The role of the external nucleus of the inferior colliculus in the construction of the superior collicular auditory space map in the guinea-pig

Abstract

In the guinea-pig two subcortical structures have been shown to contain representations of auditory space, the deep layers of the superior colliculus (SC) and the external nucleus of the inferior colliculus (ICX). The following investigation was undertaken to determine if the ICX influences the development of the SC auditory space map. A portion of the ICX was lesioned unilaterally when guinea-pigs were between 8 and 13 days old. Following a period of recovery, the SC mapping experiments commenced in the adult guinea-pig. Electrophysiological auditory responses were recorded from the deep layers of the SC to free-field azimuthal auditory stimulation. The data from the ICX lesioned animals revealed that the spatial tuning parameter values of the auditory receptive fields from the SC were significantly greater than those recorded in the normal control group of animals. The pooled data from the ICX lesioned group indicated that the peak angle of the response of the auditory receptive fields did not display normal topographic order. Furthermore, in several cases where the ICX lesion site was small, the position of the lesion could be correlated with the site of SC auditory space map disruption. These data indicate that, during early development, the presence of the ICX is essential for the normal emergence of an azimuthal map of auditory space in the deep layers of the guinea-pig SC.