Abstract
The superior colliculus serves to orient an animal to a target in the opposite visual hemifield. Commissural connections between the bilateral superior colliculi (SCs) are known to help maintain visual fixation and suppress saccades. Sprague (1966) showed that after transection of the commissure between the SCs, cats that had been made blind by prior ablation of the occipitotemporal cortex once again visually oriented to objects in a hemifield. This outcome, called the Sprague effect, suggests that the commissural connection of the SC may play an important role in visual-orienting behavior, and may mediate mutual suppression between the two SCs to prevent competing responses in the opposite direction.Later electrophysiological studies confirmed the inhibitory nature of the commissural tectotectal projection. Our recent study, using intracellular recording combined with electrical stimulation of the SC, showed that tectoreticular neurons (TRNs) in the caudal ‘saccade zone’ mainly received monosynaptic inhibition from the contralateral rostral SC and mono- or disynaptic inhibition from its caudal part, whereas TRNs in the rostral SC received monosynaptic excitation from the contralateral rostral SC, and mono- or disynaptic inhibition from all rostrocaudal levels of the contralateral SC. This review summarizes the characteristic features of commissural inhibition and excitation between the two SCs on various output TRNs terminating on different kinds of interneurons in the horizontal saccade generator in the brainstem of the cat.
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