Evidence of a functionally segregated pathway from dorsal cochlear nucleus to inferior colliculus.

Type O units in the central nucleus of the inferior colliculus (ICC) of decerebrate cats are excited by best frequency (BF) tones near threshold, but are inhibited by high-level tones at all frequencies. Dorsal cochlear nucleus (DCN) principal cells display similar response map features and project directly to the ICC, and are thus supposed to be the dominant source of excitatory input for type O units. To test this hypothesis, the responses of type O units were compared before and after two pharmacological manipulations. When DCN to ICC axons were blocked by pressure injections of lidocaine, most type O units (approximately 80%) were silenced or showed substantially reduced activity, but some units showed increased activity. All of the former units had low maximal rates to BF tones, whereas the latter units had high rates. When local circuit inhibitory mechanisms in the ICC were blocked by iontophoretic application of bicuculline or strychnine, type O unit responses also fell into two classes: low-rate units that showed increased spontaneous and driven activities and high-rate units that showed, in addition, altered response map features. Taken together, these results demonstrate that low-rate type O units are part of a functionally segregated pathway initiated by the DCN, whereas high-rate type O units are created at the level of the ICC.