Afterimages, tools, and the body schema
Stone, Brian William
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Positive afterimages can be reliably induced in an observer by exposure to a bright flash. If an observer is sitting in total darkness, then they will perceive an afterimage of whatever they were looking at when the flash occurred, even well after the room has returned to total darkness. If the afterimage is of a part of the observer's body, then subsequently moving that body part causes the afterimage of the moved part to fade, due to conflict between proprioceptive feedback (of movement) and illusory visual feedback (a non-moving afterimage) that are part of a multi-sensory system for tracking body parts. Recent studies have found that this multi-sensory effect also occurs with held objects (like tools); the visual afterimage of a held object disappears when that object is displaced in total darkness. While some argue that this is evidence that held objects are incorporated into the body schema, the series of experiments I present herein demonstrate that this is not the most likely explanation for why visual afterimages of objects fade when moved in total darkness, and I propose to explain the effect through a more general multi-sensory object-tracking mechanism. These experiments generate important new data for basic research into positive afterimages. I demonstrate that auditory feedback of object movement can induce afterimage fading, that the fading effect scales with the magnitude of proprioceptive feedback when other sensory input is held constant, and that tactile feedback alone is sufficient for the effect. Together, these data show that the visual percept of a positive afterimage is constructed not just from visual input of the scene when light reached the eyes, but in conjunction with input from the other senses.