If an area of the retina is stimulated by any means—externally by light or internally by mechanical pressure or electrical processes—the resulting sensation is always one of light, and the light is subjectively localized as coming from a specific visual direction in space. The imaginary line connecting the fixation point and the fovea is termed the visual axis, and normally, with central fixation, it is subjectively localized straight ahead.
If the stimulated areas of the retinas in the 2 eyes, or retinal locations, share a common subjective visual direction—that is, if stimulation of these 2 points results in the subjective sensation that the stimulating target or targets lie in the same direction—these retinal locations are said to be corresponding. When the image of an object in space falls on corresponding points, it is perceived as a single object. On the other hand, stimulation of noncorresponding or disparate retinal points results in the sensation of 2 visual directions for the same target, or diplopia.
With normal retinal correspondence, the foveae of the 2 eyes are corresponding points. Retinal areas in each eye that are essentially equidistant to the right or left and above or below the fovea are also corresponding points. The locus of points in space that stimulate corresponding points in each retina is known as the horopter. With symmetric convergence, the geometric relationship between corresponding points—for example, a point 1° nasal to the fovea in 1 eye would correspond to a point 1° temporal to the fovea in the other eye—gives a circle that passes through the nodal point of each eye and the point of fixation. This theoretical horopter is known as the Vieth-Müller circle. When the horopter is determined experimentally, the locus of points that are seen singly falls not on a circle but on a curve called the empirical horopter (Fig 5-1).
The horopter exists in both the horizontal and vertical planes. Although it might seem that the horopter would be a surface in space, the horizontal separation of the eyes causes points in the oblique quadrants to be vertically disparate. For symmetric convergence, the 3-dimensional horopter of points having both horizontal and vertical correspondence consists of a curved horizontal line and a sloped vertical line that intersect at the fixation point. Each fixation point has a unique horopter centered on that point.
Figure 5-1 Empirical horopter. F = fixation point; FL and FR = left and right foveae, respectively. Point 1, falling within Panum’s area, is seen singly and stereoscopically. Point 2 falls outside Panum’s area and is therefore seen doubly.
If the horopter includes all points in space that stimulate corresponding retinal points, double vision would be expected when the target does not lie on the horopter. However, the visual system can combine slightly disparate points within a limited area surrounding the horopter, called Panum’s area of single binocular vision (see Fig 5-1). Objects within Panum’s area do not result in diplopia. Objects outside Panum’s area stimulate widely disparate retinal points, resulting in physiologic diplopia. If an object is distal to Panum’s area, uncrossed physiologic diplopia will result; if an object is proximal to Panum’s area, crossed physiologic diplopia will result.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.