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ROLE OF THE VISUAL SYSTEM: OPTICAL AND OCULOMOTOR, RETINAL, AND CENTRAL NEURAL FACTORS
ROLE OF THE VISUAL SYSTEM: OPTICAL AND OCULOMOTOR, RETINAL, AND CENTRAL NEURAL FACTORS
Pages 35-180
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From page 35... ...
Much of the available information concerning the effects of heredity and environment on the development of the optical characteristics of the eye is based on studies of subhuman primates, primarily chimpanzees and monkeys. However, a recent study of the Eskimo population at Barrow, Alaska, which supports the results of studies of subhuman primates, suggests strongly that the early visual environment and early visual experience play an important role in developing and modifying the optical characteristics of the eye, that the reaction of the eye to its visual environment plays a determining role in the development of the optical characteristics necessary for seeing and reading, that the mechanism of this role should be investigated in all children as they approach the reading age, and that the mechanism cannot be effectively assessed by determining the Snellen visual acuity at 20 ft or at 20 in.
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From page 36... ...
Hirsch6 has shown that, if one correlates the refractive error determined by retinoscopy with the Snellen visual acuity, one can demonstrate a considerable correlation (+0.95) in persons who show some degree of myopia.
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From page 37... ...
An eye that requires a minus lens will be a "myopic eye"; no lens, an "emmetropic eye"; and a plus lens, a "hyperopic eye." The distinction between myopia and nonmyopia is arbitrarily taken as the use of a minus lens to neutralize the movement of the retinoscope shadow or to achieve best visual acuity subjectively. OPTICAL CHARACTERISTICS The determination of the refractive characteristics of the eye, although a great improvement over the determination of simple visual acuity, does not adequately describe the optical characteristics of the eye.
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From page 38... ...
. The combination of measures of refractive error, corneal curvature, depth of anterior chamber, front lens surface curvature, thickness of lens, rear lens surface curvature, depth of vitreous, and total axial length provides an accurate description of the optical characteristics of the eye; anything less than this combination does not.
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From page 39... ...
However, as soon as blood pressure drops, the intraocular pressure drops and the eye becomes quite flaccid; furthermore, most in vitro measurements, even when the eye is perfused to reinstate a probably normal intraocular pressure, can vary widely from the measurements that would have been obtained in vivo. Sorsby and Sheridan17 have provided possibly the best measurements available on the sagittal or anterior-posterior axial length of the eye of the newborn and of children 1-6 days old.
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From page 40... ...
Shape In the hyperopic eye, a comparison of the transverse, vertical, and axial dimensions based on radiographic measurements of 11 male adult eyes by Deller et a/.4 showed no differences between the transverse and vertical dimensions, but did show an axial length significantly longer than either at the 1% level of confidence. In the myopic eye, also, we find that the only significant deviation from sphericity occurs in the axial diameter.
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From page 41... ...
The development of the myopic eye, with its exaggerated axial length, probably depends on the operation of other variables, although the three mentioned -- genetics, scleral elasticity, and intraocular pressure -- may also play a contributory role. Sorsby et al.1S also found a second growth period, the juvenile phase, during which the eye grows at a lower but measurable rate to reach its maximum growth at about 13-14 years of age.
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From page 42... ...
Ware described the effects of fitting concave or minus lenses to nearsighted persons as follows: It should be remembered, that, for common purposes, every near sighted eye can see with nearly equal accuracy through two glasses, one of which is one number deeper than the other; and though the sight be in a slight degree more assisted by the deepest of these than by the other, yet on its being first used, the deepest number always occasions an uneasy sensation, as if the eye was strained. If, therefore, the glass that is most concave be at first employed, the eye, in a little time, will be accommodated to it, and then a glass one number deeper may be used with similar advantage to the sight; and if the wish for enjoying the most perfect vision be indulged, this glass may soon be changed for one that is a number still deeper, and so in succession, until at length it will be difficult to obtain a glass sufficiently concave to afford the assistance that the eye requires.
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From page 43... ...
Most of the studies that have attempted to demonstrate a relationship between intelligence and refractive error have found none, except when intelligence was measured by written tests.7'30 There is a positive relationship between performance on such a test and refractive error: myopic persons tend to score higher than nonmyopic. However, when reading ability is statistically adjusted for, the correlation of refractive error and intelligence approximates zero.
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From page 44... ...
Apparently, the personality characteristics precede the development of myopia, inasmuch as the characteristics may be distinguished as early as the kindergarten and first-grade years, whereas the myopia usually does not develop until the fifth- and sixthgrade years. In spite of the consistent pattern that has been described, it is not possible to conclude without more definite investigations that reading leads to myopia.
