Oculocentric Visual Direction

The visual direction of an object can be represented by a line that joins the object and the fovea called the Principal Visual Direction or visual axis. On the basis of the principal visual direction, the direction of all other objects in the subject's visual field is determined. This is the called oculocentric visual direction. Therefore, each point of the retina can be considered to have it own sense of direction. For example, when we look at an object, the object is imaged on the fovea. Other objects imaged above the fovea are seen as "below", and those imaged below the fovea are seen as "above". Visual sense of direction is organized about the fovea. For a given position of the eye, objects having superimposed retinal images will be seen as being in alignment in the visual field, but at a different distance from the eye (Fig. 1).

Figure 1

Oculocentric visual direction.

Egocentric Visual Direction

Egocentric visual direction refers to the direction of an object in space relative to one self, rather than the eyes. Egocentric direction is determined by retinal position, proprioceptive information about the eye, head and body position, and the vestibular apparatus. All this information allows us to determine whether a change in retinal position is due to object movement or due to eye or head movement. In Fig. 2a, a stationary object is imaged on the fovea with the head and the body stationary. When the eye moves, the stationary object is then imaged on a new retinal position. Therefore, oculocentric direction has changed, but the egocentric direction has not changed as the object has remained stationary. In another example, the eye tracks a moving object (Fig. 2b). As the object is imaged on the fovea at all times, the oculocentric direction is the same, but the egocentric direction is changing.

Figure 2

(a) Different oculocentric direction but the same egocentric direction as the object is stationary. (b) Same oculocentric direction but egocentric direction is changing because the object is moving.

In binocular vision, the idea of corresponding retinal points has been used to describe the principle visual direction. Corresponding retinal points are points stimulated on the retina that give rise to the same visual direction. When objects stimulate non-corresponding points, this gives rise to different visual directions. These retinal points are called disparate points. Therefore, corresponding points have the same principle visual direction, and non-corresponding points have different visual directions (Fig. 3).

Figure 3

Corresponding points of the two eyes.

As we see the world single and not double, binocular vision can be represented by a single eye, the cyclopean eye. The cyclopean eye is an imaginary eye situated midway between the two eyes (Fig. 4). Disparate points give rise to physiological diplopia (double vision). In Fig. 5, it can be seen that point A stimulates disparate points (non-corresponding retinal points).

Figure 4

The cyclopean eye is used to determine the direction of point A and point B. Point A stimulating the temporal retina of right eye and the nasal retina of the left eye, that is, stimulates a retinal point to the right of the fovea.

Figure 5

Point A and point B stimulating disparate points. Point A stimulates the nasal retina of both eyes.

Using the cyclopean eye, crossed and uncrossed diplopia can be explored. For an object closer than the fixation point such as point B in Fig. 6a, crossed diplopia occurs as the point B is imaged on the temporal retina of both eyes. This is termed crossed diplopia because the image in the left eye is seen on the right side. For an object located further than the fixation point, the image of the object falls on the nasal retina of both eyes, producing uncrossed diplopia. This is termed uncrossed diplopia because the image in the left eye is seen on the left side (Fig. 6b).

Figure 6

Demonstrating crossed (a) and uncrossed (b) diplopia using the cyclopean eye.

The principle of the cyclopean eye can be applied to patients with strabismus (a turned eye). Patients with strabismus are usually classified according to the direction of the eye turn. Two common types of strabismus are patients with an esotropia, their eye(s) turned in, and patients with exotropia, their eye(s) turned out. Patients with an exotropia will have crossed diplopia, whereas patients with an esotropia will have uncrossed diplopia (Fig. 7).

Figure 7

(a) Uncrossed diplopia with an esotropia. (b) Crossed diplopia with an exotropia.

The Vieth-Muller Circle

The Vieth-Muller circle is a theoretical horopter. All points on this circle should stimulate corresponding points on the retina and lead to single vision, provided that the fixation point lies on the center of the circle and the eyes rotate about its nodal point (instead of their center of rotation). The Vieth-Muller circle assumes there is angular symmetry of the corresponding points (Fig. 8).

Figure 8

Vieth-Muller circle. The circle represents the theoretical locus of points in space that stimulates corresponding retinal points.

Measuring the Horopter

The horopter can be measured through several methods. These methods include:

• Haplopic method
• Nonius method
• Apparent front-parallel plane (AFPP) method

The Nonius and AFPP methods directly determine the longitudinal horopter, whereas the haplopic method does not. Instead, the haplopic method determines the inner and outer boundaries of single binocular vision, and the horopter is taken as the midline.

Haplopic Method

The haplopic method (method of the region of singular binocular vision) is based on the primary definition of corresponding points; retinal points that correspond give rise to identical visual directions and, as a consequence, single vision. Thus, if diplopia is observed, disparate points are being stimulated. Therefore, the method involves determining the boundaries of single binocular vision (Fig. 9).

Figure 9

Result of the horopter determined by the haplopic method at a viewing distance of 40 cm. From Moses and Hart (1) and Ogle (2).

Nonius Method

Because corresponding points give rise to identical visual directions, the position of an object that stimulates a pair of corresponding points can be located if each eye sees a different part of the object. If the two parts are seen in the same direction, then the objects are in that position where they stimulate corresponding points. This is the basis of the Nonius method (method of equating visual directions) (Fig. 10).

Figure 10

The horopter as determined by the Nonius method. From Moses and Hart (1) and Ogle (2).

The Apparent Fronto-Parallel Plane (AFPP) Method

The theory of stereopsis holds that stimulation of disparate points is necessary for the perception of relative depth by stereopsis. If there is no depth difference between an object and the fixation point, then they stimulate corresponding points. Thus, if the subject is asked to arrange a series of objects so that they appear to be in a fronto-parallel plane (i.e., no depth difference between them), then they will lie on the horopter. This is the apparent fronto-parallel plane method (Fig. 11). Note the change in shape of the horopter at different distances.

Figure 11

The horopter as determined by the apparent fronto-parallel plane method at different distances (25 cm, 40 cm, and 1 m). From Ogle (2).

Relationship of the Horopter to Panum's Fusional Area

The haplopic method demonstrates the existence of Panum's fusional area. This concept allows for single binocular vision about the point of fixation, even when corresponding retinal points are not being stimulated. An image on the retina of one eye can be fused (and seen as single) with a similar image on the retina of the other eye, although disparity in the retinal image exists. Panum's fusional area is needed for stereopsis; if images do not fall in Panum's area, then diplopia results—and so Panum's fusional area defines the zone of stereovision.