Rapid adaptation to a prismatic distortion

Among several shape distortions caused by a wedge prism is one where a pattern of evenly spaced lines that are parallel to the prism base reaches the eye with distances seen through the base region contracted and distances seen through the apex region expanded. Rapid partial adaptation to this distortion was obtained by having subjects who wore the prism in vertical orientation move the head up and down during the exposure period. The head movements caused the distortion to shift up and down through the line pattern. When, with subjects’ heads stationary, the distortion was made to shift up and down by artificial means, rapid adaptation failed to develop. It is proposed that head movements are here needed for adaptation to develop, because the covariance between them and the shifting of the distortion serves as a signal that the distortions are not objective facts and causes the initiation of the adaptation process.