STIMULUS-INDUCED SPIKE BURSTS IN 2 FIELDS OF CAT AUDITORY-CORTEX

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.