Changes in straight-ahead eye position during adaptation to wedge prisms

If S is instructed to look straight ahead before adapting to laterally displaced vision, he does so without noticeable error. After adapting, however, in response to the same instruction, he may rotate his eyes as much as 8° toward the the displaced visual target. This is the change in judgment of the direction of gaze which Helmholtz identified in 1867 as an important physiological mechanism in adaptation to prisms. It leads to more accurate reaching behavior by causing S to make a visual judgment that the target is closer to straight ahead than it was when he first looked through the prisms. This type of adaptive change (change in judgment of the direction of gaze, oculomotor change) can be measured either by manual judgments (difference between successive “straight ahead” and “visual target” judgments) or by changes in straight-ahead eye position. It may be described as a parametric adjustment in the oculomotor control system, and is closely analogous to the eye movement which subserves the recovery of binocular fusion in prism vergence.