Cerebro-cerebellar projections from the lateral suprasylvian visual area in the cat.

1. A projection from the medial bank of the lateral suprasylvian visual area, one of the targets of the cerebello-cerebral projection, back to the cerebellar cortex was demonstrated electrophysiologically in the cat. The anatomical pathways underlying this projection were investigated using orthograde and retrograde transport of wheatgerm-agglutinin-conjugated horseradish peroxidase (WGA-HRP). 2. Responses were recorded in the cerebellar cortex on stimulation of the medial bank of the lateral suprasylvian area, and were compared with those evoked by stimulation of the motor cortex and the crown part of the parietal association cortex. 3. Responses induced by stimulation of the lateral suprasylvian area were shown to consist of early mossy and late climbing fibre responses. The mossy fibre response was evoked, at a latency of 2-3 ms, predominantly in the lateral part of the contralateral cerebellar cortex (mainly, crus I, crus II, dorsal paraflocculus and paramedian lobule) and the posterior part of the vermis (mainly, lobules VII and VIII). Climbing fibre responses were elicited with the preceding mossy fibre responses and were elicited at a much longer latency than the motor cortex-induced climbing fibre response. 4. The orthograde and retrograde HRP studies suggested that the mossy fibre response is mediated by the pontine grey whereas the climbing fibre response is conveyed indirectly to the inferior olive which sends the climbing fibres to the cerebellar cortex. After WGA-HRP injections into both the medial bank of the lateral suprasylvian area and the cerebellar responsive area, orthogradely labelled terminals of cortico-pontine projection fibres and retrogradely labelled ponto-cerebellar neurones were found in the pontine grey, where distributions of the two kinds of labelling overlapped. On the other hand, retrograde neuronal labelling alone was found in the inferior olive, implying that the climbing fibre responses evoked from the lateral suprasylvian area were relayed via indirect cortico-olivary pathways.