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From page 45... ...
EXPERIMENTAL FINDINGS A series of studies has demonstrated that subhuman primates, particularly monkeys and chimpanzees, develop myopia under experimental conditions that restrict visual space to a distance of less than 20 in. from the eye.2s,26,as-as About three fourths of all adult monkeys placed in the restrictive space show an increase in minus refractive error, and more than half the animals show approximately one-half diopter of myopia or more within 3 months after being placed in it.
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From page 46... ...
In all groups of animals, approximately 65-75% show myopic changes under these experimental conditions. When a group of newly captured rhesus monkeys were examined, only 12 of 600 eyes were found to have any myopia.34 Among wild and laboratory monkeys, the mean and median refractive errors were found to be significantly more hyperopic in the newly captured rhesus monkeys, the younger monkeys, and wild monkeys in general.
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From page 47... ...
Furthermore, when 50 inside-cage animals were matched against 50 outside-pen animals on the basis of species, age, sex, weight, diet, time in captivity, and time spent in cage or pen, the caged animals were significantly more myopic than the outside-pen animals. Because any hereditary contributions were confounded by the random factors involved in capture and placement of the monkeys, and the influences of age, sex, and diet were reduced by the matching procedures used, the conclusion that restriction of visual environment has an effect on the refractive characteristics seems to be supported.
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From page 48... ...
All these findings have been derived from studies on animals that can be placed in a situation of controlled visual environment. However, we are concerned primarily with human subjects, and it is important to demonstrate that the optical characteristics of animals are similar to 48
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From page 49... ...
The majority have negligible refractive errors and a small number have low hypermetropia."3 Skeller has indicated that in the Ang49
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From page 50... ...
The proportions of various refractive errors are similar in both groups. Figure 1, based on data from the original group, presents the proportion of persons requiring a minus lens for correction, by age groupings from 6 to 88 years of age.
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From page 51... ...
It can be seen again that most of the older persons are hyperopic, but those between 11 and 25 years old are myopic. The average refractive error of the 21- to 25-year-olds is approximately -2 diopters.
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From page 52... ...
FRANCIS A YOUNG -2.00 6-10 11-15 16-2021-25 26-30 31-35 36-4041-4546-50 51-5556-60 61-65 66-70 71-88 AGE,YEARS FIGURE 2 Mean refractive error in different age groups of Barrow Eskimos.
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From page 53... ...
If such a person is examined with a Snellen chart for near vision, he is likely to show normal visual acuity for short periods, and his visual problems are likely to be ignored. The emmetropic person should be able to read effectively as well as to see clearly at long distance and should not have to exert any more accommodation to read than a corrected myope would have to exert.
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From page 54... ...
Any child who has reading difficulties should be given a thorough examination by a qualified orthoptist as well as a qualified optometrist or ophthalmologist. The accommodation-convergence relationship should be carefully investigated; all refractive errors should be corrected and deficiencies in accommodation and convergence should be determined.
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From page 55... ...
J Relation of visual acuity to myopia.
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From page 56... ...
Refractive errors in the eyes of children as determined by retinoscopic examination with a cycloplegic. Results of eye examinations of 1,860 white school children in Washington, D.C.
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From page 57... ...
A Visual refractive errors of wild and laboratory monkeys.
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From page 58... ...
There are also people who read early and do a lot of reading, and still do not develop myopia. You mention the refractive errors under the effect of a cycloplegic, a drug that paralyzes the muscle inside the eye that operate the lens (the ocular accommodative mechanism)
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From page 59... ...
The measurement of refractive error or the optical characteristics of the eye under the cycloplegic condition would determine the cycloplegic error, and measurement under the plus lens, the manifest error. If the drug and the plus lens are equally effective, the refractive errors should be the same under both conditions except for errors of measurement.
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From page 60... ...
When persons with myopic refractive errors are compared under the two relaxation conditions, major differences are found. The myopic subjects in our study showed the same results as those demonstrated by the hyperopic subjects: 75% had more hyperopia, 15% no change, and 10%lesshyperopia.
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From page 61... ...
Development of Optical Characteristics for Seeing 55-57,1966) has shown that, during the period of ciliary muscle contraction, the movement of the aqueous from the anterior chamber back through the posterior parts of the eye just beneath the sclera is blocked.
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From page 62... ...
In clinical refraction,4 visual acuity is determined by having the subject identify letters of various sizes at optical infinity (20 ft)
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From page 63... ...
Artificial myopia was induced by convex spherical lenses in increments of 0.25 diopter until the subjects began to suffer stress from the refractive error. The results are shown in Tables 1-3.
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From page 64... ...
