Rapid adaptation of saccadic amplitude in humans and monkeys.

To study adaptive motor learning in the saccadic system we have used a psychophysical procedure that introduces a "visuomotor mismatch" between the retinal error signal (retinal distance between fovea and target image) and the motor error signal (movement required to accurately foveate the target). The saccadic system responds to this visuomotor mismatch by rapidly modifying the amplitude of the saccade. We will refer to this procedure as induced saccadic dysmetria. In our paradigm, the saccadic dysmetria is produced by electronically adding or subtracting a fraction of the eye position signal to adjust the target's position. Thus the original visual error signal that initiates the saccade no longer elicits an appropriately sized saccade; there is a mismatch between target step and saccade amplitude. We find that the human and nonhuman primate saccadic systems respond to this error by rapidly and adaptively adjusting the amplitude of saccades. Such adaptive adjustments are not the result of changes in saccade strategy but represent a genuine recalibration. We conclude that induced saccadic dysmetria provides us with a tool for the study of adaptive motor learning in the oculomotor system. It is hypothesized that the adaptive mechanism may use either or both of two signals: a visual error signal representing the retinal distance of the target from the fovea after the initial saccade and/or a motor error signal represented by the amplitude of the corrective saccade.