CONVERGENCE OF SOMATOSENSORY AND AUDITORY PROJECTIONS IN THE AVIAN TORUS SEMICIRCULARIS, INCLUDING THE CENTRAL AUDITORY NUCLEUS

Projections of dorsal column, spinal, and cochlear nuclei upon the cen tral nucleus of the torus semicircularis (otherwise known as nucleus m esencephalicus lateralis, pars dorsalis, for MLd) and upon other toral nuclei were investigated in pigeon by anterograde and retrograde trac ing and electrophysiological methods. The anatomical results showed th at caudal regions of the dorsal column nuclei and medial lamina V of t he upper four cervical spinal segments have extensive projections upon the contralateral central auditory nucleus and upon other nuclei of t he torus, in particular the core portion of the preisthmic superficial area of Puelles et al. (L. Puelles, C. Rrobles, M. Martiez-de-la-Torr e, and S. Martinez, 1994, J. Comp. Neurol. 340:98-125). The projection s of nucleus angularis were found to terminate throughout most of the contralateral central nucleus except the dorsomedial portion at rostra l levels, where the majority of the projections of nucleus laminaris w ere concentrated. Nucleus angularis (and to a lesser extent nucleus la minaris) was also found to have substantial projections to certain non central toral nuclei, in particular to the caudomedial shell nucleus o f Puelles et al. (1994). As shown positively with both Nissl and cytoc hrome oxidase staining and negatively with substance P labeling, this nucleus is a medial extension of more caudal regions of the central nu cleus, and it is suggested that it should be included as part of the a uditory midbrain. The electrophysiological results confirmed the anato mical findings by showing that evoked potentials and multiunit activit y can be recorded throughout the central and noncentral toral nuclei b y using electrical stimulation of the radial nerve and auditory click stimuli. The core portion of the preisthmic superficial area, however, can be regarded as a distinct somatosensory nucleus of the midbrain. It is concluded that there is substantial convergence of somatosensory and auditory inputs within both central auditory and noncentral nucle i of the torus semicircularis in pigeon. (C) 1995 Wiley-Liss, Inc.