Your Brain Has Perfect Pitch
Scientists have a knack for asking questions about things most of us take for granted. “The whole orchestra tunes up to an A note from the oboe – but how do our brains tell that all the different sounds are the same pitch?” asks Robert J. Zatorre in Nature.1 This is a puzzling question to neurologists. There’s more, as Zatorre illustrates with a Disney story:
As Pythagoras knew, if you pluck a string, it will vibrate in its entire extent, as well as in halves, thirds and so on, and each of those vibrational modes will result in a separate harmonic frequency. Yet we usually perceive the pitch as corresponding to the lowest of these, which is the fundamental. For a simple demonstration of the ‘missing fundamental’ effect, pick up a phone. Most telephone lines cut off the lower frequencies, resulting in a slightly tinny sound, yet the fundamental pitch does not change; a male voice does not sound like Mickey Mouse. The brain seems to figure out the missing pitch. (Emphasis added in all quotes.)
Is this just learned behavior, or what? Apparently not. Researchers working with marmosets have found neurons that are pitch-sensitive:
Bendor and Wang studied the auditory cortex (the region of the brain that enables perception of sound) in the marmoset monkey. They show that there are neurons in this region that respond in essentially the same way to a variety of sounds that all have the same fundamental but do not share any frequencies. For example, a neuron that responds to 200 hertz also responds to the combination of 800, 1,000, and 1,200 hertz because all correspond to the same fundamental. This effect is unusual because neurons usually respond only within their receptive field, which is typically a narrow range of frequencies. The marmoset neurons, however, responded not only to frequencies in their receptive fields, but also when there was no frequency within the receptive field but the other frequencies in the stimulus were harmonically related to the missing one. This property makes psychologists happy, because it provides evidence (if not yet a mechanism) for perceptual constancy. These neurons respond to an abstract property – pitch – derived from, but not identical to, physical sound features. Presumably, therefore, it is thanks to such neurons that we can follow a tune as the instruments change.
That leads to an evolutionary follow-up question, which Zatorre attempts to answer:
One might wonder why marmosets need such a system, given that they don’t spend much time listening to iPods. But periodic sounds are important in the natural environment because they are almost exclusively produced by other animals, and so pitch is a good cue to segregate these sounds from background noise. Marmosets are highly vocal creatures, and the development of pitch-sensitive neurons would also be central to communication. From an evolutionary perspective, these abilities could be seen as precursors to human pitch perception, which has led to our unique development of music and is similarly crucial for speech.
That’s that for now; he quickly changes the subject: “Now that we know that there are pitch-sensitive neural units, we have to discover how they work.” He has a long list of unanswered questions: How does the ear keep the information intact through the transformations between eardrum and cochlea? How does the brain extract details from the overall fabric of sound? What are the inputs to these pitch-sensitive neurons? – are they hierarchical, or built up from multiple inputs from other structures in the brain? Do inputs from the higher cognitive regions of the brain participate? Are these neuronal properties hard-wired or learned? The list of answers is shorter: we don’t know.
1Robert J. Zatorre, “Neuroscience: Finding the missing fundamental,” Nature 436, 1093-1094 (25 August 2005) | doi: 10.1038/4361093a.
This article almost earned a Dumb award for its useless evolutionary speculations. Zatorre committed the plostrum ante equum fallacy (cart before the horse), assuming that necessity was a sufficient mother of invention. Aside from the empty evolutionary fluff, though, the article underscored a fascinating aspect of hearing that merely hints at the engineering necessary to make it work. Music doesn’t make evolutionary sense because it is a gift of God. If Bach appreciated that fact, how much more so should modern anatomists, physiologists and neurologists.