Effects of eye movements upon excitability of the superior colliculus in cats

Influences of saccadic eye movements upon excitability of cat superior colliculus (SC) were examined in chronic cats carrying implanted electrodes and in encephale isole cats. Excitability of SC was assessed by recording orthodromic responses of SC (SC ortho-response) to electrical stimulation of the optic chiasm. The SC ortho-response consisted of four components; i.e. presynaptic T1 and postsynaptic C1, C2 and C3. Eye movements suppressed the first two components (T1 and C1) and enhanced the latter two (C2 and C3). Retinal impulses caused by movements of images were found responsible for suppression of T1 and C1: 1) This was seen when cats moved eyes seeing external objects in light. In dark, neither T1 nor C1 received any influence from eye movements. 2) By moving visual stimuli rapidly before eyes of paralysed cats the same suppression of T1 and C1 was obtained. Suppression of C1 was deemed as a postsynaptic sign of SC for saccadic suppression. In contrast to suppression of T1 and C1, enhancement of C2 and C3 was dependent upon a central factor: 1) Enhancement of C2 and C3 followed eye movements, independent of the ambient illumination. 2) Saccade-like movements of a slit of light in front of immobilized eyes never induced enhancement of C2 and C3. 3) PRF stimulation, effective to cause saccade-like eye movements, always elicited enhancement of C2 and C3, independent of the ambient condition and even after eyes had been immobilized. This phenomenon was interpreted as indicating that corollary discharges are sent to SC.