Perceived speed differences explain apparent compression in slit viewing

When a figure moves behind a stationary narrow slit, observers often report seeing the figure as an integrated whole, a phenomenon known as slit viewing or anorthoscopic perception. Interestingly, in slit viewing, the figure is perceived compressed along the axis of motion, e.g., a circle is perceived as an ellipse. Underestimation of the speed of the moving object was offered as an explanation for this apparent compression. We measured perceived speed and compression in anorthoscopic perception and found results that are inconsistent with this hypothesis. We found evidence for an alternative hypothesis according to which apparent compression results from perceived speed differences between different parts of the figure, viz., the trailing parts are perceived to move faster than the leading parts. These differences in the perceived speeds of the trailing and the leading edges may be due to differences in the visibilities of the leading and trailing parts. We discuss our findings within a non-retinotopic framework of form analysis for moving objects.