AZIMUTH CODING IN PRIMARY AUDITORY-CORTEX OF THE CAT - II - RELATIVE LATENCY AND INTERSPIKE INTERVAL REPRESENTATION

This study was designed to explore a potential representation of sound azimuth in the primary auditory cortex (AI) of the cat by the relativ e latencies of a population of neurons. An analysis of interspike inte rvals (ISI) was done to asses azimuth information in the firings of th e neurons after the first spike. Thus latencies of simultaneously reco rded single-unit (SU) spikes and local field potentials (LFP) in Al of cats were evaluated for sound presented from nine speakers arranged h orizontally in the frontal half field in a semicircular array with a r adius of 55 cm and the cat's head in the center. SU poststimulus time histograms (PSTH) were made for each speaker location for a 100-ms win dow after noise-burst onset using 1-ms bins. PSTH peak response latenc ies for SUs and LFPs decreased monotonically with intensity, and most of the change occurred within 15 dB of the threshold at that particula r azimuth. After correction for threshold differences, all latency-int ensity functions had roughly the same shape, independent of sound azim uth. Differences with the minimum spike latency observed in an animal at each intensity were calculated for all azimuth-intensity combinatio ns. This relative latency showed a weakly sigmoidal dependence on azim uth that was independent of intensity level >40 dB SPL. SU latency dif ferences also were measured with respect to the latencies of the LFP t riggers, simultaneously recorded on the same electrode. This differenc e was independent of stimulus intensity and showed a nearly linear dep endence on sound azimuth. The mean differences across animals for both measures, however, were only significant between contralateral azimut hs on one hand and frontal and ipsilateral azimuths on the other hand. Mean unit-LFP latency differences showed a monotonic dependence on az imuth with nearly constant variance and may provide the potential for an unbiased conversion of azimuth into neural firing times. The genera l trend for the modal ISI was the same as for relative spike latency: the shortest ISIs were found for contralateral azimuths (ISI usually 3 ms) and the longer ones for ipsilateral azimuths (the most frequent I SI was 4 ms, occasionally 5 ms was found). This trend was also indepen dent of intensity level. This suggests that there is little extra info rmation in the timing of extra spikes in addition to that found in the peak PSTH latency.