Music vs physics

How come physicists think they know everything about everything? Is it just pure arrogance that makes them think they can explain how brains work, as well as things like black holes?

Well, brain cells, or neurons, do send signals to each other by building up an electrical voltage to a high enough level to discharge, which is all to do with physics. And the networks they form are just the kind of thing that physicists are good at modelling with mathematics.

As an example, in a recently published paper, Russian and Italian physicists have modelled how neurons in our auditory system might work to help us distinguish between harmonious and non-harmonious combinations of musical notes (Ushakov YV, Dubkov AA & Spagnolo B 2011, “Regularity of spike trains and harmony perception in a model of the auditory system”, Physical Review Letters, vol. 107, 108103).

The auditory system is good for this kind of study, because it’s a relatively simple system from a wiring point of view – plus, of course, we have a good understanding of how to turn sound into electrical signals.

Anatomical diagram of the human auditory system, showing the outer, middle and inner ear (click to find out more)
Anatomy of the human ear. Sound waves are converted into electrical signals by hair cells in the cochlea (the purple bit). The rest of it mostly serves to focus and amplify the sound. (Image by Chittka L, Brockmann, via Wikimedia Commons)

For humans, this is done by hair cells in our inner ears, which, as the name suggests, have tiny hairs or cilia that pick up amplified sound waves and then turn them into electrical signals.

Of course, physicists like to simplify things to understand them, so in this case they used a model with only two receptor neurons, both connected to a third interneuron that passes the signal to the rest of the brain.

Nerve signals typically appear as a sequence of electrical impulses called spike trains, created when they build up a voltage, fire, relax and build up again. In the model the physicists used, the interneuron mixes different frequency spike trains from the two receptor neurons into a combined signal.

What they found was that, when the two tones were harmonious, the interneuron produced a nice, regular, coherent spike train. But when they were discordant, the spikes were all messy and blurred into each other.

Now, we’ve known since the days of Pythagoras in about 500 BC that pairs of notes with simple frequency relationships go well together – like octaves (2:1) or perfect fifths (3:2). But this research shows that those mathematical relationships create strong, regular electrical signals in the brain.

It also fits in with what people hear when two notes are combined, which is often a low frequency that wasn’t really there in the original sound waves. This third note could well be the combined, regular spike train that the interneurons create.

Of course, this theoretical 3 neuron system is a vastly simplified model. But it does show how networks of neurons can give rise to phenomena that we see as subjective experience.


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