Localization of pitch in the brain?

Matt Jones jonesmat at physiology.wisc.edu
Thu Jul 19 17:40:10 EST 2001


"Didier A. Depireux" <didier at klezmer.isr.umd.edu> wrote in message news:<9j77cp$slt$1 at hecate.umd.edu>...

>See, if I make a harmonic sequence, with sounds at freqs 1800Hz,
2000Hz,
>2200Hz, you will get a sound that has a pitch of 200 Hz. So at some
level
>in your brain, "pitch of 200Hz tone = pitch of above harmonic
sequence". So
>you would like to get a cell somewhere that responds equally to the
200 Hz
>pure tone and to the harmonic sequence. Except for Riquimaroux, no
one has
>found such a cell.
>
>Of course, if I record from a 2000Hz cell in primary auditory cortex
and
>below, the cell will likely have a tuning of about 1/3 octave to 1
octave.
>Hence when I present the harmonic sequence above, because of the
beating
>between neighboring tones, I will see that my 2000Hz cell responds,
and its
>response is modulated at 200Hz (though a 200Hz-modulation-following
cell is
>rare in cortex, but anyway). That means that the cell has all the
>information in its firing rate, but it doesn't mean that you have
found a
>pitch cell. I have a poster,
http://www.isr.umd.edu/CAAR/posters/ARO97.pdf
>on that



I see the distinction you are making. You would define a "pitch cell"
as a cell that is tuned to pitch only, and does not represent
harmonics in its firing at all. This is indeed different from a cell
that represents the pitch along with the harmonics, because such a
response could occur through superposition of the components of the
sound wave, rather than a distillation of the pitch information by the
circuitry, as in your beating example.

But in your beating example, where the high frequency firing was
modulated at 200 Hz, the all-interval distribution as used by Cariani
& Delgutte wouldn't show the most prominent intervals at 200 Hz, would
it? I would expect that the all-interval distribution in that case
would have most of its mass near the freqs that were actually in the
signal (am I wrong about this?). Thus the cell responding to the
beating wouldn't have been a good estimator of the pitch under their
analysis, whereas their auditory nerve fiber populations were good
estimators. Of course, this would be a "pitch population", not a pitch
cell.


Another question though: is it necessary that there exist "pitch
cells" by your definition at any level in the brain?  That is, can
pitch not be represented as a population code, possibly even a
multiplexed code where the same cells are also representing other
aspects as well?


Matt




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