Word Size Diopters of Induced Myopia (as Snellen fraction) Piano +0.25 +0.50 +0.75 +1.00 +1.25 +1.50 20/20 20/25 20/40 20/80 5.18 7.58 5.42 4.92 9.62 9.25 8.33 10.00 9.75 9.46 3.17 6.00 8.96 4.46 8.54 7.17 5.50 TABLE 2 Percentage of Responses Falling below 80 Percent Accuracy Level Word Size (as Snellen Diopters of Induced Myopia fraction)
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From page 65... ...
Romine9 thought that "it would seem most important to correct any marked degree of astigmatism," and Cleland,5 sharing that view, stated that "in severe cases of astigmatism it was found to be closely allied with reading failure." Several of the foregoing studies involved a comparison of the visual characteristics of students who were successful and unsuccessful in reading, and the investigators did not state whether the refractive errors were corrected or uncorrected at the time of testing. If the subjects were fully corrected, they would be emmetropic, and comparisons would therefore have involved reading ability with normal vision.
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From page 66... ...
Therefore, for our study, we induced 1.00 diopter of astigmatism with the rule while our subjects, 35 seniors from the Los Angeles College of Optometry, from 22 to 47 years old, performed an intelligence test. At the conclusion of the test, each student recorded his own subjective reactions to the induced astigmatism.
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From page 67... ...
J Relation of visual acuity to myopia.
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From page 68... ...
Induced myopia and far-point perception, pp.
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From page 69... ...
These were children who complained of vision problems, even though no refractive errors were found by cooperating specialists, who said that the lenses would help. Psychologically, the results were apparently very good, in the sense that the headaches disappeared and the reading improved.
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From page 70... ...
COGAN / JERRY B WURSTER Normal and Abnormal Ocular Movements Ocular movements are customarily divided, according to the psychosensory stimulus that evokes them, into vestibular, regard, pursuit, and command categories.
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From page 71... ...
The ocular movements to fixate an object of attention are quick and simulate command movements -- with which they may, in fact, be iden71
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From page 72... ...
are never fully developed. Pursuit Movements Ocular movements in following a moving object are called pursuit or optokinetic movements.
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From page 73... ...
ABNORMAL OCULAR MOVEMENTS The most common examples of abnormal oculomotor development are congenital nystagmus, strabismus, palsies of conjugate gaze, and oculomotor apraxia. Because these do not necessarily have any relationship with one another, they will be described separately.
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From page 74... ...
Latent congenital nystagmus is so called because it is elicited by covering one eye. Customarily, the eyes are stationary, but covering either eye evokes a conjugate jerk nystagmus with a fast component toward the side of the covered eye.
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From page 75... ...
This in turn produces an excessive stimulation of the correlated convergence mechanism, with consequent turning-in of one eye. The eye with the greater refractive error is almost invariably the one that turns.
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From page 76... ...
Convergence is unaffected and, except for the compensatory head movements and the expressionless facies, the defect causes no functional handicap. Congenital Oculomotor Apraxia This is a condition in which, despite full random and vestibulogenic movements, a person is unable to move his eyes efficiently at will or in following a moving object to either side.
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From page 77... ...
NORMAL APRACT 1C FIGURE 1 Comparison of head-eye movements in the normal and apractic child on gaze to the left. Whereas the eyes precede the head on eccentric gaze in the normal person, the head precedes the eyes in the child with congenital apraxia.
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From page 78... ...
An apparatus for grading the visual acuity of infants on the basis of opticokinetic nystagmus. Pediatrics 19:10881092, 1957.
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From page 79... ...
, combined with the ideas I shall present, shows that one way of viewing reading disabilities and perceptual disorders is in terms of ineffective programming of visual input related to faulty functioning of eye movement mechanisms. To understand this fully, it is necessary to grasp the extent to which the perceptual process depends on eye movements.
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From page 80... ...
We shall arbitrarily choose printed language as an example of another information-bearing system, partly because it will lead back to further consideration of one focus of this meeting -- reading. DISCONTINUITY OF VISUAL INPUT There are two major reasons why scientists have not recognized the discontinuity of visual input before.9 The first is that vision is subjectively experienced as continuous over time and that our conceptual construct of the real external material world is overwhelmingly one of temporal continuity.
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From page 81... ...
Before describing evidence of discontinuity of visual information input and processing, I would like to illustrate the manner in which eye movements affect the time course of the perceptual process. Figure 2 shows the effects of gross jumping of the eye about a simple scene.
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From page 82... ...
At upper left is the scene with five numbered dots and connecting lines superimposed. The dots represent five successive fixations and the lines represent the track of the eye jumps between these fixations.
