Ascending vestibular projections: further results at cortical and thalamic levels in the cat

The purpose of this work was to precisely delineate the vestibular projection area in the cat's cortex and to trace electrophysiologically its afferent pathways at thalamic and mesencephalic levels. On anaesthetized animals, macropotentials evoked by electrical stimulation of the ampullar nerve from the horizontal semi-circular canal were recorded at three levels: on the cortex, the vestibular zone is restricted to the lower lip of the suprasylvian gyrus; in the thalamus, our results demonstrate the vestibular character of the magnocellular part of the medial geniculate body; at the mesencephalic level, our recordings indicate the existence of an ascending vestibular pathway situated laterally and ventrally with respect to the medial longitudinal bundle. The discussion focuses on two main points: 1. the precise cortical projection area of the horizontal ampullar nerve in relation to cytoarchitectonic areas and its possible functional role in motor programming. 2. the thalamic relays of the vestibulo-cortical pathway with regard to the hypothesis of two parallel input channels.