THE POSTERIOR FIELD-P OF CAT AUDITORY-CORTEX - CODING OF ENVELOPE TRANSIENTS

The posterior field (P) of the cat auditory cortex contains a very hig h proportion of neurons whose responses change non monotonically with the sound pressure level (SPL) of tonal stimuli, leading to circumscri bed frequency-SPL response areas, and it has therefore been suggested that field P may be specialized for processing of sound intensity. We demonstrate here a great diversity of response areas in field P. Furth ermore, by varying tone SPL and rise time, we show that, as in primary auditory cortex (AI), the onset response of a field P neuron is bette r described as a function of the instantaneous peak pressure (envelope ) at the time of response generation than of the steady-state SPL of t he stimulus. Such responses could be used to track transients or repre sent envelopes in more general terms, rather than to code SPL. Compare d with Al, field P neurons have relatively long minimum latencies alon g with a large jitter in spike timing. Tracking would therefore be mos t effective for slowly varying envelopes, and one function of the inhi bition that generates nonmonotonicity in field P may be to suppress te mporally sluggish responses to rapid transients, such as the onsets of high-SPL, short rise time tones. Field P may thus be specialized for coding slowly varying signals.