Discrimination of communication calls in the squirrel monkey: "call detectors" or "cell ensembles"?

Single cells in the primary auditory cortex of the awake squirrel monkey were tested for their responses to intraspecific communication calls presented to the monkey normally ("calls") and backwards ("llacs"). These two groups of signals were similarly effective in eliciting responses, and response patterns were of the same nature and equally diverse. In about 2% of the cells the time structure of a response to at least one "llac" was virtually a "mirror image" of the response to the corresponding "call". In about 34% of the cells, for at least one vocalization, at one intensity or other, the time distribution of response peaks closely approximated in time with the envelope of a particular spectral component of the call, corresponding with the cell's best frequency. These results suggest that complex sounds may be represented in the auditory cortex by the synchronized activity of functional cell ensembles in which differently tuned individual members are distributed throughout the cochleotopic space according to their best frequencies.