The Double-Drift Illusion Affects Both the Perception of Where the Target IS and the Memory of Where it WAS
Gilbert, Sydney Jane
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Gilbert, Sydney Jane
To successfully hit a curveball, how does your conscious perception of the curving ball effect where your bat actually swings? From evading car accidents to using basic hand-eye coordination, we often rely on our perceptions of the world to help guide our actions. Successfully perceiving and interacting with a moving object requires the brain to encode how the object’s edges (global motion) and the object’s internal texture (local motion) are moving through space. In order to quickly process moving objects, the brain typically assumes that these motions are in agreement. However, this assumption is a simple shortcut that does not always reflect the true physical world, often leading to a visual illusion. Previous research has shown that the perceived trajectory of an object with contrasting global and local motion is a combination of the two motion directions. The purpose of this thesis was therefore to investigate the relative influence of the local and global motions over time and how the memory of the stimulus’ previous locations are affected by the perceived trajectory. We assessed the change in the observer’s memory of the trajectory’s starting location by asking the observer to compare the onset location with a probe that could be presented before or after motion onset (-250, 0, 250, 500 or 1000ms). Participants maintained fixation in the center of the screen while an object containing leftward, rightward, or no internal motion traveled upward for 500ms in the periphery. The global motion of the stimulus was adjusted for each observer so that the perceived double-drift trajectory appeared purely vertical. For probes presented 250ms before motion onset, the local motion induced a small but significant distortion of the perceived starting location. This bias grew significantly with later probe presentations, reaching a plateau for delays of 250ms or longer. Given that a delay period enhances the effect of the illusion, these results suggest that at least a portion of the distortion in the perceived trajectory of a double-drift stimulus is caused by a bias in the memory of its earlier locations, which are pushed in a direction opposite the local motion.