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From page 83... ...
Because these fixation eye jumps are much smaller than in the previous example, they do not have the effect of causing the same massive transformation of the central retinal area as in the gross viewing eye jumps. Instead, they result in changes at the edges of objects on the retinal image, imitated here by a photographic technique.
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From page 84... ...
This simulates the change of the retinal image produced by small eye jumps (indicated by the arrows)
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From page 85... ...
Bottom, net change of edge produced by the jump. tions on the text; the third lines, arbitrary central retinal areas around these fixations; and the fourth lines, the time sequence of presentation to the retina and brain of the contents of these successive central retinal areas in a particular chain.
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From page 86... ...
Chunking or Packaging of Visual Input With this picture of the physical facts of the eye-movement system, we can now examine some of the other evidence for and against the idea of visual perception as a discontinuous process mediated by eye jumps. I shall refer to five sets of experiments and their implications.
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From page 87... ...
Another argument holds that, if perceptual input is intermittent, there must be inhibition of vision during the periods when input is not being processed, i.e., during eye jumps. This is found to be the case during jumps: visual thresholds are raised and inhibitory neurons are activated in the lateral geniculate nucleus.4'18 4.
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From page 88... ...
The direct analogy to time-shared computer technology,12 which uses short-term buffer memory storage, should be noted. To recapitulate briefly what has been shown, the input of visual information during perception is not continuous, but is interrupted several times each second by eye jumps, which naturally divide the input into chunks or packages; these packages are reassembled by the brain into a spatiotemporally continuous visual world including, for example, the continuous line of text read from a page; finally, the packages represent natural physiologic units -- a step in the direction of reducing phenomena to units whose measurement reflects their intrinsic nature.
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From page 89... ...
We will attempt simple definitions of these terms mainly by illustration. Hierarchy The three main elements of the analogy to printed language are the hierarchy of levels, intrinsic units, and formation of chains.
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From page 90... ...
We will now look at how analogy to written language helps us to understand visual perception. The package of visual input mediated by eye jumps is analogous in several respects to the intermediate level of the language hierarchy.
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From page 91... ...
For example, as you look at an object, the chain of your eye-jump packages will constitute a sentence that is completed when you glance at the next object and begin a new sentence in organizing a percept of it. We can be specific at two levels as to the nature of the visual system units: the lowest level of vision is a nerve spike, analogous to the lowest level of a written language, the alphabetic letter; and a higher level of visual perception is the package of information (mediated by the eye jump)
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From page 92... ...
states,8 whereas both too high and too low rates would lead to interference from other spheres. Our model of visual perception has strong implications for a model of sensory processing and behavior in general that can help us to understand these interferences.
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From page 93... ...
Interpretive study of evoked responses elicited by gross saccadic eye movements. Percept.
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From page 94... ...
In the normal situation, clearly, the "computer" knows where the eyes are going to go next and, as a consequence, is able to get a good deal more out of the visual input from successive fixations than could otherwise be possible.
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From page 95... ...
ROBERT M BOYNTON Retinal Contrast Mechanisms The verb "contrast," according to Webster's New Collegiate Dictionary, means "to exhibit noticeable differences when compared or set side by side." Implicit is the idea that small differences will become less noticeable or unnoticeable if the items being compared are separated.
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From page 96... ...
One purpose of this presentation is to review some of what is known about the retinal contrast mechanisms that underlie this and other observations. Another purpose is to relate this knowledge to the perception of small dark details against a brighter background, which is characteristic of the typical reading task.
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From page 97... ...
of Arc) FIGURE 1 Line- and point-spread functions on the human retina, as determined from the experimental data of Westheimer and Campbell21 by direct physical measurement on the human eye.
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From page 98... ...
The retinal contrast produced by lines of various widths is shown in Figure 4. We can now see that, when we are concerned with the vision of fine lines of high physical contrast, we are, nevertheless, dealing with low retinal contrast.
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From page 99... ...
Boldface looks blacker because retinal contrast is higher, although the objective contrast is the same as for regular type (produced with the same ink)
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From page 100... ...
The bizarre inks and backgrounds now used for artistic purposes in some popular magazines sometimes produce very low physical and retinal contrasts and can thus be very difficult to read. Despite the low retinal contrasts that fine lines produce, our ability to resolve such lines is remarkable.
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From page 101... ...
CONTRAST DETECTION AND LIGHT INTENSITY Although retinal contrast does not depend on light intensity, the threshold of detectable contrast does. We are all familiar with this from everyday experience.
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From page 102... ...
