Recent understanding of binocular vision in the natural environment with clinical implications

Abstract

Technological advances in recent decades have allowed us to measure both the information available to the visual system in the natural environment and the rich array of behaviors that the visual system supports. This review highlights the tasks undertaken by the binocular visual system in particular and how, for much of human activity, these tasks differ from those considered when an observer fixates a static target on the midline. The everyday motor and perceptual challenges involved in generating a stable, useful binocular percept of the environment are discussed, together with how these challenges are but minimally addressed by much of current clinical interpretation of binocular function. The implications for new technology, such as virtual reality, are also highlighted in terms of clinical and basic research application.

Keywords: Binocular vision; Depth perception; Eye movements; Natural image statistics; Stereopsis; Strabismus.

Figure 1:. A classical illustration of interocular retinal disparity.

Panel A presents a series of colored objects. The eyes are fixating the dark blue one on the mid-saggital plane and the theoretical horopter for that fixation distance is illustrated by the dashed grey circle. The images of this object sit on the fovea in each eye, with the images of the other objects falling at eccentric locations. The grey object provides a simple example of uncrossed disparity from the horopter. If the eyes were to now align at that object, its absolute uncrossed disparity would reduce to zero, while its relative disparity to the blue object would remain constant irrespective of the fixation and alignment distance of the eyes. The green and red objects illustrate the more typical natural situation of objects located away from the mid-saggital plane, in uncrossed or crossed disparity, respectively, from the horopter. Panel B illustrates that changing fixation from the green object to the red object involves an eye movement with both a lateral and depth component, and that the required angular rotations of the eyes are not equal and symmetric (purple arrows).

Figure 2:. An illustration of the presence of both horizontal and vertical retinal disparities

when an object is located off the mid-saggital plane between the eyes (e.g. at the location of the red object in Figure 1). The left eye’s image is colored in the combination image for clarity, and it is assumed that the eyes are fixating the middle knuckle of the robot’s forefinger.

Figure 3:. An illustration of the impact of vision loss in the central visual field

(e.g. resulting from age-related macular degeneration, AMD). If the patient is attempting to view the shirt on the table, as seen in the left panel, their vision loss results in a scotoma at that point of fixation as shown in the right panel. Interestingly, under- or over-converging your eyes to align and fuse the images in these two panels provides a demonstration of the challenges of combining non-corresponding information with unilateral retinal pathology.

Figure 4:. An illustration of the binocular integration of images

from eyes that are aligned (row A) or misaligned (row B). The images in the left column illustrate the scene with simulated visual fields for the left (red) and right (blue) eyes. The aligned case illustrates the typical central binocular overlapping field with monocular regions in the temporal extremes. The misaligned simulated exotropia in row B illustrates the reduced binocular overlap and extended monocular regions present with the divergent deviation of the right eye. The middle column provides the relevant visual fields for each eye and a reminder that binocular integration is not merely a question of overlaying these fields/retinal images. The right column follows the basic principle of mapping in primary visual cortex, where the binocular visual field is represented with aligned information and appropriate transition into the monocular crescents for aligned eyes and conflicting information in the misaligned case. The cortical representation for the patient with exotropia might imply diplopia and confusion in the nominally binocular cortex, as shown in the top example in row B, or, if the image from the right eye is suppressed in binocular cortex, shown below, any percept from the right eye’s monocular crescent would result in an apparent missing section of the visual scene (as illustrated by the grey region in the left column scene). These potential cortical ‘images’ illustrate some of the challenges faced by the brains of these strabismic patients in compiling a stable unified percept of the world.

Figure 5:. An illustration of the concept of interocular velocity difference.

Panel a represents the two eyes and an object moving along the red arrow in front of them. Panels b & c demonstrate the different retinal information generated by this trajectory in the two eyes, by viewing from behind the right (panel b) and left (panel c) eyes.

Figure 6:. An illustration of the different percepts with distance from the horopter.

The top pencil is moved forward or backward from vertical alignment with the bottom pencil, while binocular fixation remains on the point of the bottom pencil. Panel A: An illustration of the overlaid retinal images in each region. Panel B: The orange region represents the horopter; the green region is classical stereopsis, where the top pencil generates retinal disparity but the percept is fused and single with a depth difference from the bottom pencil; in the blue region the percept is double but still includes a depth difference from the bottom pencil; in the purple region the percept is double with no sense of depth. The approximate angular extents of these regions are provided, but their exact values will depend on the characteristics of the stimuli.

Figure 7:. An illustration of a typical clinical fixation disparity test.

The images presented to the left and right eyes are shown in panel A. When the eyes are aligned accurately, the combined percept would be as seen in panel B. More commonly observers note a perceived offset of the monocularly presented lines, as shown in panels C & D, even though the overlaid retinal images for the corresponding small misalignments of the eyes would be as shown in panels E & F.