CONTRAST EFFECTS ON SMOOTH-PURSUIT EYE-MOVEMENT VELOCITY

These experiments show that the smooth-pursuit system [in humans] responds to changes in contrast in a similar way to the known response of direction-specific mechanisms, suggesting that the smooth pursuit system uses the signal generated by these mechanisms. Smooth pursuit eye movement velocity and saccadic latency were measured as a function of target contrast in 2 experiments using ramp target motion between 5-40/s. In the 1st experiment, the target velocity was predictable: in the 2nd, it was unpredictable. Smooth pursuit eye velocity in response to predictable ramp targets was independent of target contrast. Saccadic latency decreased dramatically as contrast increased. Eye velocity in response to unpredictable ramp targets increased with increasing target contrast over a narrow range of contrast (0.3 log units) above contrast threshold and then the response saturated.