Ординатура / Офтальмология / Английские материалы / Handbook of Pediatric Strabismus and Amblyopia_Wright, Spiegel, Thompson_2006

When a subject with normal eye alignment fixes on a target, that target falls on both foveas. Mathematical theory predicts that objects peripheral to the fixation target (points A and B in Fig. 3-1) will project to corresponding retinal points if the peripheral objects lie on a circle that passes through the optical centers of each eye. This mathematically determined circle of points is called the Vieth–Müller circle. As is often the case for mathematical explanations of biological phenomena, physiological experiments have shown that the Vieth–Müller mathematical model only partially works for visual perception. Psychophysical experiments indicate that the locus of points, which project to corresponding retinal points of each eye, is not a circle but actually takes the shape of an ellipse. This elliptical line of points, which project to corresponding retinal points, is the empirical horopter and is shown as a dotted line in Figure 3-1. Remember, the location of the horopter is determined by the point of fixation. Objects located in front of or behind the empirical horopter will project to noncorresponding retinal points.

In Figure 3-2A, note that point “A” is distal to the empirical horopter and stimulates binasal retina. Point “B” in Figure 3-2B, which is proximal to the horopter, stimulates bitemporal retina. These binasal and bitemporal retinal points are noncorresponding retinal points, and images falling on these points are termed disparate images. Disparate images have the potential for either producing stereoscopic vision or causing physiological diplopia.

Stereoscopic Vision

The empirical horopter is a theoretical locus of points, and is infinitely thin. All three-dimensional objects lie in front of and behind the horopter line; therefore, virtually all solid objects stimulate noncorresponding retinal points and result in disparate retina images. The brain, however, can merge or “fuse” images from slightly noncorresponding retinal points. This finite area in front of and behind the horopter line where objects stimulate noncorresponding retinal points, yet are still fusible into a single binocular image, is called Panum’s fusional area (Fig. 3-3). Stimulation of noncorresponding retinal points within Panum’s fusional area will produce three-dimensional vision. This ability for the brain to determine that images are falling on retinal points that are not exactly corresponding (i.e., disparate images) produces stereoscopic vision. Only horizontal retinal

A

B

FIGURE 3-2A,B. Empirical horopter and Panum’s fusional area. Objects that lie in front of or behind Panum’s fusional area will stimulate noncorresponding retinal points. (A) Patient fixating on the star in the center of the empirical horopter. Point A, which is distal to the horopter, stimulates the binasal retinal points that are noncorresponding. (B) Patient fixating on the same spot; however, point B is proximal to the Panum’s fusional area, and point B stimulates bitemporal retinal points that are noncorresponding. Point A in (A) would cause uncrossed diplopia, whereas point B in (B) would cause crossed diplopia. This type of diplopia is termed physiological diplopia.

}Panum's fusional

area Empirical

Horopter

Stereoscopic

Image

FIGURE 3-3. Diagrammatic representation of stereoscopic vision. Note that any three-dimensional objects will straddle the empirical horopter and parts of that object will be in front of or behind the empirical horopter; this stimulates noncorresponding retinal points that provide stereoscopic vision so long as the three-dimensional objects fall within Panum’s fusional area.

image disparities produce stereoscopic vision; vertical disparities do not. Panum’s fusional area is narrow at the center and gradually widens in the periphery reflecting the high resolu- tion–small receptive fields in the central visual field and low resolution–large receptive fields in the periphery. Large displacements are required for the peripheral retina to detect a change in receptive field.

Figure 3-3 shows a three-dimensional cube as a fixation target. Note that the cube lies in front of and behind the empiri-

cal horopter, projecting to noncorresponding retinal points. The fovea has high spatial resolution, so even small displacements off the horopter line (i.e., small image disparities) in the central visual field are detected, resulting in fine, high-grade stereoscopic vision. In contrast, as one moves to the peripheral fields, the receptive field size enlarges and the spatial resolution decreases. The peripheral binocular visual fields are sensitive to large image disparities and provide coarse stereoacuity. This retinal architecture of high central resolution versus low peripheral resolution explains the excellent stereoacuity from central fields and progressively poorer stereoacuity from peripheral binocular retinal fields.

Physiological Diplopia

If an object is too far off the horopter line and outside of Panum’s fusional area, then the images can no longer be fused and double vision may result (diplopia) (Figs. 3-2, 3-4). This type of double vision is a normal phenomenon and is termed physiological diplopia. Note that, in Figure 3-4, the pencil is in front of Panum’s fusional area and the pencil is, therefore, stimulating the temporal retinas of each eye. Because the temporal retina projects to the nasal visual field (opposite field), the observer perceives two pencils with the left image coming from the right eye and the right image coming from the left eye; this is called “crossed diplopia,” and occurs with bitemporal stimulation. Physiological diplopia would occur in everyday life; however, it is normally ignored or suppressed. You can experience physiological diplopia by simply fixating on a distant object several feet away then placing a pencil a few inches from your nose. While you are looking at the distant object, the pencil will appear double. This is crossed diplopia: when you close your right eye the left pencil disappears, and when you close the left eye the right pencil disappears. You can demonstrate that Panum’s fusional area is narrow centrally and wide in the periphery by moving the pencil held at near to the right or left, while maintaining fixation on a distance target. Observe that the physiological diplopia and image quality diminish when the pencil is moved into the peripheral binocular fields. (Remember to keep your fixation on a distant object while the pencil is held at near.) Objects distal to Panum’s fusional area stimulate binasal retinal points and can cause uncrossed diplopia (see Fig. 3-2A). You can experience uncrossed diplopia by fixating on a pencil a few

FIGURE 3-4. A pencil is seen in front of Panum’s fusional area; this stimulates noncorresponding bitemporal retinal points. Because the temporal retina projects to the opposite field (arrows), the patient perceives crossed physiological diplopia.

inches in front of you and observing that the distant objects are double (this may be difficult to see).

