Gaze-Contingent Adaptation to Prismatic Spectacles

Abstract

Ivo Kohler reported remarkable after-effects resulting from the prolonged wearing of prismatic spectacles; when the spectacles were removed the apparent size and shape of objects varied with direction of view.' How does a constant retinal image come to evoke different impressions of size and shape? The result obviously is connected with the distortion produced by prisms. A prism displaces light as a function of the angle of incidence and the thickness of glass traversed. Thus, a person wearing wedge-prisms mounted in spectacle-frames (with bases oriented in the same direction) notes a variety of deformations, both when the head is moved while fixating a target, and when the eyes are moved with the head stationary. In general, the after-effects reported by Kohler are the same distortions as those produced by prisms, but in the opposite direction. Kohler's observations of these gaze-contingent after-effects were based upon two Ss, himself and Dr. T. Erisman. Pilot studies by the present authors indicate that Ss exposed to prismatic distortion for as short as 20 days experience such after-effects but that their measurement is difficult. The purpose of the present study was to develop appropriate techniques for verifying the existence of no after-effects and for quantifying them.2 Problem. The present report focuses on the measurement of adaptation to two deformations produced by prisms. The first distortion is contingent upon change in vertical position of the eyes in the head. Specifically, horizontal lines above and below eye-level appear to converge towards the apex of the prism. This distortion is alternatively manifest in the apparent 'shearing' or tilting of a fixated horizontal target as the head is nodded up and down. The second distortion is contingent upon change in lateral position of eyes in the head. Specifically, identical targets viewed through the base and apex of the prism have different apparent widths (the base-