Re: How is musical timbre perceived?

Dear Nina

You asked a very good question. There is some relations between the 
physics of sound and perception but much remains to know. 
Electrophysiological studies have shown that single neurons in the 
auditory cortex are «tuned » to a narrow range of (sound) frequencies. 
Moreover, increasing the amplitude of sound increase the discharge rate of 
neurons. Thats mean that a single neuron will provide more action 
potentiels (discharge rate increase) when a given frequency is presented 
and when the sound is « louder ». Since there are many neurons that are 
tuned to different frequencies in the brain, it allows the subject to hear 
a given range of frequencies. Moreover, there is a specific arangement of 
neurons in the temporal lobe of the brain (the cortex of Heschl’s gyrus) : 
neurons are distributed along a stripe (gyrus) according to their preferred 
frequencies. It is likely that this is a sort of map that represents the 
cochlea (part of the inner ear) in wich ciliary cells move when a 
particuliar narrow range of frequencies are presented. As you have 
mentioned, there are also several attributes of sound (ex. : harmonics). It 
can be imagined that when you hear the same note played by a violin or a 
piano, you can distinguish the two because there are different groups of 
cells that responded in different discharge rates when you hear one or 
another instrument, just like the different harmonics of the sound have 
different amplitude. The problem of how the brain puts together the 
different attribute of sound to forme a single percept remains to know 
(sorry). It can be imagined that single neurons coding for different 
attributes of sound such as you have mentioned converge into a single cell 
that code for a more complex percept (hierarchical model) or that it is 
the fact that different cells located in different regions of the brain 
are activated simultaneously that allows to percieve the whole.

Hope that help,
Eric