Organization of the inferior colliculus of the gerbil (Meriones unguiculatus): projections from the cochlear nucleus.

Projections from the cochlear nuclear complex to the inferior colliculus in the gerbil (Meriones unguiculatus) were studied using anterograde tracing methods based on axonal transport. Methods were developed to map the results onto comparable sets of sections through the inferior colliculus so that the patterns of termination in different animals could be compared directly. Projections to the contralateral inferior colliculus are widespread and most, if not all of them, are topographically organized. Axons terminate throughout the central nucleus and also in at least three distinct regions outside the central nucleus: a caudomedial region in the dorsal cortex, the ventrolateral nucleus and the rostral pole nucleus. Projections from the dorsal and ventral cochlear nuclei appear to overlap almost completely, although those from the dorsal cochlear nucleus may be slightly more widespread at the boundaries of the central nucleus. Projections from the ipsilateral cochlear nuclei arise in both the dorsal and ventral divisions and are largely restricted to the dorsal (low-frequency) part of the inferior colliculus. In this region, the pattern of ipsilateral and contralateral projections is similar, although the terminal fields from the two sides do not appear to overlap completely. The methods developed to display the results form a framework for comparisons with the distribution of inputs from the other major sources of input to the inferior colliculus.