Stereoacuity Testing

Stereoscopic perception can be created from two-dimensional figures by presenting each eye with similar figures that are horizontally offset to produce bitemporal or binasal retinal image disparities. Bitemporal retinal stimulation within Panum’s fusional area gives the stereoscopic perception of an image coming toward the observer (Fig. 3-2B), and binasal retinal stimulation within Panum’s fusional area gives the perception of an image going away from the observer (Fig. 3-2A). Note that the upper circles in Figure 3-5 are displaced nasally. The displaced

Circles/seconds (s) of arc VA.4

with corresponding red/green figures, or polarized glasses with corresponding polarized figure plates (see Fig. 3-5). These systems provide different images to each eye separately under binocular viewing and are termed haploscopic devices. Stereoacuity can be quantified by measuring the amount of image disparity. The angle of disparity can be measured in seconds of arc. The minimum stereoscopic resolution is a disparity of approximately 30 to 40 s of arc. Stereoscopic resolution depends upon visual acuity, as poor vision in one or both eyes will decrease stereoacuity. A general guide on the effect of image blur on stereoacuity is seen in Table 3-1. Interpupillary distance also influences stereoacuity. The farther apart the two eyes, the greater the angle of visual disparity and the greater the stereoscopic potential. Additionally, the closer an object is to the eyes, the greater the angle of disparity; therefore, the better the stereoscopic view. As objects move away from the observer, the relative interpupillary distance diminishes as does the visual angle, so stereoscopic vision decreases for distance objects.

CONTOUR STEREOACUITY TEST

Contour stereoacuity tests use stereoscopic figures with a continuous contoured edge (Fig. 3-5). The Titmus test is a popular contour stereoscopic test and measures disparities from 3000 s arc (the big fly) to 40 s arc (ninth circle). Some pictures in the test are stereoscopic and others are flat (two-dimensional). The patient is required to identify which figure is stereoscopic. Contour stereoscopic figures are clinically useful because the stereoscopic effect is obvious and easy to see, but they have the disadvantage of having monocular clues. Monocular clues allow patients who are stereoblind to identify the stereoscopic figures1,3; this occurs because each stereoscopic figure is made

up of two drawings, nasally shifted off center (see Fig. 3-5). Patients with monocular vision can identify which figure is supposed to be stereoscopic, because that figure will be horizontally off center. Another type of monocular clue used by patients with alternating strabismus to falsely pass contour stereo tests is “image jump.”1 These patients alternate fixation between the two horizontally displaced figures and identify the figure that jumps back and forth. Monocular clues work for stereoscopic figures with large disparities and if the stereoscopic figure is framed so the displaced figure looks off center. The first three stereoscopic “circles” and first stereoscopic “animal” on the Titmus stereoacuity test can often be identified by using monocular clues, but stereo figures with smaller disparities are difficult to detect using monocular clues.

One way to help verify that the patient has true stereoacuity is to retest with the Titmus test book turned 90° and see if the patient still sees the stereoscopic target. With the test book turned 90°, the targets are not stereoscopic, but the monocular clues still work. If the patient again identifies the stereoscopic target, they are using monocular clues, not true stereopsis. For further verification, turn the book 180° (upside down) and see if the patient notes that the stereo targets have returned but are now projecting in an opposite direction away from the patient. The Titmus “fly” can be useful in preverbal children as young as 1 to 2 years of age. If a child startles to the fly coming out of the page, then this is suggestive of gross stereopsis. Also, if a child clearly picks up the wings of the Titmus “fly” well off the page, this is good evidence for at least some peripheral fusion.

RANDOM DOT STEREOACUITY TEST

Random stereograms consist of two fields of randomly scattered dots or specks, with one field of dots projected to each eye separately through a haploscopic device. Each field of random dots is identical except for a group of dots that is displaced nasally. The group of displaced dots can take the form of any recognizable shape, such as the square shown in Figure 3-6. The nasally displaced square of dots stimulates bitemporal retinal points and produces the perception that a single square of dots is coming up off the page. Random dot stereoacuity tests have an advantage over contour stereo tests, as random dot tests have almost no monocular clues, and a positive response indicates true stereopsis with few false-positive responses.8 The problem with

FIGURE 3-6. Diagrammatic representation of a Randot stereogram. The left eye sees one set of dots and the right eye sees a second set of dots. The dots are identical, except for the dots within the square that have been horizontally displaced (nasally in the figure). Nasal displacement stimulates bitemporal disparate retinal points and produces the stereoscopic perception that the square of circles is raised off the page. This clinical test for Randot stereoacuity consists of nasal displacement, so that the stereo images appear to come off the page.

random dot stereoacuity testing, however, is that many young, normal children and some normal adults have trouble seeing the random dot stereoscopic effect and falsely fail the test.

Monocular Depth Perception

Depth perception can occur without stereoacuity. Monocular vision can provide information regarding depth and the distance of an object. Motion parallax, shadows, object overlap, and the relative size of objects give us monocular clues of depth. Motion