Localization of pitch in the brain?
jonesmat at physiology.wisc.edu
Wed Jul 18 09:44:08 EST 2001
meshinder at aol.com (MEShinder) wrote in message news:<20010716122745.19834.00000992 at nso-bg.aol.com>...
> In article <b86268d4.0107160805.6215f470 at posting.google.com>,
> jonesmat at physiology.wisc.edu (Matt Jones) writes:
> >However, the earliest stage where neurons appear to respond to pitch
> >(as opposed to frequency - not the same thing) is probably the
> >cochlear nucleus which is the first main integrative processing stage
> >for auditory information.
> I am naive to the neuralogical difference between pitch and frequency
> processing in the cochlear nuclei. I am aware of the tonotopy in these nuclei,
> but I'm not sure how it relates to a difference.
Frequency is the physical description of the pressure wave (e.g.,
power spectral density) whereas pitch is the perceptual sensation
evoked in the listener. Most of the time these things amount to
roughly the same thing. However there are some well known situations
in which frequency and pitch are different.
For example, it is common when listening to music to "hear" pitches
that are not actually there, such as "hearing" the tonic of an
inverted chord even if it has not been played. The auditory system
appears to have a number of mechanisms for completing patterns so that
the perceived sound matches certain expectations, rather than merely
relaying the spectral content of the sound. The visual system does
this too, in the spatial frequency domain.
Another example is a thing called the "Doppler illusion", in which a
person (i.e., a substantial fraction of subjects in psychophysical
experiments, but not all) "hears" a rising pitch as a sound source is
moved away from them, despite the fact that no rising frequency
pattern is present in the sound.
A google search for "missing fundamental" or "Doppler illusion" will
turn up a bunch of info on these things.
As for the biological basis, I don't know exactly, but I'm pretty sure
that certain cell types in cochlear nucleus phase lock to the missing
fundamental. In fact, I would bet that even hair cells in the cochlea
that are tonotopically located at the position of the fundamental are
probably activated in response to sounds that are missing the
fundamental. I say this because physical systems have a tendency to
vibrate at characteristic modes given certain stimuli. Like, if you
pick the A-string on a guitar, you don't just get A, you get A plus a
bunch of integral multiples of A (the overtones or harmonics).
Further, if you -play- an A on one guitar, that sound will cause the A
string on a nearby guitar to vibrate (this is called "sympathetic
vibration". The sitar was designed with lots of sympathetic strings to
take advantage of this phenomenon. That's why it has that trippy
room-filling sound). I would guess that different parts of the
cochlea undergo sympathetic vibration in response to vibration at
other frequencies, and therefore can perform a physical pattern
completion to fill in the missing fundamental, even before any neurons
are allowed to get their grubby little hands on the auditory
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