Analysis of the frequency response of the saccadic circuit: System behavior

To more thoroughly describe the system dynamics for the saccadic circuit in monkeys, we have determined the frequency response by applying a frequency modulated train of microstimulation pulses in the superior colliculus. The resulting eye movements reflect the transfer function of the saccadic circ uit. Below input modulations of 5 cycles/s, the saccadic circuit increasing ly oscillates with multiple high-frequency, low-amplitude movements reminis cent of the "staircase saccades" evoked during the sustained step response. Between 5 and 20 cycles/s, the circuit entrains well to the input, exhibit ing one saccadic response to each sinusoidal input. Within this range there are systematic frequency-dependent changes in movement amplitudes, includi ng super-normal saccades at some input frequencies. Above 20 cycles/s, the saccadic circuit increasingly exhibits periodic failures at rates of 1:2 or higher. In addition, the circuit exhibits predictable amplitude-modulated interference patterns in response to a combined step and frequency-modulate d input. These experimental results provide insight into several biological mechanisms and serve as benchmark tests of viable models of the saccadic s ystem. The data are consistent with negative feedback models of the saccadi c system that operate as a displacement controller and inconsistent with th eories that put the superior colliculus within the lowest-order, local feed back loop. The data support theories that the circuit feedback operates wit h dynamics that simulate a "leaky integrator." In addition, the results dem onstrate how the temporal output of the superior colliculus interacts with recurrent inhibition to influence the eye movement dynamics.