Ординатура / Офтальмология / Английские материалы / Oculomotor Systems and Perception_Ebenholtz_2001

40 Oculomotor Systems

wavelengths of light (in the red region) are differentially absorbed before the remaining wavelengths are reflected back from the iris. Soon after birth, pigmented cells, chromatophores, develop on the rear surface of the iris, the portion that floats just above the lens. The density of chromatophores determines the color of the adult eye (Adler, 1965).

Two types of muscle, a dilator and constrictor or sphincter, control the quantity of light in the retinal image by varying the pupil diameter from about 1.0 to 8.0 mm. Because the retinal image is at a fixed distance from the pupil, of about 20 mm (see Chapter 2), the retinal intensity varies with the area of the aperture and hence with the square of the aperture radius. This provides a range of about 64:1 in retinal illumination. In terms of a camera analog, because f-number is defined as the ratio of focal distance to aperture diameter, the pupil provides an f-stop range from about f/20 to f/2.5, the latter being not quite as bright as the value of f/1.9 available in many cameras.

Functional Aspects

The pupillary light reflex has obvious functionality in that it brightens an image seen under dark illumination and reduces image brightness under high illumination conditions. This simple view is somewhat complicated, however, by the fact that the reflex is modulated by light adaptation, becoming less responsive as the retinal receptors decrease in light sensitivity (Lowenstein & Loewenfeld, 1959). Generally speaking, as age increases beyond about 18 years, pupillary responses diminish, yielding typically smaller pupils in dim illumination.

Many conditions influence pupil size, including fear, anxiety, pain, and vestibular and other sensory stimulation, all of which induce pupillary reflex dilatation. However, the pupil is reduced in size during sleep and in narcosis (Lowenstein & Loewenfeld, 1969). The neural transmitter acetylcholine, associated with the parasympathetic branch of the autonomic nervous system, generally is responsible for pupil constriction, whereas the sympathetic transmitter adrenalin facilitates dilation. It is easy to see, therefore, why a great variety of emotional states influence pupil size.

The pupil also takes part in what has been termed the near reflex, a threepart synergism between accommodation, vergence, and pupillary reflex. Thus, when viewing a nearby target, the two eyes converge, each eye accommodates, and the pupils constrict. As shown in Chapter 2, although a small pupil reduces retinal illuminance, it also has a very positive functional consequence – namely, that of reduction of image blur. Put another way, a small aperture increases

Vergence 41

the depth of focus, thereby obviating the need to clarify an otherwise blurred image by additional focusing.

Vergence

Definition, Functionality, Decussation, and Stereopsis

Vergence eye movements are disjunctive in the sense that each eye moves in a direction opposite that of the other eye (Fig. 3.05). They are primarily associated with species having forward-oriented eyes, such as primates and members of the cat family (Hughes, 1972), and probably represent a working solution to the problem of how single vision emerges from two eyes and therefore from two images of the same object (Walls, 1962). Because vergence movements ensure that for the object of interest the image in each eye falls on corresponding loci of each retina, both of which project to the same region of brain (Figs. 3.06 and 3.07), sensory fusion of the images ensues. In those species with panoramic vision, like the rabbit, where the eyes are almost 180 deg apart, totally different images are represented in the brain from objects within each visual field, but because each eye receives images from different objects, the problem of diplopia (double images of the same object seen at different locations) would not exist. However, many lateral-eyed species, including the rabbit, have a region of binocular vision where each monocular field overlaps the other to some extent. In this case, an object within the binocular field stimulates both eyes, but instead of a vergence eye movement being initiated, the whole animal may jump or, as in the case of the lizard or frog, a tongue may strike out to capture the object of interest.

It also is distinctly possible even in lateral-eyed species that a binocular field, where a single object is represented in images of both eyes simultaneously, forms the basis for stereopsis, the visual appreciation of solidity or of a depth interval between objects (Hughes, 1977, p. 642). The minimum requirement for stereopsis is that noncorresponding points be stimulated so as to produce a slight binocular disparity. In humans, this implies stimulation from a point on the horopter (see Fig. 3.06) and another at some small distance within or beyond it to create the disparity. Of course, if these disparate points are separated too far, they produce double images; however, if within a threshold region, the disparate points fuse but also signal a sense of depth. Overlapping visual fields, usually but not universally associated with vergence, may be a precondition for binocular disparity and depth. A quantitative measure of binocular disparity is derived in Fig. 3.08.

Vergence 43

Vergence-Stimulated Accommodation

When vergence movements occur they also provide an assist to the accommodative mechanism. With increasing vergence, accommodation also increases so as to be appropriate for near vision. Likewise, with decreasing convergence, accommodation relaxes for distant vision. This means of altering accommodation seems redundant, given the negative feedback control that focusing already has (see Fig. 3.04). However, there is added functionality here in that this convergence signal to accommodation is a feed-forward signal. Without the feedback portion of the loop, feed-forward signals tend to be much faster than those of the feedback variety. Therefore, the focusing mechanism is made ready for changes in distance without first having to sample the blur in the image and then send through an error-correcting signal.

