Visual illusions based on single-field contrast asynchronies.

A single-field contrast asynchrony refers to a stimulus configuration in which there is a single temporally modulated field and multiple sources of contrast information; the sources of contrast information modulate at different temporal phases or at different temporal frequencies. In this paper we show how single-field contrast asynchronies can lead to a wide variety of visual illusions. We investigate, in depth, the window shade/rocking disk configuration, in which a temporally modulated disk is surrounded by a split annulus (i.e., the top half is dark, and the bottom half is light). When the annulus is thick, the disk appears spatially inhomogeneous (shading); when the annulus is thin, the disk appears to rock back and forth (shifting). We measure the proportion of trials that a disk appears to shade or, on separate trials, appears to shift as a function of modulation amplitude, surround thickness, temporal frequency, and disk size. We account for the shading effects by postulating a combination of separate first- and second-order responses and/or a multi-scale spatial filtering process. We account for the shifting effects by examining four elemental motion conditions. For luminance modulation, the direction of the shift follows the same pattern as that produced by the rectified output of an array of spatial center-surround filters applied to the X, t plot. For equiluminant modulation, the direction of the shifts is consistent with a sequence-tracking (or third-order) motion response.