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 T
l and post-synaptic C
1, C
2 and C
3. Eye movements suppressed the first two components (T
1 and C
1) and enhanced the latter two (C
2 and C
3). Retinal impulses caused by movements of images were found responsible for suppression of T
1 and C
1: 1) This was seen when cats moved eyes seeing external objects in light. In dark, neither T
l nor C
l received any influence from eye movements. 2) By moving visual stimuli rapidly before eyes of paralysed cats the same suppression of T
l and C
l was obtained. Suppression of C
l was deemed as a postsynaptic sign of SC for saccadic suppression.
In contrast to suppression of T
1 and C
1, enhancement of C
2 and C
3 was dependent upon a central factor: 1) Enhancement of C
2 and C
3 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 C
2 and C
3. 3) PRF stimulation, effective to cause saccade-like eye movements, always elicited enhancement of C
2 and C
3, 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.
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