Response properties of type II and type III units in dorsal cochlear nucleus.

Type II and type III units in the dorsal cochlear nucleus (DCN) of unanesthetized (decerebrate) cats are those with excitatory responses to best frequency (BF) tones at all levels above threshold. They are distinguished from type IV units which give predominantly inhibitory responses to tones. Type II and type III units are distinguished from one another by their rates of spontaneous activity. Type II units have low rates of spontaneous activity (below 2.5 spikes/s), whereas type III units are spontaneously active at rates up to 95 spikes/s. In this paper we show that segregation of units according to spontaneous activity produces a segregation of several other properties as well. A typical type II unit responds vigorously to BF tones (median maximum rate of 242 spikes/s), has slightly nonmonotonic rate versus level function, and responds weakly or not at all to broadband noise. These units tend to be located in the deep layer of the DCN. Type III units tend to respond to BF tones at moderate rates (median maximum driven rate of 124 spikes/s), have monotonic or nonmonotonic rate versus level functions, and respond to noise at rates comparable to their tone rates. They are distributed somewhat more uniformly in all DCN layers, but a majority were found in the fusiform cell and molecular layers. Most evidence indicates that type II units are recorded from inhibitory interneurons in the DCN.