Spatial distortions in rotating radial figures

A white sector on a black rotating disk appears spatially compressed. We fo und that apparent shrinkage: (1) for sectors ranging from 15 to 150 degrees and rotating at 1.25 rps varied in an inverted U-shaped manner from 3 to 1 6 degrees and back to 11 degrees (corresponding to 20, 16, and 7.5%, respec tively); (2) increased with speed of rotation producing maximal compression s of between 7 and 30 degrees for velocities ranging from 0.8 to 2 rps; and (3) affected the leading and the trailing portions of the rotating sector equally, while allowing for apparent expansion of the middle region. Consis tent with these findings we found that (4) two black lines 20 mm apart acro ss the center of the rotating disk and extending outward towards the edge a ppeared to converge when they were actually parallel and were seen as paral lel when their end points were physically diverged by 6 degrees. Our findin gs suggest a foreshortening process which ensures that the shapes of moving stimuli are perceived approximately correctly, irrespective of whether the y are actually sharp or blurred. (C) 1999 Elsevier Science Ltd. All rights reserved.