Second order auditory pathways in the chimpanzee.

Abstract

Substantial portions of the dorsal, and almost the entire posteroventral and anteroventral (Av) cochlear nuclei were aspirated unilaterally in a chimpanzee. Axonal degeneration was studied by the Fink-Heimer method. The greatest amount of degeneration was followed medially from the region of Av into the lateral part of the trapezoid body. Degeneration also coursed around the superior surface of the restiform body and was traced into the dorsal and intermediate acoustic striae. Within the superior olivary complex, degeneration was distributed to: the ipsilateral lateral superior olive; laterally and medially oriented dendrites of the ipsilateral and contralateral medial superior olivary nuclei respectively (some perisomatic degeneration also was present bilaterally); the contralateral medial trapezoid nucleus; retro-olivary and preolivary cell groups bilaterally. Abundant degeneration passed into the contralateral lateral lemniscus and was distributed largely to its ventral nucleus. The contralateral central nucleus of the inferior colliculus was a major site of termination of ascending second order auditory fibers. The caudal tip of the ipsilateral ventral nucleus of the lateral lemniscus received abundant degeneration, but this diminished rostrally. The ipsilateral inferior colliculus contained a moderate amount of degeneration. A fair number of degenerated second order auditory fibers ascended in the contralateral brachium of the inferior colliculus and were distributed both to the principle and magnocellular divisions of the medial geniculate body. This pathway appears to represent a phylogenetic advance in the brain of the great ape.