Electrophysiological evidence for a bisynaptic retinocerebellar pathway

1. Electrical microstimulation was applied to an in vitro turtle brain preparation while recording extracellular activity from the cerebellar cortex. A visual input to the cerebellum was investigated by measuring spike responses evoked by stimulation of drifting visual patterns imaged onto the contralateral retinal eyecup. A vestibular input was assessed by extracellular field potentials following brief current pulses through monopolar suction electrodes holding the eighth cranial nerve (nVIII). 2. The cortical topography of visual and vestibular inputs was first examined. Visual units and vestibular fields show considerable topographic overlap in the rostrolateral quadrant of the cerebellum. In addition, granule layer units were isolated that responded to current stimulation of nVIII (60-150 microA monopolar). In some cases, spikes occurred at short and fixed latency after each current pulse for stimulus frequencies of 100 Hz. The responses of these units suggest a direct path between the stimulating and recording electrodes without intervening synapses. Alternatively, extracellular units were also encountered that responded with longer, more variable latencies but only for low stimulation frequencies (< or = 20 Hz). Of the units that responded to nVIII stimulation, three units also responded to visual stimuli, yet those units all failed to follow high-frequency stimulation of nVIII. This cortical area may then be a site for convergence of visual and vestibular signals on postsynaptic cells. 3. The cellular identity of the visual units in the granule layer and the visual pathways leading there were next investigated.(ABSTRACT TRUNCATED AT 250 WORDS)