Convergence of labyrinthine influences on units in the vestibular nuclei of the cat. I. Natural stimulation

Abstract

Neurons in the vestibular nuclei were examined by extracellular recording in cats with high cervical spinal cord transections and were physiologically identified by their response to angular acceleration stimulation in the plane of the horizontal canal. Units which responded to horizontal canal stimulation were then tested firstly by slow constant velocity tilts and statically maintained positions in space (otolithic tests) and then by angular accelerations (canicular tests) in the plane of the left anterior-right posterior canals and in the plane of the left posterior-right anterior canals. In this way the extent of convergence from various vestibular sensory regions onto horizontal canal neurons was determined. In some cats an attempt was made to eliminate input from vestibular sensory regions contralateral to the recording site by making a midline section of the floor of the fourth ventricle from the aqueduct to the obex. Approximately one-third of the units in both sectioned and non-sectioned animals responded solely to horizontal canal stimulation. The remaining two-thirds exhibited convergence from other sensory regions in varying degrees of complexity — most responded to only one other sensory region, but some responded to stimulation of 2 or 3 other sensory regions. Approximately half of all units in both sectioned and non-sectioned animals responded to otolithic stimulation. Likewise, approximately half of all units responded to stimulation of at least on vertical canal, however, midline sectioning appeared to reduce this proportion. It is suggested that vestibular convergence is due to neural interaction within the vestibular nuclei rather than being due to spread of stimulation or being instances of unusual modes of operation of particular vestibular receptors. The comparatively negligible effect of midline section may be consistent with the hypothesis that vestibular commissural fibers convey ‘mirror image’information from vestibular receptors on the contralateral side, and that this information, in the animal with bilaterally intact labyrinths, has an enhancing or ‘sharpening’ effect.