Separation Of Form Perception And Stereopsis

Abstract

This study addresses the question of whether static and dynamic stereopsis require the perception of form. The retinal image requirements of the visual mechanisms subserving form perception and stereopsis are not only distinct but potentially antagonistic. Form perception requires the retinal image to have luminance gradients that are steep enough to produce suprathreshold temporal transients in the receptors during normal eye movements. Stereopsis, on the other hand, requires identification of corresponding luminance gradients in the two retinal images so that their retinal disparity can be calculated. Thus, while the motion of the retinal image caused by normal eye movements is essential to form perception, it may be detrimental to stereopsis. We eliminated the motion of the retinal image that would normally have occurred during eye movements by using a pair of SRI dual-Purkinje-image eyetrackers and stimulus deflectors to stabilize the retinal images of selected form elements. We examined the thresholds for perceiving motion in depth under stabilized and unstabilized conditions and found that the perception of motion in depth continues in the absence of monocular form perception. Likewise, when we stabilized the disparate images of a line stereogram, stereopsis persisted in the absence of form perception of those elements whose retinal disparity deter-mined their perceived depth. These results imply profound separation of the form-perception and stereopsis mechanisms.