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Re: pitch neurons

Dear Martin and List,
       Dave McAlpine presented a poster at ARO (2002, asbtract no. 154) in which
he suggests (rather convincingly, IMHO) that the responses to
high-carrier sinusoidally-modulated tones (HC-SAMs) are due to cochlear
combination tones. The discussion in the Biebel and Langner paper does
not addresses, but not really answer, this critique. It's the other
piece of evidence that they bring (very wide inhibitory sidebands) that
is for me the more convincing evidence for wide-band integration by
these neurons, but then such wide bandwidth can already be demonstrated
in the dorsal cochlear nucleus (e.g. Spirou and Young, J Neurophysiol.
1991 Nov;66(5):1750-68 and Spirou et al., J Neurophysiol. 1999
Aug;82(2):648-63) which is an important input to the inferior colliculus.
       Another piece of evidence against the interpretation of responses to
HC-SAMs as related to pitch comes from some optical imaging results that
we recently collected in ferret auditory cortex (to be presented at the
Neuroscience meeting in Orlando, FL in November). We used HC-SAMs with a
carrier at 6 kHz, that produce essentially no pitch sensation, and also
high-pass clicks (at 4 kHz, with rather shallow slope) that do produce a
very strong pitch perception. We demonstrated a response in
low-frequency AI that was sensitive to the modulation frequency of the
HC-SAM, but was insensitive to the period of the high-passed clicks.
This result essentially reproduces the data of Schultze et al. (Eur. J.
Neurosci. 15:1077-1084, 2002) who showed activation of low-frequency
auditory cortex by HC-SAM in gerbils, but the additional test using
high-passed clicks is a serious reason to doubt the interpretation of
this map as related to pitch.
       All in all, I think that the cautious position at the moment is still
that there is no unequivocal evidence for pitch sensitivity up to and
including the level of primary auditory cortex in animals, independent
of the spectral content of the sounds.

Israel Nelken                     | Tel: Int-44-1865-272438
Dept. of Physiology,              | Fax: Int-44-1865-272469
Hebrew University -               |
       Hadassah Medical School    |
P.O.Box 12272, Jerusalem 91120    | Email:israel@md.huji.ac.il
ISRAEL                            |