Abstract
Over fifty years ago, MacKay reported on effects of viewing a pattern of finely-spaced black and white radial sectors. While he was primarily concerned with the aftereffects resulting from viewing these patterns, he also noted a variety of effects that occurred during direct viewing. Similar effects have been noted with regard to other images containing finely spaced lines, in particular the op-art painting “Fall” by Bridget Riley. An explanation has been recently proposed which invokes small involuntary eye movements and low-level motion detectors (Zanker, Hermens & Walker, 2010). An alternative explanation, which does not involve motion detectors, has been proposed by von Tonder (2010).
The “phantom spokes illusion” (Mulligan, 2012) is an effect which produces the same shimmering sensation via real motion of a regular array of light or dark spots. Bands of light and dark (the “spokes”) are seen in regions where the motion of the pattern causes the trajectories of nearby spots to coincide. The effects can be qualitatively modeled by a temporal filter followed by a nonlinearity, similar to the proposal from von Tonder (2010). A compressive nonlinearity produces a dimming, while an expansive nonlinearity produces a lightening; simple temporal integration produces a single band aligned with the grid, while a biphasic temporal response (such as the psychophysically-measured temporal impulse response) produces a pair of flanking bands. Nulling of the effect in an electronic display made to rotate by optical means can provide a sensitive measure of visual nonlinearities and temporal processing.