Trade-off between spatiotopy and saccadic plasticity.

Saccadic eye movements bring objects of interest onto the high-resolution fovea. They also change the retinal location of objects, but our impression of the visual world is stable: We represent our visual world in spatiotopic coordinates. Visual stability could be the result of a null hypothesis that things do not move during a saccade, or of realigning retinal images based on an internal copy of the eye movement (remapping). The current studies disentangled these hypotheses. Subjects saccaded to peripheral targets that were displaced by different amounts during execution, and detected or discriminated displacement direction. Evidence for a null hypothesis was provided by the relatively poor perceptual performance, and evidence for remapping by the independence of performance from saccade endpoint. There was a trade-off between spatiotopic performance and saccadic plasticity: Good performance (perception of displacements) led to small compensative modifications in saccade amplitude on the next trial while poor performance led to larger modifications. Results also showed that variations in saccade amplitude also depended on the size of the retinal error and of the saccade itself. These results suggest that when faced with a discrepancy between the saccade endpoint and visual target, the visual system attributes the discrepancy to object displacement or to saccade error, in which case the subsequent saccade is corrected. This result reconciles the two hypotheses by suggesting that accurate remapping serves oculomotor accuracy but not visual stability. Internal copies of eye movements may thus be used separately to establish spatiotopic representations and to maintain oculomotor accuracy.