Displacement of Apparent Straight Ahead as an Aftereffect of Deviation of the Eyes from Normal Position

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Abstract
Helmholtz's proprioceptive theory of apparent visual direction predicts a displacement of egocentric straight ahead as an aftereffect of deviation of the eyes from normal frontal position. In a test of this prediction, 91 Ss (a) selected from a line of lighted discs that disc which appeared to be straight ahead, (b) fixated the eyes for 30 sec. on a point in the line of discs which was either 30° from frontal position or at the most extreme position attainable (approximately 45° from frontal position), (c) returned the eyes to what seemed to be frontal position and selected the disc which appeared to be straight ahead. Ocular deviation produced as an aftereffect a displacement of apparent straight ahead which had a mean value of 3.12° and occurred in the same meridian and in the same direction as the eyes had been deviated. The amount of displacement was not significantly affected by the degree of prior ocular deviation or by the orientation of the line of discs (vertical, horizontal, or diagonal).