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L. K. Cormack, S. B. Stevenson; Exploring motion aliasing under steady illumination. Journal of Vision 2001;1(3):248. doi: 10.1167/1.3.248.
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WHY. Surprisingly, motion aliasing occurs under steady light (Purves et al., 1996), but the causes and characteristics are not well understood. Properties of the illusion include a reversal of the perceived motion and supernumerosity (a perceived increase in, e.g., the number of spokes in a rotating wheel). We wished to explore this wagonwheel-in-steady-light (WISL) illusion in the hopes of grounding it in the spatiotemporal properties of the visual system. HOW. Rotating periodic stimuli were viewed under indirect sunlight. The temporal frequencies that produced perceptual transitions (WISL stops) were determined by ascending and descending method of limits (numerosity change and motion reversals were measured in separate runs). The relationship between perceived numerosity and frequency (a WISL stop tour) was determined using a matching paradigm with the method of constant stimuli. WHAT. The percepts of reversed motion and supernumerosity both occurred at a stimulation frequency of ∼5 Hz. (“stimulation frequency” refers to the dominant temporal frequency of the stimulus measured at any one point). The matching paradigm revealed a reliable increase in perceived numerosity as a function of stimulation frequency, indicating that this kind of judgment might prove a robust way to examine temporal aliasing in the visual system. SO? The WISL phenomenon has received little attention in the literature despite its implications for motion processing. For example, it is common to compute motion responses by convolving a spatiotemporal receptive field with a space-time representation of the stimulus. While useful in many respects, this analysis fails to predict motion reversals observed in periodic rotating stimuli under steady illumination. Others have proposed discrete temporal sampling as an explanation. However, continuous temporal sampling combined with discrete spatial sampling, which may be more plausible, can also produce motion reversals consistent with our results.
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