Visual directions

Stereopsis is stimulated by horizontal disparity that results from the slightly different perspective views of the two eyes. Each eye views slightly more of the temporal than the nasal visual field and also sees more of the ipsilateral than the contralateral side of a binocularly viewed object. When viewed with one eye at a time, the retinal images of the three-dimensional (3D) scene have slightly different locations relative to the fovea of each eye, and they are seen in slightly different directions relative to the line of sight (oculocentric directions). However, each eye perceives the targets in a common direction relative to the head (egocentric direction). The perceived egocentric direction of an object is equal to the average of its oculocentric directions in the two eyes combined with the average of right and left eye positions (conjugate eye position). Movements of the eyes in opposite directions (convergence) have no influence on perceived egocentric direction. Thus, when the two eyes fixate on near objects that lie to the left or right of the midline (straight ahead) in asymmetrical convergence, only the conjugate component of the two eyes' positions (produced by movements of the two eyes in the same direction) contributes to perceived direction. Ewald Hering summarized these facets of egocentric direction as five rules of visual direction, and Howard (1982) has restated them. The laws are mainly concerned with targets imaged on corresponding retinal regions (i.e., stimulation of corresponding retinal points in the two eyes results in percepts in identical visual directions).

How are oculocentric visual directions of the two eyes combined in a common visual space, sometimes referred to as the cyclopean eye? When the target lies near the plane of fixation, its egocentric direction is based upon the similar retinal image locations of the two eyes. When the target is slightly nearer to or farther from the plane of fixation, its monocular images are formed at different horizontal eccentricities from the foveas; however, its averaged direction is seen as single (allelotropia). The range of disparities that produce singleness is referred to as Panum's fusional area (Schor and Tyler, 1981). The consequence of averaging monocular visual directions of disparate targets is that binocular visual directions are perceived correctly from a reference point midway between the eyes (cyclopean eye). Binocular visual directions can be judged accurately only for targets with averaged visual directions. When targets are positioned far in front of or behind the plane of fixation, their retinal image disparity becomes large and the disparate targets appear diplopic (i.e., they are perceived in two separate directions). The directions of monocular components of the diplopic pair are perceived inaccurately as though each of the images had an invisible paired image formed on a corresponding retinal point in the other eye. There are ambiguous circumstances in which a target in the peripheral region of a binocular field is seen by only one eye because of partial occlusion of the other eye by the nose. The monocular target could lie at a range of viewing distances. However, its direction is judged as though it was located in the plane of fixation such that if it were seen binocularly, its images would be formed on corresponding retinal points.

Several violations of Hering's rules for visual direction have been observed in both abnormal and normal binocular vision. A common binocular abnormality is strabismus (an eye turn or deviation of one eye from the intended binocular fixation point). In violation of Hering's rules, people who have a constant turn of one eye (unilateral strabismus) can have constant diplopia, and they use the position of their preferred fixation eye to judge visual direction, regardless of whether they fixate a target with their preferred or deviating eye. People who have an alternating strabismus (either eye is used for fixation, but only one at a time) use the position of the fixating eye to judge the direction of objects (Mann et al., 1979). People with both types of strabismus use the position of only one eye to judge direction, whereas people with normal binocular eye alignment use the average position of the two eyes to judge direction with either eye alone.

Two exceptions in normal binocular vision involve monocular images, and a third exception involves judgment of direction of binocular-disparate targets. Hering's rules predict that if a target is fixated with one eye while the other is covered, the target will appear to move in the temporalward direction (toward the ear nearest the open eye) if the eyes accommodate (focus on a near target), even if the monocular position of the open eye remains stationary. The perceived temporalward movement results from the nasalward movement of the covered eye caused by the cross-coupling between accommodation and convergence (Müller, 1843). When the open eye focuses on a near target, the covered eye automatically converges (i.e., turns nasalward) (cross-coupling). Hering's rules predict that the average horizontal position of the two eyes determines the egocentric direction of the foveated target. The first exception occurs when the apparently temporalward motion is greater during accommodation on a near target when one eye is occluded compared to when the other eye is occluded. This asymmetry between the eyes resembles the biased perception of egocentric direction in constant-unilateral strabismus, and it may be related to an extreme form of eye dominance.

The second exception to Hering's rules occurs when the retinal image of a target that is seen monocularly is imaged near a disparate target seen by both eyes (binocular view). Monocular views occur naturally in the peripheral visual field when one eye's view is occluded by the nose. The direction of the monocular target is judged as though it was positioned at the same depth as any nearby disparate binocular target rather than at the plane of fixation. The visual system assumes that there is an occluded counterpart of the monocular target in the contralateral eye that has the same disparity as the nearby binocular target, even though the image is seen only by one eye (Erkelens and van Ee, 1997).

The third exception to Hering's rules is demonstrated by the biased visual direction of a fused disparate target when its monocular image components have unequal contrast (Banks et al., 1997). Greater weight is given to the retinal locus of the image that has the higher contrast. The average location of the two disparate retinal sites is biased toward the monocular direction of the higher-contrast image. These are minor violations that occur mainly for targets lying nearer to or farther from the plane of fixation or distance of convergence. Since visual directions of off-horopter targets are mislocalized, even when Hering's rules of visual direction are obeyed, the violations have only minor consequences.