The response characteristics of auditory neurons in the multimodal midbrain torus semicircularis of rainbow trout were analyzed to examine their topography and functional differences. This analysis included the localization of recording sites, the measurement of spontaneous activity, the ratio of transient/sustained activity, and the synchronization, latency, preferred direction and directional range of the response. On average, units with a directionally selective (DS) response are positioned 60 microm more dorsally than other auditory units. Directionally selective units usually have a higher response rate, a higher transient/sustained activity ratio and are better synchronized. Auditory units encountered within the same electrode track tend to be either all DS or all non-DS. Within a track, there is no uniformity of the response characteristics observed except that the preferred direction of DS units appears to be the same. The anatomical stratification of the torus, containing 66 000 somata (5-10 microm in diameter), does not match the electrophysiologically observed vertical distribution of functionally distinct units. On the basis of the topographical distribution of response characteristics, two types of well-synchronized DS units can be distinguished, hypothetically representing separate channels for the processing of acoustic motion and (mainly) pressure information. A third type of DS unit which receives input from both these channels and actually encodes the source direction uniquely for all directions is postulated.

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