An orientation anisotropy in the effects of scaling vertical disparities

Garding et al. (Vis Res 1995;35:703-722) proposed a two-stage theory of ste reopsis. The first uses horizontal disparities for relief computations afte r they have been subjected to a process called disparity correction that ut ilises vertical disparities. The second stage, termed disparity normalisati on, is concerned with computing metric representations from the output of s tage one. It uses vertical disparities to a much lesser extent, if at all, for small field stimuli. We report two psychophysical experiments that test ed whether human vision implements this two-stage theory. They tested the p rediction that scaling vertical disparities to simulate different viewing d istances to the fixation point should affect the perceived amplitudes of ve rtically but not horizontally oriented ridges. The first used elliptical ha lf-cylinders and the 'apparently circular cylinder' judgement task of Johns ton (Vis Res 1991;31:1351-1360). The second experiment used parabolic ridge s and the amplitude judgement task of Buckley and Frisby (Vis Res 1993;33:9 19-934). Both studies broadly confirmed the anisotropy prediction by findin g that large scalings of vertical disparities simulating near distances had a strong effect on the perceived amplitudes of the vertically oriented sti muli but little effect on the horizontal ones. When distances > 25 cm were simulated there were no significant differential effects and various method ological reasons are offered for this departure from expectations. (C) 1998 Elsevier Science Ltd. All rights reserved.