Interactions between natural and electrically-evoked saccades in the head-free cat

Fixed-vector saccades obtained upon electrical stimulation--while the eyes are steady--of any central oculomotor system structure can be derived from 2 different signals: (1) a retinal error signal, directing the eyes toward a specific location relative to the retina; (2) a motor error signal determining the movement vector itself. Theoretically, the two mechanisms can be segregated by delivering the stimulation during a naturally-triggered saccade (colliding saccade paradigm, Schlag-Rey et al., 1989), thereby creating a difference between the eyes' position at stimulation onset and their position at saccade outset. If derived from a retinal error signal, the electrically-evoked vector trajectory should be modified to compensate for the intervening change in eye position. If the saccade is derived from a motor error signal, the vector trajectory should be the same as when stimulating the same site while the eyes are immobile. This study describes the results of the application of the collision test in conjunction with the stimulation of 16 sites throughout the Superior Colliculus of 3 head-free cats. The main result of these tests--whether applied in conjunction with stimulation of superficial (where cells have visual activity) or deeper layers (where cells have motor activity) sites--is an absence of compensation indicating that the gaze saccades were derived from a motor error signal. Compensating saccades were only found in 6% of the cases with superficial layer stimulation sites while they were found in about 12% of the cases with deeper layer sites