Spatial distortions in rotating radial figures

A white sector on a black rotating disk appears spatially compressed. We found 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 16 degrees and back to 11 degrees (corresponding to 20, 16, and 7.5%, respectively); (2) increased with speed of rotation producing maximal compressions 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. Consistent with these findings we found that (4) two black lines 20 mm apart across the center of the rotating disk and extending outward towards the edge appeared to converge when they were actually parallel and were seen as parallel when their end points were physically diverged by 6 degrees. Our findings suggest a foreshortening process which ensures that the shapes of moving stimuli are perceived approximately correctly, irrespective of whether they are actually sharp or blurred.