In a series of seventeen rhesus monkeys attempts were made to produce discrete stereotaxic lesions in the anteroventral cochlear nucleus (Av). Anterograde degeneration was described in detail in four cases with lesions confined within the cochlear complex to Av. Fibers decussating at pontine levels coursed exclusively in the trapezoid body. Degenerated fibers projected: ipsilaterally to the lateral superior olivary nucleus; bilaterally to the preolivary nuclei; to the lateral side of the ipsilateral medial superior olive and the medial side of the contralateral medial superior olive; and to the contralateral medial trapezoid nucleus. A topographic projection upon the medial superior olive was Numerous behavioral and physiological investigations have been concerned with the mechanisms of audition in the cat and monkey. While the auditory system has been subject to many neuroanatomical studies, information about the auditory pathways in primates remains incomplete including the origins, course, and terminations of ascending second order fibers. Lorente de No ('33) using the Golgi method, divided the cat cochlear nuclear complex into 13 subdivisions, and recently Osen ('69b) identified nine regions with Nissl material. The results of a combined morphological and physiological study by Rose, Galambos, and Hughes ('59) indicate that in the cat the cochlear complex is comprised of three major subdivisions : dorsal, posteroventral, and anteroventral nuclei, and that the complete auditory frequency spectrum is represented in each of these subdivisions. Cells located adjacent to the anteroventral nucleus within the fascicles of the cochlear nerve are referred to as the interstitial cochlear nucleus. Although this is one of Lorente de No's 13 regions, evidence suggesting a distinctive nature of this area is lacking, and it may be related J. COMP. NEUR., 143: 217-242.to the anteroventral subdivision. The present study represents an attempt to determine the course and terminations of fibers emanating from the anteroventral cochlear nucleus in rhesus monkey. MATERIALS AND METHODSSeventeen rhesus monkeys weighing approximately 2500 gm each were used in this study. In these animals attempts were made to produce unilateral stereotaxic lesions in the anteroventral cochlear nucleus. Preliminary stereotaxic coordinates were determined using dissections of formalin fixed heads in situ as described by Carpenter and Whittier ('52) and through the use of a stereotaxic atlas (Snider and Lee, '61). Following a suboccipital craniotomy, electrodes were protruded through the cerebellum in the horizontal plane of the stereotaxic instrument at a moderate angle to the axis of the brain stem. Electrodes were angled 15" towards the midline in order to enter the lateral aspect of the anteroventral cochlear nucleus and avoid other
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