The control of saccadic adaptation: implications for the scanning of natural visual scenes.
Accurate scanning of natural scenes depends on: (1) attentional selection of the target; (2) spatial pooling over the attended target to compute the precise landing position; and (3) adaptive modification of saccades to ensure saccadic accuracy. The present experiments studied adaptation. Adaptive modifications were induced by displacing the target during saccades. Adaptation was found to be: (1) similar for a small target point and a large target circle, despite the differences in the spatial pattern of landing position errors for each; (2) unaffected by instructions to look part way to the target, even though such instructions altered landing position error relative to the target; and (3) insensitive to symbolic cues disclosing the direction of the intra-saccadic displacement. Briefly delaying the presentation of the post-saccadic target greatly reduced adaptation. Neither corrective saccades, nor the position errors that trigger corrections, were involved in adaptation because corrective saccades rarely occurred with a large target circle even though the circle produced as much adaptation as the single point. Taken together, the results do not support the traditional notion that post-saccadic retinal position error controls adaptation. We propose that adaptation relies on a comparison of the actual post-saccadic retinal image with the post-saccadic image that would be predicted based on a representation of the planned saccade. Such a comparison: (1) is consistent with our results; (2) may be more effective than retinal position error in controlling adaptation in natural visual scenes containing large targets and backgrounds; and (3) is similar to the motion-based adaptive mechanisms associated with the VOR. Similarity between the adaptive control of saccades and adaptive control of the VOR raises the possibility that the most important role of saccadic adaptation may be the coordination of eye and head movements during shifts of gaze.