Abstract
The gain control of the flocculus is executed with the aid of oculomotor function toward the direction of improving the stabilization of the retinal images during head movements. Visual suppression (VS) of caloric nystagmus occurs also in the direction of stabilizing retinal images of visual surroundings. In fact, VS of caloric nystagmus is lost after total destruction of the flocculus. Under such conditions, optokinetic nystagmus involving smooth pursuit of eye movements and fixation may also be disturbed. In the present experiment using cats, the optokinetic nystagrxmus was investigated after lesions had been induced in the superior colliculus (SC) or the inferior olive (IO), both considered to mediate visual signal to the flocculus. VS of caloric nystagmus, and optokinetic nystagmus remained normal in all IO-lesioned cats. After the SC lesion, loss of VS was always evident, and optokinetic stimuli produced directional preponderance to the left and diminished response to the right in 7 of 9 cats. Hence, the SC is considered to be the one of the main relay nuclei in the mediation of visual signals responsible for the immediate modification of VOR by visual stimuli.
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