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.