Saccadic plasticity: parametric adaptive control by retinal feedback.

Saccadic gain (the ratio of saccadic amplitude to target eccentricity) was experimentally altered as normal human observers made refixation saccades to the right, which caused step changes in the horizontal position of the target. Eye movements were monitored by diffuse infrared limbus reflection. We found that decreases in saccadic gain reached 60% of completeness, whereas increases were only 25% complete. This asymmetry in adaptive capacity may reflect the saccadic system's need to avoid overshooting a target. With a single target, adaptation is rapid (time constant = 6 saccades); if training is distributed over six different targets, adaptation is considerably slower (time constant = 57 saccades). Gain changes that result from training with a given target do not transfer strongly to other targets in the same horizontal direction and may not transfer at all to targets in the opposite direction. The gain of saccades to one target may be decreased, and simultaneously the gain of saccades to another target at a different distance in the same direction is increased. These results suggest that each element of a sensory-motor structure underlying saccadic plasticity is associated with a particular retinal or spatial sensory locus and can alter its motor response without much affecting the response of neighboring elements. This is consistent with the finding that distributed training slows adaptation.