First-trial adaptation to prism exposure: artifact of visual capture.

Terminal target-pointing error on the 1st trial of exposure to optical displacement is usually less than is expected from the optical displacement magnitude. The authors confirmed 1st-trial adaptation in the task of pointing toward optically displaced targets while visual feedback was delayed until movement completion. Measurement of head-shoulder posture while participants (N = 24) viewed the optically displaced field revealed that their shoulders felt turned in the direction opposite to the displacement (visual capture), accounting for all but about 4% to 10% of 1st-trial adaptation. First-trial adaptation was unrelated to realignment aftereffects. First-trial adaptation is largely an artifact of the asymmetry of the structured visual field produced by optical displacement, which induces a felt body rotation, thereby reducing the effective optical displacement.