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I will define a "tone" as  a steady periodic sound. As an example, I consider a sinusoidal wave to be a tone. By "tonal processing", I mean ratio relationships between the notes. For instance, if I play 220Hz sine wave then a 440Hz wave will be perceived as an octave higher.

I don't understand why humans have the ability to perceive frequency relationships between tones. I have read several evolutionary anthropology books like The Singing Neanderthal and Finding Our Tongues and, between these two books, they suggest that the obvious evolutionary purpose of music is for communication. Each author takes a slightly different approach but the basic underlying point remains that tonal processing exists because it gives us advantageous communication abilities. But even accepting this hypothesis, it doesn't explain why humans have a tonal processing mechanism that is specific enough to perceive ratio differences between notes. The tonotopic organization of both the basilar membrane and the precortical auditory pathways might partially account for the machinery used for tonal processing and why the advanced forms in music could have developed. But this still leaves us with the question as to why would such information be useful?

I remember reading somewhere one person hypothesized that the purpose of perceiving these differences was to identify "relative pitch", i.e. the ability to identify or re-create a given musical note by comparing it to a reference note and identifying the interval between those two notes. But this hypothesis doesn't really make complete sense to me because it doesn't explain why the brin can have such specificity when identifying the ratio between two notes.

Another phenomena that seems pertinent is the idea of a "drifting reference point" which Glimcher discusses in Foundations of Neuroeconomic Analysis. In Chapter 12, he explains how our perception of color isn't fixed but depends on context and in this sense drifts. IIRC, there is a literature that discusses possible neural mechanisms for this. Perhaps something similar would be relevant here for explaining why humans can process tones so precisely.

My Question:

What are the accepted arguments for why would ratios between tones would be be useful information for social communication?

Just to clarify, I am expecting the answer to this question to involve a discussion of the physical properties of tones and why a hominid brain would find this useful information for communication purposes.

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  • $\begingroup$ I don't understand the question. Is it "why can we process multiple frequencies" or "why do we appreciate music". If the former, then you have to realize that not only music contains various frequencies, but e.g. speech contains a range of frequencies as well. $\endgroup$ – AliceD Sep 3 '15 at 21:07
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I think the whole issue is that you put all your focus into music. The ability to tell apart different tones hasn't evolved with the culture of music playing. It is much older.

Ability to distinguish different tones in non-human animals

Humans are of course not the only animals that are able to recognize tones. Mice for example can as well accurately tell the difference between two close tones (ref.).

Perceiving its environment

So why is the ability to tell different tones apart important. It is important because it allows one organism to better perceive the elements in its surrounding. For example, consider being in the savanna and you can't tell the difference between a lion roaring and a water fall. It seems to be a pretty important handicap.

Tones recognition in social contexts

A very large part of animal's communication is based on tons. Very primitive forms of language are mostly based on tones. Courtship display for example is often based on the tone of the other gender (the tone is correlated with the size of the organ producing the tone). A very large part of human's communication is based on tone as well. You might want to have a look at Albert Mehrabian.

If you have more questions about the importance of non-verbal communication you will need to ask your questions to psychologists (on CognitiveScience.SE) and not to biologists. Note btw, that the condition of being unable of recognizing different tones is called tone deafness.

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  • $\begingroup$ +1 Interesting. I didn't realize mice could perceive differences between tones. $\endgroup$ – Stan Shunpike Sep 4 '15 at 4:19
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In short, I suspect that the answer is not so much that it is useful to recognize tonal relationships, but that it is easy.

It sounds like why or how we can distinguish pitch at all is a separate question which you have already accepted. So let's take as a given that whatever purpose our hearing serves, what we have ended up with is machinery which can usually tell higher tones from lower tones.

Now, it turns out that this by itself is not a simple problem to solve. Real, natural sound is never a simple sinusoidal wave. If you hear a pure sinusoidal wave played over a speaker, it sounds synthetic, like maybe a pitch fork, a singing bowl, or a computer - definitely something man-made. That's because absolutely nothing natural makes a sound like that. Consider any natural thing that for even a moment holds a single tone, like a long tone from within birdsong, or the long tone at the peak of a wolf's howl, or wind whistling through a cavern. If you heard samples of each at exactly the same pitch, they would still be distinguishable, just as a clarinet is distinguishable from a flute, or in fact, to the trained ear, two clarinets are distinguishable from one another. This is because the wave that reaches your ear is actually much more complex than a simple sine wave.

In fact, if you saw the high-fidelity digital wave representation of almost any sound which we would consider to be a single tone of one steady pitch, you might have a lot of trouble identifying the period of that wave (which correlates to it's pitch). In digital processing, to identify the tone in a digital wave, or for that matter, the many tones represented in a piece of music, we run a complicated calculation on the wave called a Fourier Transform.

Our aural machinery came up with a fascinating solution to the problem of distinguishing frequencies. Rather than discovering the mathematical method for calculating a "Fast Fourier Transform", our inner ears contain thousands of stereocilia - little hairs of varying lengths which vibrate at different frequencies. Rather than trying to determine the periodicity of a wave by any sort of analysis, these hairs automatically vibrate when any frequency within the wave resonates precisely with their length. This allows us to hear the various frequencies within the sound wave that reaches our ear, so that we can take in the whole ensemble of our environment.

Now here's the catch. A hair that vibrates at exactly 440Hz, also vibrates pretty well at 220Hz. In a sense, this actually represents a weakness in our hearing strategy. Even though the one frequency is twice as fast as the other, they actually sound a lot alike to us. It is harder for us to distinguish middle C (C4) from the C an octave above (C5) than it is for us to distinguish middle C from the D-flat right above it. We can still recognize the difference, but when we do, part of what we recognize is their likeness. Other intervals share the same characteristic in different ways. For example, double a frequency is one octave higher, but three times that same frequency is a fifth above that.

So, from an evolutionary perspective, this is a coincidence of the handy solution found to distinguish tones. However, the joy we find in music is so serendipitous as to beg for a more divine explanation, which you will find echoed throughout our history and surely our future, and within any moment in which you find yourself appreciating the beauty of well-tempered harmony.

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  • $\begingroup$ This was really interesting! Thanks very much. A bunch of things you mentioned I knew already but you helped me fit them together very nicely. Like I hadn't completely gotten what Fourier transforms were used for. Thx :D $\endgroup$ – Stan Shunpike Sep 4 '15 at 4:07
  • $\begingroup$ As I understand it, Fourier Transforms are used in the breakdown and analysis of all kinds of signals. But I personally had the opportunity in school to see how they operate on the vibration of a single guitar string. G, I think it was. The transform breaks down the wave (which is a function of point intensity over time) into a display of frequency intensity (where frequency is on the X axis and cumulative intensity is on Y). I was fascinated to see that although the correct frequency for that G was the strongest frequency present, numerous other undertones and harmonics were in the signal. $\endgroup$ – Mark Bailey Sep 4 '15 at 16:25

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