pitch neurons

["Dennis P. Phillips, Ph.D." <Dennis.Phillips@xxxxxx>]

Dear List:

A few related thoughts on the "pitch neuron" thread.

First, I'm in broad agreement with Dr. Nelken.  Pitch percepts arise
from a host of very diverse stimulus configurations.  So, if
individual neurons are going to respond to a "pitch" irrespective of
its origin, then the level of abstraction required for that "code" is
probably something beyond what we've seen to date in the mammalian
auditory system.  (See also Jont Allen's posting of October 9.)

Second, it is a separate question as to whether the pitches
associated with different kinds of stimulus arrangements need to
encoded by the same neurons (Chris Chambers' posting of October 8).
Thus, the spatial quality of percepts can arise from a number of
stimulus configurations (various interaural time disparities,
interaural level disparities, monaural spectral cues), and there is
good evidence that the neural mechanisms/circuits encoding these
stimulus features are somewhat non-overlapping.  There is
psychophysical evidence (Phillips & Hall, 2001, JASA, 110, 1539-1547;
Phillips et al., 2002, Perception, 31, 875-885; likely among work by
many others) and possibly imaging evidence (Pavani et al., 2002,
Current Biology, 12:18:1584-1590) for representations of space
spanning those proximal stimulus dimensions, but those
representations seem by definition to be "higher level" and they
quite possibly are located outside the central auditory system as we
normally think of it.  Perhaps the same general picture is true for
pitch.

Third, in the pitch case, I thought that the work of Schwarz and
Tomlinson (1990, J. Neurophysiol., 64, 282-298) was important in
casting some doubt that auditory cortical neurons in awake primates
were sensitive to the pitch of missing fundamental stimuli - even in
animals trained in a missing fundamental task.  Perhaps sadly, the
animals were not performing the missing fundamental task at the time
the recordings were made; it's an empirical question as to whether
that matters.  See also Fishman et al. (1998, Brain Res., 786,
18-30).  Robert Zatorre et al (1992, Science, 256, 846-849) have
presented imaging data in man showing that brain regions
differentially activated during a pitch-processing task using speech
materials lie outside the auditory cortex (in the right prefrontal
area).

Anyway, I hope that this helps.  All good wishes,

Dennis Phillips