Analysis of the step response of the saccadic feedback: system behavior.

We used prolonged stimulation of the monkey superior colliculus to elicit staircase eye movements. By changing the parameters of the stimulating current we were able to obtain movements with different dynamics. An increase in the current frequency resulted in the shortening of the intersaccadic interval and a decrease of the amplitudes in the staircase. In cases of high stimulation, after an initial saccade of fixed metrics, the eyes moved in an apparently smooth fashion. The movement was conjugate and in the same direction as the first saccade. By analyzing the velocity trace we found that the movement consisted of a chain of small saccades, each of which started before the previous one ended. We conducted a quantitative analysis of the staircase movements including the cases of apparently smooth movement of the eyes. We conclude that all of the movements elicited by prolonged SC stimulation were generated by the saccadic feedback circuitry. The dynamic profiles of the elicited movements changed continuously with the stimulating current parameters. On one end of the continuum we observed the classically, described staircase movements with individual movements separated in time. On the other end of the continuum we saw the apparent smooth movement as the limit case produced by high stimulation of the SC.