Single unit analysis of the posteroventral cochlear nucleus of the decerebrate cat.

Single unit recordings were obtained in the cochlear nuclear complex of the unanesthetized, decerebrate cat. Sixty-six of 282 units were localized to the posteroventral cochlear nucleus, 17 from the multipolar cell area and 49 from the octopus cell area. Spontaneous rates ranged from less than 1 to 75 spikes per second in the multipolar cell area and from less than 1 to 135 spikes per second in the octopus cell area. Poststimulus time histograms revealed four response types, at the best frequency, in the posteroventral cochlear nucleus. These responses were: (1) primary-like (maximum response shortly after the stimulus onset, followed by a reduction in activity to a steady state); (2) chopper (similar to primary-like but with multiple peaks in the first 10-15 milliseconds); (3) onset-ex (onset response followed by a low level of excitation); and (4) onset-in (onset response followed by inhibition). The onset-in responses represented the first observations of inhibition, at best frequency, for onset units in the mammalian cochlear nuclear complex. Analysis of interspike interval distributions showed that both spontaneous and driven activity consisted of irregular intervals for all four response types. Activity-intensity functions for primary-like, chopper and onset-ex units showed monotonic increases with increases in stimulus intensity. Activity-intensity functions for onset-in units were non-monotonic. Latency-intensity functions for primary-like, chopper and onset-ex units exhibited monotonic decreases with increases in intensity. Latency-intensity functions for onset-in units were non-monotonic. In contrast to primary-like, chopper and onset-ex units, onset-in units do not retain the intensity and temporal information coded in the eighth nerve, as least for stimuli above 2 kilohertz. It is hypothesized that a depolarization block, caused by the massive eighth nerve input to octopus cells, is responsible for the inhibition observed from onset-in units.