ARCHITECTURE OF A GAIN CONTROLLER IN THE PURSUIT SYSTEM

A monkey can pursue faster target oscillations if they appear during o ngoing smooth pursuit than if they appear while the monkey is fixating a stationary target. Others have proposed a switch in the pursuit cir cuit to account for this bistable sensitivity to high frequency target s. It is hypothesized that the switch is closed only during pursuit, p ermitting the retinal motion signal to pass through the circuit at ful l gain. Losses in pursuit gain caused by certain cortical lesions do m imic the effect of a switch jammed open. To explore this gain adjustme nt mechanism further, we measured in monkeys the smooth eye movements in response to a high frequency sinusoidal target (called 'humm') pres ented under a variety of testing conditions. Pursuit gain measured in response to this humm was not merely bistable. Rather, a graded gain m odulation of the pursuit system was possible. Furthermore, the gain ad justment had some directional sensitivity to it, enhancing the respons e to humm along one axis more than the other. In exploring the factors which gated the gain adjustment, it appeared that the movement of the eyes and not the image motion that occurs during pursuit was paramoun t for enhancing pursuit gain. Gain was not enhanced by saccadic but on ly by smooth pursuit tracking movements. Finally, gain could be modula ted somewhat by covert signals such as the expectation of future smoot h pursuit movements.