The sound-evoked responses of extracellularly recorded cat primary aud
itory cortical neurons usually consist of a single spike or a short-te
rm burst of 2-4 spikes, irrespective of the nature of the acoustic sig
nal. In the cat's auditory cortex, the properties of such responses ha
ve to date been described only for cells in the primary field (AI). Th
e purpose of the present study was to describe the properties of stimu
lus-evoked spike-burst responses seen in neurons of the posterior audi
tory field (P) and to compare those properties with those of a sample
of AI neurons studied under similar conditions. The data come from 80
field P and 31 AI neurons studied with tonal and noise-burst stimuli i
n barbiturate-anesthetized cats, using calibrated, sealed stimulus del
ivery systems and conventional extracellular recording techniques. The
mean inter-spike intervals (ISI) seen in the transient burst response
s of posterior field cells were typically short (2-5 ms) and, where it
was possible to test them, independent of the rise time of tonal sign
als, suggesting that they were also independent of the onset spectrum
of the stimulus. The mean ISIs were often independent of the stimulus
amplitude, even though the signal level had profound effects on the nu
mber of spikes evoked and the latency and regularity with which the re
sponses were initiated. Each neuron was assigned a 'characteristic ISI
', i.e., the mean ISI seen in the most vigorous responses. The distrib
ution of characteristic ISIs for AI and P neurons overlapped, but were
significantly different, with the characteristic ISIs of field P neur
ons being longer, In both AI and P populations, characteristic ISI was
significantly correlated with minimal first-spike latency. The slopes
of the regression lines of characteristic ISI on minimal latency for
AI and for P cells were not significantly different from each other. S
ince the minimal latencies of AI neurons were usually shorter than tho
se of field P neurons, the shorter characteristic ISIs of AI cells may
thus be interpreted as secondary to their shorter latent periods. The
general properties of stimulus-evoked spike bursts seen in field P ne
urons were thus very similar those previously described for AI cells.
These data are consistent with the view that the majority of extracell
ular recordings in the cat's auditory cortex come from pyramidal neuro
ns and an appropriate as a specialization for transfer of information
to nonpyramidal, inhibitory interneurons.