Tuning properties of auditory cortex cells in the awake squirrel monkey

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Abstract
Pure tone bursts elicited in primary auditory cortex (AI) cells of the awake squirrel monkey a wide range of response patterns which consisted of one or more excitatory or inhibitory temporal response components. In almost 60% of these cells, response patterns were frequency and/or intensity dependent. Response components such as early and late onset excitation, offset excitation and on-off excitation; as well as tonic excitation or inhibition often varied independently with changes in these stimulus parameters. Individual cells were therefore considered as multiple bandpass filters, and each discrete response component was analyzed separately for its tuning properties. A correlation between best frequencies of the various excitatory components (BEF), and between BEFs and best frequencies of inhibitory components (BIF), in cells which responded with more than one discrete response component, disclosed a significantly higher correlation between BEF/BIF pairs compared with BEF/BEF pairs, presumably reflecting certain “lateral inhibition like” processes. Applying Q10dB factor, and √Hf-√Lf bandwidth at 10 dB above threshold, as measures of the “sharpness” of response areas, revealed that approximately 65% of all response areas could be defined as “narrow” by either one of these 2 measures, with no distinction, in that regard, between excitatory and inhibitory components. The average response bandwidths of the narrowly and the broadly tuned components, at 10 dB above threshold, were 0.4±0.18 and 1.42±0.68 octaves respectively. A comparison with the medial geniculate body (MGB) of the squirrel monkey, applying the √Hf-√Lf measure of sharpness of tuning, showed a significantly higher proportion of narrow response areas in the AI. “Narrow” response areas in both these regions were equally narrow, whereas the “broad” response areas of MGB cells were significantly broader. These results suggest a sharpening of response areas throughout the geniculo-cortical transformation.