Superior colliculus projections to tactile areas of rat cerebellar hemispheres

The spatial organization of the projections of the superior colliculus (SC) to the contralateral cerebellar hemisphere and posterior vermis was examined using micromapping techniques in barbiturate or ketamine anesthetized rats. Electrical stimulation of the tactile-responsive intermediate layers of SC evoked responses in tactile regions of the contralateral granule cell (GC) layer of cerebellar cortex. In most experiments, the SC stimulating electrode remained stationary while the cerebellar electrode mapped the region of the GC layer activated by SC stimulation. In a few experiments, the cerebellar recording electrode remained fixed while the SC electrode mapped the area of effective stimulation. Puncture densities up to 60/sq.mm were necessary to fully determine the spatial pattern of these projections. Major results are: (1) cerebellar responses to SC stimulation are found in the GC layer of contralateral crus I, crus II, lobulus simplex, and the paramedian lobule; no responses were seen in the uvula. The anterior lobe was not examined; (2) peripheral receptive fields (RFs) of the interconnected SC and GC loci consist exclusively of facial structures (especially vibrissae, crown, bridge of nose, and eyelid); (3) SC sources project within the confines of GC layer patches with similar RFs; (4) GC layer responses typically consist of an early, low amplitude potential, followed by a multiple-unit response; and (5) latencies of GC layer responses to SC stimulation are 2.2-2.5 msec for the early potential, and 2.5-3.5 msec for the multiple-unit response. This is the first physiological demonstration of projections from the superior colliculus to the cerebellar hemispheres. The results of this and other studies in our laboratory indicate that the tactile areas of the cerebellar hemispheres receive convergent inputs from peripheral, tectal, and SI cortical tactile sources.