Stereopsis and binocular rivalry.

Abstract

Stereopsis and binocular rivalry appear to be incompatible. Rivalry involves suppression of input from one eye at each location in space and time, whereas Stereopsis requires the combination of information from both eyes to yield a sensation of depth. This article presents a theory of the relationship of these phenomena. Evidence from previous work and from experiments reported here shows that (a) the pathways mediating Stereopsis are separable from those mediating rivalry; (b) Stereopsis and rivalry can coexist at the same point in space and time; (c) rivalry occurs inevitably, whenever visual stimuli are present, even if identical stimuli are presented to both eyes; and (d) Stereopsis can be disrupted without disrupting rivalry and vice versa (termed double dissociation). On the basis of the evidence, it is proposed that rivalry and Stereopsis involve independent and parallel pathways through the early stages of visual processing and that binocular perception is a simple combination (probably a weighted average) of the output of the pathways mediating Stereopsis and rivalry. Stereopsis and binocular rivalry, two of the central phenomena of binocular vision, are seemingly incompatible. In Stereopsis, matched or nearly matched retinal images are combined to form a image (Helmholtz, 1909/1924; Julesz, 1971). The cyclopean percept is identical to neither monocular image but requires contributions from both. Binocular rivalry is most easily demonstrated when each eye is presented with a different image. At each location in the visual field, one monocular image will be dominant and visible while the other is suppressed and unseen (Breese, 1899). Over time, the pattern of dominance and suppression will change. At any given point in space and time, the stimulus presented to one eye is seen while that presented to the other eye is lost to sight. The suppression of one monocular input would eliminate the basis for Stereopsis. The fusion of the two inputs into a cyclopean image would eliminate the basis for rivalry. How, then, do the two phenomena coexist in human vision? One class of theoretical solution is to deny the existence of one of these two. Verhoeff(1935, 1959), for example, held that binocular vision could be explained without allowing for the fusion of the two monocular images into a unique binocular image. At the present time, it is more common to argue that rivalry and Stereopsis cannot coexist at the same point in space and time and that Stereopsis takes precedence; that is, rivalry occurs when Stereopsis cannot occur (e.g., Julesz, 1971, p. 23; Julesz & Tyler, 1976).