The interconnections of the inferior colliculi through their commissure

Much is known of the sources and manner of termination of ascending and descending input to the inferior colliculus (IC) but its commissural connections are less well understood. Most studies of the commissure have utilized small lesions or tracer deposits; while all agree that commissural axons terminating in the IC do so in its superficial and dorsomedial sectors, it is not clear where projecting cell bodies are located in the IC. The present study attempted total infiltration of the commissure of the cat IC with horseradish peroxidase (HRP) in an effort to label all neuronal somas whose axons cross in the commissure. The distribution of labeled cells after the brachium of the IC (BIC) was cut unilaterally and infiltrated with HRP was also examined to enable comparison of the locations and approximate proportions of cells projecting to the contralateral IC and medial geniculate body (MGB). The cells giving rise to commissural axons form an array tilted dorsally from caudal to rostral that spreads mediolaterally through the central nucleus into the external nucleus of the IC, but largely excludes the dorsomedial sector at posterior levels. A similar distribution of labeled cells, but with reduced numbers, is found when large HRP deposits are made in the contralateral BIC. These results, in conjunction with those from studies of the terminations of commissural axons made by others, suggest that the interconnections of the inferior colliculi through their commissure are complementary, rather than reciprocal.