The psychophysical inquiry into binocular summation

Abstract

Experiments that compare monocular and binocular visual performance of human psychophysical Os on a variety of visual tasks are reviewed, The review attempts to include all experiments published in English in this century, excluding work on stereopsis, rivalry, and evoked potentials, The concept of probability summation as a baseline for assessing the presence of neural summation is discussed, and the assumptions of several models for estimating probability summation are considered. Experiments are classified in terms of visual task, major categories being increment detection, flicker fusion, brightness magnitude, and contour resolution. A major conclusion is that binocular performance is superior for essentially all task categories and in most cases by a magnitude greater than that predicted by appropriate probability summation models.The cardinal feature of the binocular visual system in man is the unification of the two separate monocular views into a binocular cyclopean view that betrays little trace of its monocular origins. Relatively little is known about the mechanism responsible for unification, although an examination of the physiology of the visual system reveals a number of features that seem expressly designed to promote unification. Some of the more prominent features are the partial crossing of the optic tracts at the optic chiasma and the elaborate system for the maintenance of conjugate eye movements. The existence of these features in man, and in binocular vertebrates in general, suggests an adaptive function of binocularity of sufficient magnitude to justify the survival of this complicated physiological organization. Yet the immediate advantage of having two eyes functioning as one is not obvious. The simple experiment of closing one eye does not result in any dramatic change in the appearance of the visual world. Rather, only by more stringent experimentation are the subtle advantages of binocularly joined eyes revealed.The purpose of this paper is to review those experiments undertaken in the 20th century that have compared monocular and binocular performance on a variety of visual tasks. The rubric under which such experiments are typically grouped is "binocular summation." These experiments compare the relative performance on some visual task (such as detection or form resolution) by a human S using one and both eyes. In the literature, this kind of inquiry into binocular vision is treated separately from investigations of stereopsis and of binocular rivalry; following that distinction, the voluminous research on these binocular phenomena is not included. A further exclusion is data

Stereopsis and binocular rivalry.

“…There has long been considerable interest in the experimental situation wherein a patch of one luminance is presented to one eye while another luminance is presented to the other (see reviews by Blake & Fox, 1973;and Levelt, 1965). The resulting apparent brightness is almost never a simple average of both retinal inputs.…”

“…These include detection of stimuli and brightness summation (reviewed in Blake & Fox, 1973;Blake, Sloane, & Fox, 1981) and several perceptual and visual motor tasks (Jones & Lee, 1981). These phenomena probably reflect the contribution of the Stereopsis pathway, as all of them require a fairly close match between the monocular inputs.…”

The precedence of binocular fusion over binocular rivalry

“…This process of combination is often referred to as "binocular fusion. " Working together in this fashion, the two eyes outperform either eye alone, providing an observer with enhanced visual sensitivity (Blake & Fox, 1973), improved visuomotor coordination (Jones & Lee, 1981), and keenly accurate depth information (Ogle, 1964). When, however, the two eyes receive incompatible information, stable single vision gives way to fluctuations in dominance between the two eyes (Breese, 1899).…”

On utrocular discrimination

“…Performance on this task was measured at I cycle/deg, a value at which both observers tested excel on utrocular discrimination, and at 8 cycles/deg, where their Binocular summation. On a variety of visual tasks involving threshold measures, binocular performance exceeds monocular performance by an amount greater than that expected on the basis of probability summation (see Blake & Fox, 1973, for a review of this literature). This superiority of two eyes over one, known as binocular summation, is generally believed to reflect the involvement of neural summation between monocular inputs.…”