Pitch perception and beyond

American symphony orchestras tune their instruments to A 440 Hz (in Europe they tend to go a little sharper with A 444Hz). However, no instrument produces a single frequency. Instead, they tune their instruments so that it sounds like an A. Humans can do this quite easily but if you were to look at the spectrum you would see a mess of frequencies. If you removed the fundamental frequency and just listened to the harmonics of a sound you would still identify the pitch you hear as that of the fundamental. It has always been quite puzzling as to how the brain does it. In this week's issue of Nature, a group from Johns Hopkins reports that it has found neurons in the auditory cortex of the marmoset (Callithrix jacchus) that respond to pitch, the way a human responds. These neurons will fire when the animal is presented with the pure tone fundamental or a set of harmonics without the fundamental.

This is without a doubt a very nice and illuminating piece of work but I wouldn't say it was surprising. Ever since Hubel and Wiesel discovered orientation selective neurons in the visual cortex in the late fifties and early sixties, much of systems neuroscience has been driven to providing more and more exotic stimuli and looking for neurons that respond to them. Given the data from the past forty years, I would venture that for anything which we can sense, perceive or ideate, there exists some neuron who's activity is directly correlated to that thing. That is not to say there is just one neuron that responds to some given concept. The other lesson we learned from Hubel and Wiesel is that neurons are broadly tuned over some category. Thus, for any type of perception there will be a population of active neurons.

By any type of perception, I mean all aspects of a thought. So if you are viewing a Cezanne landscape for example, there will be neurons responding to primitive elements like shapes, lines, colours and so forth. Simultaneously, there will be other neurons that respond only to more specific things like a coloured square or to a pair of adjacent coloured squares. Then there will be neurons that respond just to trees or houses and if you know enough about art just to Cezanne paintings. What we don't know is what sorts of neural architectures and learning rules can give rise to such behaviour and how all this cacophony of activity gets sorted out. While there are some candidate ideas floating around, the jury is definitely still out.