A test spot subtending only 1 min of visual angle and lasting for only 1 msec was used. The function labeled AL/I shows that the contrast required just to see the test spot drops from more than 1000% at the lowest luminance used to slightly more than 1% at the highest.
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From page 103... ...
RETINAL CONTRAST VERSUS SUBJECTIVE CONTRAST One further psychophysical observation bears reporting before we turn to physiologic mechanisms: the highly nonlinear relationship between retinal contrast and its visual effects. This relationship is most easily examined in a large bipartite field, where physical contrast and retinal contrast are essentially the same.
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From page 104... ...
Some years ago, we measured the ability of observers to seek out and recognize complex critical targets presented against a background of pseudo-targets. Figure 7 shows that the main improvement in visual performance is associated with the lower range of physical contrasts.
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From page 105... ...
In the human eye, there are about 125 times as many receptors (rods and cones) as optic nerve fibers.
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From page 106... ...
When we fixate on a stimulus, we move our eyes to place its image in a highly specialized region of the retina, the fovea centralis. Only cone receptors are found in this region, and it is here that our visual acuity is by far the best.
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From page 107... ...
Retinal Contrast Mechanisms FIGURE 8 Summary diagram of the interconnections among the neural cells in the retina. A, amacrine; H, horizontal; C, cone; R, rod; MB, midget bipolar; RB, rod bipolar; FB, flat bipolar; MG, midget ganglion; DG, diffuse ganglion.
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From page 108... ...
Light A can stimulate only ommatidium A In the absence of other stimulation, this produces a particular frequency of firing in the optic nerve fiber A
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From page 109... ...
As shown in the lower graph of Figure 10, this activity in fiber B is associated with a decrease in the frequency of firing in fiber A, which is still being directly activated only by the original light intensity delivered to ommatidium A It turns out that, once some threshold frequency is reached, the inhibitory effect of B (as measured by the decrease in frequency of firing in fiber A)
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From page 110... ...
If field C is turned on, it produces a marked inhibition of fiber B, as reflected in the middle part of the uppermost record. This reduction in the response rate of fiber B releases in turn some of the original inhibitory effect of B on A, as revealed by the fact that, while field C is turned on, the response to A increases.
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From page 111... ...
. This is very important, because it strongly sugA • B ( C © 5 mm A ' alone FIGURE 12 Oscillographic records of electrical activity of two optic nerve fibers (A and B)
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From page 112... ...
RECEPTIVE FIELDS AND TRIGGER FEATURES In addition to the purely summative types of receptive fields already mentioned, which are found in the dark-adapted eye, much more complex arrangements are typical in warm-blooded vertebrates. Figure 14 shows one example.
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From page 113... ...
As Lettvin et al. remark, it is tempting to call such a specialized unit a "bug detector." More recently, the name "trigger feature" has become associated with the peculiar, com 1 mm FIGURE 14 Receptive field of a cat ganglion cell, obtained by Kuffler.14 This unit has an ON center and an OFF surround.
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From page 114... ...
Trigger features have been examined in detail in the primate visual cortex, especially by Hubel and Wiesel in a long series of experiments (see Hubel11)
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From page 115... ...
It has perhaps not been emphasized enough that these inhibitory mechanisms appear, on the basis of both behavioral and electrophysiologic evidence in vertebrates, to be specific to the light-adapted eye. In the dark-adapted state, in which the task of the eye is to gather as many photons as possible, receptive fields are purely excitatory.
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From page 116... ...
H The visual cortex of the brain.
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From page 117... ...
When I used the term "contrast," I referred specifically to the physical, objective definition, the difference in luminance between two areas divided by the luminance of the larger of these two areas. Physical contrast so defined is independent of the illumination on a reflecting target, such as a letter on a page.
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From page 118... ...
I tried to point out that there are species differences. I picked Limulus for a detailed illustration largely in deference to historical values: it was the first experimental animal in which clearly defined retinal interaction mechanisms were demonstrated, although many workers had felt for many years that they must be present in the human retina.
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From page 119... ...
MATHEW ALPERN The Pupillary Light Reflex and Binocular Interaction I am going to discuss some recent experiments on one aspect of the cross-talk between the two eyes in what I am constantly reminded is thought to be the simplest of all reflexes, the pupillary reflex to light. I want to speak about binocular cross-talk in this "simple" reflex, as well as about some findings on a disorder of perception -- something not associated with the reading problem in any proper sense of the term, but an interesting perceptual disorder nonetheless.
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From page 120... ...
MATHEW ALPERN R.W. 12° Field N = 400 0 Monocular • Binocular -1 LOG 10 012345 RETINAL ILLUMINANCE (TROLANDS)
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From page 121... ...
