Localization of pitch in the brain?
Didier A. Depireux
didier at klezmer.isr.umd.edu
Thu Jul 19 13:03:05 EST 2001
Matt Jones <jonesmat at physiology.wisc.edu> wrote:
> Well, I was sort of just making that up as I went along. But after
> you asked I did a websearch for "missing fundamental cochlear nucleus"
The papers you quote are the most relevant ones. The one by Riquimaroux is
the only one that actually shows what people would call a 'pitch cell'.
Other papers, esp the excellent paper of Cariani and Delgutte, only
purports to show (which is already hard enough) that the information needed
to extract the pitch of a sound is present in the auditory nerve, taken as
a population of neurons.
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
The problem with knowing the level at which pitch cells have occasionally
been found in the auditory pathway is that if you blast the ear with
90-100dB sounds (esp in an anesthetized animal in which the acoustic reflex
is gone), you are likely to get a response to anything. In other papers
where the animal was awake, the loudspeaker was of inferior quality. If
there's a distortion of the sound, then the harmonic sequence above will
have distortion products at 200 Hz (from saturation, for instance) so that
when you think you are presented a missing fundamental sound, the
loudspeaker distortion induces the funcdamental in the sound actually
perceived by the subject.
Didier A Depireux didier at isr.umd.edu
Neural Systems Lab http://www.isr.umd.edu/~didier
Institute for Systems Research Phone: 301-405-6557 (off)
University of Maryland -6596 (lab)
College Park MD 20742 USA Fax: 1-301-314-9920
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