Apparent motion distorts the shape of a stimulus briefly presented along the motion path.

We examined whether motion blur accompanying apparent motion (AM) affects the shape of a stimulus presented in the motion path. In a two-alternative forced-choice procedure, observers judged the shape of a Gaussian test stimulus flashed in the path of motion, relative to a reference stimulus, which was a circular Gaussian stimulus located away from the path of motion. In Experiment 1, we report that the test stimulus was affected by AM and its perceived width was wider than its actual width, and counteracting this distortion, shape discrimination thresholds coincided with a test stimulus that was physically "thinner" than the reference stimulus. Shape distortion correlated with the strength of AM (Experiment 2) and increased within the range of inter-stimulus intervals used to induce AM and with retinal eccentricity but was eliminated when the test stimulus was made "hard-edged" (Experiment 3) or when the stimulus does not overlap with the motion path (Experiment 4). In Experiment 5, we demonstrate that the effect is present for dichoptic presentations. These results can be accounted for by a process in which the neural representation of AM generated by higher cortical areas feedback to interfere with the coding of stimulus shape by units located along the trajectory of AM.