Running Head: Early processing yields the spoke brightness illusionEarly processing yields the spoke brightness illusion When a bright white disc revolves around a fixation point on a grey background, observers perceive a “spoke”: a dark grey region which connects the disc with the fixation point. Our first experiment suggests that motion across the retina is both necessary and sufficient for spokes: the illusion occurs when a disc moves across the retina even though it is perceived to be stationary, but the illusion does not occur when the disc appears to move while remaining stationary on the retina. A second experiment shows that the strength of the illusion decreases with decreasing luminance contrast until subjective equiluminance, where little or no spoke is perceived. These results suggest that spokes originate at an early, predominantly luminance-based stage of motion processing, before the visual system discounts retinal motion caused by smooth pursuit. 2 Early processing yields the spoke brightness illusio
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