Cutting the eye muscles of a newborn kitten results in an alternating divergent strabismus. After this strabismus had developed, Hubel and Wiesel measured the percentage of cells in the visual cortex that were binocularly driven.
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From page 122... ...
MATHEW ALPERN -1012345 LOG 10 RETINAL ILLUMINANCE (TROLANOS) FIGURE 3 Change in diameter of the pupil of the left eye for equal binocular retinal illuminance.
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From page 123... ...
The vast majority of cells in their visual cortex are only monocularly driven. This looks like a paradigm for testing binocular pupil additivity.
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From page 124... ...
MATHEW ALPERN 1/1 ce 1 6 _.
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From page 125... ...
The Pupillary Light Reflex and Binocular Interaction 4 E c i 3 Oi ' 1*
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From page 127... ...
What is the relationship of these ideas to the problems of reading? In the process of reading, one makes eye movements of very high velocity, saccades; in making a saccade, the visual system also undergoes a momentary turning down of activity (it takes more light to produce a threshold than when the eye is immobile)
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From page 128... ...
Binocular interaction in striate cortex of kittens reared with artificial squint.
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From page 129... ...
How do they manage to read, to get a retinal outline down? DR.ALPERN: I have no evidence on this except the subjective reports of a professor of mathematics at the University of Michigan who has very poor visual acuity and a constant nystagmus.
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From page 130... ...
We know from psychophysical studies, such as those of Blough and Schrier,1'12 that monkey vision is quite similar to our own; hence, we should be able to understand a good deal about the human eye from studies on monkeys. Figure 1 is a low-power picture of the monkey eye, showing the typical results of histologic procedures for fixation, embedding, and staining of eyes.
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From page 131... ...
Many animals have almost exclusively one or the other type of receptor. I would like to discuss some aspects qf comparative anatomy of the retina of mammals -- both because it is interesting in itself and because comparative study can show in a simple way something about our human retina.
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From page 132... ...
MITCHELL GLICKSTEIN FIGURE 2 Higher-power view of monkey fovea; same section as in Figure 1. (about X 150)
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From page 133... ...
There is summation from many receptors onto a smaller number of cells in the inner nuclear layer, and summation in turn from inner nuclear cells onto ganglion cells. By contrast, Figure 6 illustrates the ratios in a tree shrew.
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From page 134... ...
MITCHELL GLICKSTEIN FIGURE 4 Oil-immersion photomicrograph of kinkajou (Potos flavus) retina.
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From page 135... ...
Figure 8 shows the lateral geniculate nucleus of a squirrel monkey. If one eye is removed and a sufficient amount of time elapses, a covert lamination is revealed in the geniculate.4 Atrophied cells are smaller than those seen in the lateral geniculate nucleus of a normal animal.
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From page 136... ...
PROJECTION TO CORTEX I would like now to consider the projections from the lateral geniculate nucleus to the cortex. Classical teachings would hold that there is only one representation of the visual fields in the cortex.
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From page 137... ...
Talbot and Marshall14 first began to study systematically the potentials evoked in the cat's brain by flashes of light. On the basis of their observations and those of later workers, we know that gross evoked potentials to flash can be recorded not only in area 17, the striate cortex, but also in area 18: Indeed, Doty3 showed that the evoked potentials in area 18 are of short latency and actually of higher amplitude than those in area 17.
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From page 138... ...
- v^'.5* l FIGURE 8 Low-power photomicrograph of lateral geniculate nucleus of squirrel monkey (Saimiri sciureus)
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From page 139... ...
Visual recall is thought to be a function of connections from primary visual cortex to association cortex nearby. Current theories of receptive-field organization,8'9 for example, suggest that cells in area 18 derive their complex receptive fields from simpler receptive fields of cells in area 17.
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From page 140... ...
Receptive fields, binocular interaction and functional architecture in the cat's visual cortex.
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From page 141... ...
There was a slight amount left that might have damaged fibers going into area 17. The amount of retrograde degeneration in man involves degeneration from area 18, occurring in a little nuclear group abutting the lateral geniculate nucleus on its median dorsal edge.
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From page 142... ...
169:107-126, 1967] in which retrograde degeneration was found in the lateral geniculate after lesions were placed in area 18, so I think the retrograde studies are going to confirm the antegrade studies.
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From page 143... ...
The most disturbing observation is that the topologic system in the neocortex of cats (areas 17 and 18) can be fully removed without apparent detriment to such complex phenomena as pattern vision and visual estimation of distance, provided the removal takes place in the neonatal period.6 The same appears to be true in tree shrews, even as adults,17 and possibly to a lesser degree in adult cats.19 Thus, the exquisitely refined neural circuitry of the topologic matrix has no necessary relevance to pattern vision, although it would be difficult to be143
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From page 144... ...
