pitch neurons ("Dennis P. Phillips, Ph.D." )

Subject: pitch neurons
From:    "Dennis P. Phillips, Ph.D."  <Dennis.Phillips(at)DAL.CA>
Date:    Wed, 9 Oct 2002 13:53:36 -0300

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

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Electrical Engineering Dept., Columbia University