The spoke brightness illusion originates at an early motion processing stage

When a bright white disk revolves around a fixation point on a gray backgro und, observers perceive a "spoke": a dark gray region that connects the dis k with the fixation point. Our first experiment suggests that motion across the retina is both necessary and sufficient for spokes: The illusion occur s when a disk moves across the retina even though it is perceived to be sta tionary, but the illusion does not occur when the disk appears to move whil e remaining stationary on the retina. A second experiment shows that the st rength of the illusion decreases with decreasing luminance contrast until s ubjective equiluminance, where little or no spoke is perceived. These resul ts suggest that spokes originate at an early predominantly luminance-based stage of motion processing, before the visual system discounts retinal moti on caused by smooth pursuit.