Individuals differ in the quantity of accommodation triggered by convergence. A low ratio of convergence accommodation per unit of convergence (CA/C ratio) requires more effort from the negative feedback loops of the focusing mechanism than would a high CA/C ratio. This added burden is a potential cause of asthenopia, or eyestrain (see Appendix). It is interesting that adaptive systems characteristic of oculomotor control seem to be organized around the general principle that wherever possible, feedback control should be kept to a minimum. Frequently, this is accomplished by substituting altered values of feed-forward signals in their place so that feedback loops may become quiescent. The stimulation of error-correcting feedback loops may thereby serve as the precondition for subsequent adaptation, whereas altered values of parameters in the feed-forward portion of the control loop constitute the adaptive response itself.

As a case in point, consider that the CA/C ratio itself is subject to adaptation. Miles, Judge, and Optican (1987) had subjects wear a device, represented in principle in Fig. 3.09, that effectively increased the interocular axis (the separation between the eyes). Thus, the device increased, by a multiplicative factor the convergence angle of every object the subject viewed. Because of the convergence-accommodation link, the device increased the accommodation as well. Such an unnecessary increase in accommodation, along with error-correcting feedback, probably triggered an adaptive response in the focusing system requiring less accommodation for each unit of convergence than previously. Lowering the level of accommodation would then yield a lowered CA/C ratio, which in fact was found.

44 Oculomotor Systems

O

A

γγ

X

γ

d

Figure 3.06. The Horizontal Horopter. (From Linksz, 1952, Physiology of the Eye, Vol. 2, p. 441. New York: Grune & Stratton. Reprinted with permission of W.B. Saunders Co.) Within an error of about 7.8 mm it is useful to assume that the schematic nodal point, 5.7 mm behind the cornea, and the center of ocular rotation, 13.5 mm behind the cornea, exist at the same point in space. Given this fiction, then an equi-convergence curve can be drawn through the nodal points N having the simple shape of a circle. Binocular fixation on any point on this circular locus produces the identical convergence angle, as does fixation at any other point, as long as the distance d is not altered. Another significant property of this curve is that the angular separation between images of points in one eye are exactly equal to their separation in the other eye. For example, the angles φ in the two eyes must be equal because they are included angles intercepting the same arc, OA. Because point O is at the fovea F and the images of O and A are equidistant in each eye, they can be identified as falling on corresponding retinal points and therefore are seen as fused or single. Points in space, the locus of which forms a circle passing through the nodal points and stimulate corresponding retinal points, define a boundary or horopter of single vision. Although the term was coined and the concept first introduced by a Jesuit monk, Francisco Aguilonius, in 1613 (von Noorden, 1980, p. 21), the horopter was first properly described by Johannes Muller¨ (1826) and G. U. A. Vieth in 1818. Accordingly, this form of the horopter has become known as the Vieth–Muller¨ circle. It is interesting to note that stereoscopic depth perception results when points are present at some small distance in front of or behind the horopter locus, whereas large displacements produce noticeable double images.

46 Oculomotor Systems

P

δ γ2

O

γ1

α

d

β

Figure 3.08. Binocular Disparity. Because coordinated vergence movements keep the two foveas closely oriented on the same detail O, other details such as P , in a different depth plane not on the horopter, stimulate noncorresponding points, thus causing binocular disparity and stereopsis. The difference in retinal angles (β − α) defines the magnitude of disparity, traditionally designated (η). An expression for η may be derived from the identity (β − α) = (γ1 − γ2). As shown in Fig. 3.05, in radian measure:

Assuming δ to be small relative to d allows one to drop the dδ term, yielding:

η = adδ2 .

Thus, disparity varies inversely with the square of the distance to the nearer target and varies directly with the length of the interocular axis a, and the actual depth interval δ. The equation also indicates that those individuals with the larger interocular axis a will have the greater disparity η. Because disparity controls the amount of depth experienced, one might expect adults to experience a greater sense of depth than young children, whose interocular axes are relatively small, but this has not yet been proved to be the case.

Vergence 47

D

D1

Figure 3.09. Stretching the Interocular Axis. (Adapted from Fisher & Ciuffreda, 1990, Adaptation to optically increased interocular separation under naturalistic viewing conditions, Perception, 19, 171–180. Reprinted with permission of Pion, London.) The interocular axis may effectively be increased by presenting to each eye the view it would receive if the eyes were more separated than they actually are. In the figure, the outer mirrors are separated by about twice the typical interocular separation of 63.0 mm. These images are then reflected into the eyes at fovea f via the pair of inner mirrors. Because convergence varies directly with the size of the interocular axis (see Fig. 3.05), the target at D would be seen with twice the convergence it would elicit if seen directly without mirrors. This is equivalent to viewing the target as if it were at D one-half the distance to D.

will and, of course, repeat the feat in light as well. Figure 3.10(a) shows the author’s eyes verging and Fig. 3.10(b) shows the eyes looking straight ahead in darkness in pictures taken under infrared illumination. Voluntary vergence can be trained and these procedures are helpful in the treatment of vergence disorders such as vergence insufficiency (Cooper et al., 1983). Until recently, it was not entirely clear, however, whether such voluntary control is exerted over the vergence system directly or indirectly via accommodation because both responses tend to occur together. McLin and Schor (1988) found evidence for an indirect route when they compared the ratio of convergence to accommodation measured during the voluntary effort in darkness to the ratio