Adaptation to disparity produced by vertical magnification causes a slant bias at the perceptual level and biased azimuth signals from eye position.

Abstract

Vertical magnification of one ocular image introduces a cue for azimuth that produces horizontal stereo-slant toward the magnified eye (induced effect) and an aftereffect with slant in the opposite direction. We investigated biases of retinal and extra-retinal signals for azimuth and perceptual biases caused by adaptation to the induced effect. In two experiments, subjects adapted to vertical magnification of one ocular image of a tall dichoptic random dot stereogram. Adaptation was quantified by nulling the aftereffect with horizontal magnification of one ocular image. The first experiment utilized a property of slant from horizontal binocular disparity; i.e. for a constant horizontal disparity pattern, slant scales with distance. The aftereffect was tested with a tall stimulus to reveal biases in vertical disparity signals and perceived slant. If adaptation introduces a vertical disparity magnification bias, then we predict that the after-effect expressed in units of horizontal disparity will be independent of viewing distance. If adaptation introduces a perceptual bias, then we predict that the after-effect expressed in units of slant will be independent of viewing distance. We found that the after-effect expressed in units of slant was independent of distance. Thus adaptation produced a perceptual bias but not a bias of vertical disparity signals. The second experiment evaluated eye position and perceptual bias aftereffects. We tested aftereffects with a short height stimulus to increase the weight of eye position signals over the weight of vertical disparity signals for azimuth. Eye position aftereffects and perceptual aftereffects are predicted to produce slant biases in opposite directions. We found the direction of the after-effect with short test stimuli had the reverse sign of the direction with the tall test stimuli. Thus, there is also a large eye position aftereffect that is in the same direction as the azimuth cue from the induced effect.