Illusions tell us something about the inner working of the brain. There are unwanted consequences of the design of the visual system, which has to serve many different purposes at the same time: from recognizing objects to helping to stabilize the body while moving around.
The freezing rotation illusion arises when a figure is continuously rotating in front of a back and forth rotating ground. The term “freezing rotation” designates the decrease in the perceived rotation speed of a figure when the figure and the ground are turning in equal directions (Dürsteler, 2005). The illusion is related to the motion freeze illusion (Mesland and Wertheim, 1996) and Duncker's (1929) illusion. The mechanisms responsible for the freezing motion illusion may help to partition a complex scene. Smaller motion differences between an object and a normally stable surround or a small object moving slowly on a large object should by ignored to avoid detection of spurious motion and favor the recognition of figure parts with common motion.
The term “stereo rotation standstill” refers to a new illusion: a rapidly rotating spoked wheel defined by disparity cues only is perceived as a stationary three-dimensional structure with some local wiggling of the spokes. Stereo rotation standstill is seen best when looking at the center of rotation. When looking at a single spoke, one starts tracking it and can therefore infer the wheel’s rotation. A stereo pseudo-rotation is seen when the dot pattern containing the disparity cues is rotating, but the wheel is stopped.
Recently I found a similiar standstill illusion in the color domain. A wheel made out of 16 equally seized sectors painted alternately in isoluminant red and cyan is turning behind a stationary transparent pattern with black random dots. When looking at the center of rotation, the wheel is perceived as stopped ( 'color rotation standstill'), when looking at a single sector, it appears as moving rapidly; by tracking it one could infer the wheel's rotation. A color pseudo-rotation is seen when the transparent random dot pattern overlaying the colored wheel is rotating, but the wheel is stopped.
Color and stereo standstill illusions appear not only in rotation but also in other types of complex motion such as expansion and contraction ('color expansion/contraction standstill' and 'stereo expansion/contraction standstill').
Rotation standstill is different from motion standstill, where a rapidly shifting pattern is perceived as motionless (for stereo motion standstill Tseng et al. 2006). We hypothesize that there exist motion detectors with an input from disparity/color detectors (cf. Lu et al. 2002), which are not inhibited by a stationary luminance surface texture, whereas complex motion (rotation, contraction, expansion, spiraling motion) detectors with disparity or color input (the human equivalent of the high order rotation detectors found by Tanaka et al 1989 and Duffy et al. 1991 in monkey’s MSTd) are inhibited by a stationary luminance texture. Alternatively, assuming that complex motion detection for stimuli defined by disparity or color differences only depends on a previous figure-ground separation, the stationary random dot pattern may obfuscate line terminators and therefore interfere with figure extraction; the complex motion perceived is the complex motion of the random dot pattern.
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