. In cats, many of these areas of neocortex remain responsive to photic stimulation even after total extirpation of visual cortex and degeneration of the lateral geniculate nucleus, pars dorsalis.7 Perhaps equally disturbing to any simplistic concept that the visual system operates via a mere topologic hierarchically organized matrix is the fact that visual information can be drastically modified at the first and subsequent central relays by action of the centrencephalic system.
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From page 145... ...
Surgical levels of anesthesia severely depress synaptic transmission at the lateral geniculate nucleus (LGN ) , and even one-tenth the anesthetic dose of Nembutal lengthens the recovery time of synaptic transmission from 15 msec to 50-100 msec.
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From page 146... ...
Indeed, when the monkey is relaxed and probably dozing, transmission through the LGN almost ceases for single volleys coming over the optic tract, and the cortical response is correspondingly reduced. The reduced cortical response, however, belies the true state of the cortex, in that stimulation of the optic radiation at such times produces a severalfold increase in the cortical response.
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From page 147... ...
It does not seem to follow the brachium of the superior colliculus, and it survives extensive brain-stem lesions in the area between the two structures. The significance of this control of visual input is equally obscure.
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From page 148... ...
The MRF stimulation that facilitates transmission through the LGN is not consistently linked to loci controlling eye movements, and the potentials that appear in LGN in response to eye movements also occur in response to tactile or auditory stimulation.11 In cats, potentials in the LGN also occur during eye movements and are correlated with discharge in the pons,3 discharges of neurons in the visual cortex,18 and presynaptic inhibition of the LGN.2 To gain some insight into the meaning of the modulation of LGN excitability by the MRF, John Bartlett in my laboratory has been studying the effect of MRF stimulation on responses of single units in the area striata of painlessly immobilized, unanesthetized squirrel monkeys. A natural stimulus, such as a moving line, is presented, to which the unit responds; 50 msec before every other presentation, the MRF is stimulated with six pulses, which in the nonimmobilized animal produces a mild alerting.
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From page 149... ...
Auditory, somatic and visual input to association and motor cortex of the squirrel monkey.
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From page 150... ...
Cohen. Electrical activity in the lateral geniculate body of the alert monkey associated with eye movements.
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From page 151... ...
Each electrode is made from a 50-ju straight tungsten wire etched to a l-n tip and coated with multiple layers of Isonel 31. They may be included in an indwelling microdrive that can move them from one neural unit to another in the cortex, or they may be left in a fixed cortical locus, in which case electrical pickup of single units is likely because of the large number of active neurons at the tip (Figure 2)
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From page 152... ...
5 msec FIGURE 2 Oscillogram recorded from a single unit in the human visual cortex.
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From page 153... ...
IL 220 2 01 0 fc30 B _i o ^ 40 f_ a: LU 50 . , , , , 1 50 40 30 20 10 0 10 20 30 Left Field Right Field HORIZONTAL CM FROM FIXATION POINT FIGURE 3 Receptive fields recorded from single units in the human visual cortex.
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From page 154... ...
Monocular and binocular fields were plotted, and we detected in these patients the various degrees of dominance that have been reported in laboratory animals. All the receptive fields that we plotted had some characteristics in common.
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From page 155... ...
"E" is a disk-shaped and "F" is a bar-shaped binocularly equal receptive field; "F" showed marked habituation. "G," "H," and "J" are binocularly equal receptive fields; only "G" showed strong habituation.
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From page 156... ...
Rutkin. Receptive fields of cells in the human visual cortex.
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From page 157... ...
The study of the primary system does not encompass all that we know about the spread of visual information through the cortex or subcortical structures. Animal experiments have clearly indicated for more than 20 years that visual projections exist in many structures outside the primary pathway.
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From page 158... ...
At the reticular level of the mesencephalon, the data show visual inputs, with longer latencies and far greater variability than in the primary pathway, including the superior colliculus.9'19 Among extrageniculate thalamic nuclei that respond to stimulation of the retina, some belong to the "associative" group, as defined anatomically, such as the lateralis posterior and pulvinar. However, visual responses have also been characterized within another group of thalamic structures, namely those belonging to the group of nonspecific, diffusely projecting nuclei, as defined through physiologic methods by Dempsey and Morison20 and later by Jasper31 and others.
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From page 159... ...
This may occur through a facilitation of the visual cortex or, electrophysiologically, by addition of a multisynaptic late visual input to the earlier input transmitted through the primary channel. The importance of these various processes, which are not mutually exclusive but complementary, remains to be determined.
