensations.
Still, combined with head and body movements, this small asymmetry might just do the trick. Spatial sensation might be nothing else but the impact of a sound on one ear before the other. This is I think much more plausible than the topological arguments used by Rauschecker and Tian in "Mechanisms and streams for processing of ‘‘what’’ and ‘‘where’’ in auditory cortex" (2000). I really have no idea how computations can create any sensation at all, and certainly not the sensation of "where", even if parts of brain (and not a human individual) could make use of geometrical equations. A view that does not cease to amaze me. Auditory Field and External Space
What would happen if the only factors allowing us to localize, approximately, sounds were ILD or ITD? All sounds reaching the right or left ear would seem like not only coming from the same direction, but also from the same location. What is strange is that sounds coming from a loudspeaker are in fact all from the same location. Nonetheless, we do not have the sensation that they all occupy the same space.  I think we make much too easily the amalgam between auditory space and real external space. Vision has spoiled us so to speak. What we see is also where we see it.  All Sounds at One Location?
Two sounds of different frequency will be felt at different locations. This the basis of the place theory as advocated by Bekesy. It seems to work quite well for certain frequencies, but stops being of any use at some fundamental frequencies like those used by speech and at very high frequencies.
The problem really is the fact that we still have on clue where the spatial codes can be hidden. How does the fact that neurons activated at specific places from the basilar membrane explain the spatial character of auditory sensations?
That the auditory field, like the visual field, moves with our own head is understandable.
But while we expect a perfect correspondent between the visual field and 3D external space, that relationship is lost when it comes to sound. 
Frequencies seem to have a location in auditory space that turns out very often to be completely unrelated to real sound sources.
We localize sounds in the objective external space by ear (ILD and ITD as sensations, not computations), but we hear different sounds at different locations in auditory space instead of throwing them all in a heap. Distinguishing them "audio-spatially" help us interpret them better [something indispensable for speech production and comprehension] even if it creates some confusion as to where they are in real space. Does sound explain hearing?
The basilar membrane definitely reacts to sound. But that is certainly not the case of auditive fibers. They are activated by hair cells which react, just like with touch, to movements of their cilia. So yes, hearing certainly involves sound indirectly, but maybe we should consider hearing more like a particular modus of touch. That would in a way explain why we feel sounds as belonging to an auditory space. Maybe what we are feeling is comparable to what we feel when something touches our skin, or our body hair. There is also always a spatial sensation accompanying the touch sensation. That would also explain why those spatial sensations do not give us precise knowledge concerning the source of sounds in the external space. They convey the spatiality of our cochlea, just like touch conveys the spatiality of our body as a whole. We feel where we have been touched, or where our skin itches. What makes such a view especially interesting is that it can perhaps be empirically falsified. This could be done by showing that there are no correlations between the objective position of the part of the cochlea admittedly involved in the perception of a specific frequency and the spatial sensation. 
I do not think we can hope to get clear-cut statistics that would show a definitive relationship between sensations and spatial position of parts of the cochlea. I am not sure the position of the different parts of the cochlea relative to the body would allow such unambiguous conclusions. But it might be possible to find some correlation between auditory sources and cochlea positions. In other words, that when we feel that a sound is coming from behind us, we are actually feeling a part of our cochlea that could be said to lie caudally, to the back, relative to where we are facing.
A negative result would not infirm the view that auditory spatial sensations are cochlea and not space related, but it would leave it a little bit dangling in the air, like any other speculative idea.
[I could not find any data concerning the localization ability of sounds among patients with a cochlear implant. All the efforts are concentrated around speech comprehension, music being a far second. But If am right, this kind of patients should have more difficulties in localizing sounds. The problem will be to to prove that it has anything to do with the number of electrodes. After all, if they can learn to recognize speech maybe the idea that each auditive fiber can convey all or many acoustic sensations can be complemented with the corresponding spatial sensations. In other words, it might be almost impossible to find definitive answers to all the questions we have. A pragmatic attitude, certainly concerning implants, might be our best bet.]