Saccades to stationary and moving targets differ in the monkey

Saccade characteristics in response to moving and stationary targets were studied in three monkeys (Macaca mulatta) that had been trained to look at a target, which after an initial jump either remained in place or moved forward or backward with constant velocity (10 degrees /s). Eye movements were recorded using a search coil. The contribution of smooth pursuit to the saccade amplitude was small (<0.25 degrees ). Saccades having the same amplitude (5.67-6.83 degrees for different monkeys) to forward and backward moving targets were compared. Peak velocity was higher (37-42 degrees /s on average for different monkeys) and saccade duration was shorter (8-10 ms on average) for backward saccades than for forward saccades These differences were highly significant (t-test: P<0.001). Thus, forward and backward saccades are not on the same main sequence. This suggests that saccade dynamics are affected not only by the retinal position error but also by target motion. Further analysis revealed that saccade peak velocity mainly depends on the retinal position error, but saccade amplitude also depends on a stimulus-related velocity factor, which affects the saccade mainly during deceleration. This velocity factor could be retinal slip or target velocity, which was the same under our conditions. Our results experimentally support recent models that propose that the saccade acceleration in response to moving targets might be controlled by the superior colliculus, whereas the deceleration changes are fine-tuned by the cerebellum. This prediction must still be tested on a neuronal level.