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From page 160... ...
Depending on the experimental procedure, these modulatory influences act in various ways, although they often are facilitating rather than inhibiting. This is the case with the visual evoked potential recorded through macroelectrodes and at the single-cell level; thus the temporal discrimination of cells in the visual cortex of cat has been shown to increase during stimulation of the nonspecific system.34 Participation in Input Another category of data emphasizes the participation of the nonspecific system in visual input to the visual cortex.
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From page 161... ...
FUNCTIONAL CORRELATIONS The last group of investigations to be dealt with herein should concern the functional role of visual projections to nonvisual cortex -- i.e., to 161
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From page 162... ...
The transfer of optic information through the lateral geniculate body of the rabbit, pp.
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From page 163... ...
Average evoked potentials and reaction times to visual stimuli. Electroenceph.
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From page 164... ...
DR. BUSER: Yes, of course, but as long as that has not been demonstrated, my personal bias would favor a parallel system more than a system in series with the visual cortex.
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From page 165... ...
You might say that the cortex is like a giant screen, on every part of which there is some manifestation of a response or an increment of visual input. Then, to speculate further, perhaps the input from one's sensory system is manifested in some way on almost every part of his cortex.
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From page 166... ...
By comparing the single-unit response in the nonprimary central medianum nucleus of the thalamus with that in the lateral geniculate, we can show that the state of sleep or wakefulness of an animal is changing all the time. Of course, when the animal is strongly aroused, it is very difficult to record something outside the primary fields.
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From page 167... ...
Ronald Saul and me4 on the effects on visual perception of congenital absence of the corpus callosum in man. The behavioral symptoms seen with congenital absence of the callosum will be compared with those produced by surgical elimination of the callosum and other cerebral commissures.
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From page 168... ...
This patient exemplifies the functional plasticity of neural maturation7 that is presumed basic to many phenomena in which early experience is critical in the shaping of adult behavior. Patients deprived of the corpus callosum by surgery are unable to describe in speech or writing things that they see in the left half-field of vision.
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From page 169... ...
The minor hemisphere's comprehension is expressed in nonverbal tests in which the subject selects the correct name of the stimulus or a matching picture or object by pointing. Ability of the subject to retrieve by touch alone objects pictured in the left half-field and emotional responses to left-field stimuli also show that the left-field stimuli, about which the subject verbally disclaims any knowledge, are actually seen and recognized in the minor hemisphere.
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From page 170... ...
ROGER W SPERRY FIGURE 2 Schematic diagram of visual fields, optic tracts, and associated brain areas, showing left and right lateralization in man.
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From page 171... ...
A profound absence of awareness in each hemisphere of the mental experiences of the other is consistently evident in the test data. Along with the immediate visual perception, visual memories and all kinds of mental associations of vision with language, with calculation, and with other sensory modes -- including touch, hearing, and olfaction -- are all confined to the same hemisphere.
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From page 172... ...
Although the anterior commissure often is also absent in such cases, it appears to be present in this person, judging by her x-rays, and to be slightly enlarged, as is not uncommon among cases of agenesis of the corpus callosum. The extra fibers in the anterior commissure might thus be a contributing factor.
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From page 173... ...
The thinness of the cerebral aqueduct and the ease with which it becomes blocked make one wonder about the presence of an atypical hypertrophy among the midbrain centers. In addition to a purely functional reinforcement of whatever connection possibilities exist at these lower levels, there might also be purely embryonic reactions associated with the agenesis of the neocortical system that would make for an enhanced development of the older brain-stem systems that handled higher visual, auditory, somatesthetic, and other functions before the neocortex evolved.
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From page 174... ...
Some preliminary evoked-potential records taken during visual performance suggest that only her left hemisphere is active in vision. It is conceivable that speech, somatesthesis, audition, and vision are all handled in a single dominant left hemisphere.
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From page 175... ...
She also draws poorly and has difficulty with geography, block design arrangements, and matching patterns -- all specialties of the minor hemisphere in typical right-handed persons. At this stage, our evidence suggests a distinction between two somewhat different types of cross-integrational functions mediated by the corpus callosum: those which can be compensated for in congenital absence of the corpus callosum and those for which compensation is more difficult or impossible.
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From page 176... ...
On the basis of evidence collected from patients with congenital and surgical absence of the corpus callosum, as well as from the literature, this fundamental antagonism in the nature of these modes of brain functions might be a causal factor behind the evolution of cerebral dominance and lateral specialization in the human brain. What meaning this may have for problems of dyslexia remains to be seen.
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From page 177... ...
Some consequences of perinatal lesions of the visual cortex in the cat. Psychon.
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Key Terms
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