Beware of the Straight-Ahead Shift—A Nonperceptual Change in Experiments on Adaptation to Displaced Vision

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Abstract
Seen through sideways-displacing prisms, the wall of a room or testing apparatus looks displaced and slanted in depth. Consequently the viewer may involuntarily treat as straight ahead a a spatial direction that is displaced from his median plane toward the optically displaced, environmentally defined axes of space. There is evidence that such cognitive shifts in the subjective straight ahead do occur and are often larger than the adaptive shifts commonly found in experiments with prisms. A straight-ahead shift would affect performance on any task that involves judgments of the straight ahead, such as judging the visual straight ahead or pointing straight ahead with eyes closed. Thus a straight-ahead shift may give the misleading impression that there is a change in visual perception, or a ‘maladaptive’ change in position sense, when there actually is none (that is, no change in performance of any other visual or proprioceptive task). A straight-ahead shift could also produce a wide variety of other unexpected, paradoxical, or misleading findings like those that have been noted in previous experiments.