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Abstract:
We studied the physiologic properties of OFF responses in
awake macaques by sampling from auditory cortex with linear array
multielectrodes. Laminar profiles of postsynaptic potentials and
action potentials were indexed by current source density (CSD) and
multiunit activity analyses, respectively. Best frequency at each
recording site was defined using binaural tones (70dB;100ms with
5ms on/off ramps), ranging from 0.5-40kHz. In both A1 and
surrounding regions, most recording sites that responded robustly
to clicks displayed clear OFF responses. OFF responses consisted of
either net excitation (OFF-E) or net inhibition (OFF-I). The
polarity of the OFF response was not obviously related to the
polarity of the preceding ON response. Often OFF-E and OFF-I could
be elicited from the same recording site by different frequency
stimuli, but in neither case were their best frequencies
necessarily related to the frequency of the corresponding ON
response. Decreasing the inter-stimulus interval from 9s to 150 ms
reduced the ON but not the OFF response. Limited manipulations of
stimulus duration (50-250 ms) revealed no clear interdependence of
ON and OFF responses. These findings suggest that, 1) in auditory
cortex ON and OFF responses are dissociable, and 2) that specific
processes in cortical ensembles respond in distinctive ways to
different aspects of the same stimulus. (Supported by MH61989 and
MH60357)
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