Night Vision Current Research and Future Directions, Symposium Proceedings (1987) / Chapter Skim
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Oculomotor and Spatial Orientation Factors
Oculomotor and Spatial Orientation Factors
Pages 81-140
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From page 83... ...
The important message underlying these papers is that vision is multidimensional and that it involves different kinds of tasks, particularly recognition and spatial orientation, with different functional relationships to ambient illuminance. It is also clear that to predict visual performance under demanding conditions, it is also necessary to consider the interactions among the visual, oculomotor, vestibular, and proprioceptive systems.
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From page 84... ...
84 cursory comparison of the two literatures reveals the increased scope of the contemporary problem. Meeting this challenge will both increase our understanding of basic mechanisms and, at the same time, assure us that recommendations and procedures for military personnel operating in visually demanding environments can be carried out efficiently and effectively.
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From page 85... ...
These normal variations of oculomotor behavior contribute to a number of problems of night vision, and they may help to explain and predict individual differences that are characteristic of nighttime visual performance. THE RESTING STATE OF THE EYES The resting state of the eyes is a key concept for understanding normal variations of oculomotor behavior.
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From page 86... ...
Relation to Clinical Indices While individual differences of the dark focus and dark vergence are not detected by standard clinical measures, there is some rather inconsistent evidence for a relation between the resting states and clinical indices. Two investigations reported evidence that the dark focus is inversely related to ametropia; that is, when far points are equated optically, hyperopes tend to have a nearer dark focus than myopes (Maddock et al., 1981; Epstein et al., 19811.
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From page 87... ...
87 _ ~0 Cal , ~ IX CY CD ~ Dud n be JO E t' .
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From page 88... ...
THE RESTING STATE AND VARIATIONS OF OCULOMOTOR PERFORMANCE The intermediate resting state concept has helped to clarify normal variations of oculomotor performance that occur under degraded stimulus conditions. In general, accommodation and vergence responses tend to be biased toward the subject's characteristic resting posture, and these biases increase progressively as stimulation is reduced.
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From page 89... ...
As luminance decreases, accommodative responses are increasingly iased toward the subject' s dark focus, producing anomalous ref. ractive errors for targets farther ~ night myopia)
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From page 90... ...
FIGURE 3 Scatter diagrams illustrating the relationship between indiv~dual differences in the dark focus and night myopia. Each diagram compares the sub jects' dark focus with their accommodative responses for a distant outdoor scene at the indicated light level.
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From page 91... ...
or a video display terminal (Murch, 1982~. Dark Vergence Bias Although studied less extensively, analogous biases of vergence toward the subject's dark vergence posture appear when fusional stimr uli are reduced.
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From page 92... ...
Other studies suggest that the dark vergence bias is related to problems of visual localization that occur under low illumination. These findings indicate that assessment of the dark focus and dark vergence may provide valuable information for predicting and enhancing nighttime visual performance.
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From page 93... ...
The anomalous refractive errors that result from this dark focus bias depend on the individual's dark focus. Therefore, it should be possible to predict and correct anomalous refractive errors, such as night myopia, by simply measuring a person's dark focus and providing a negative optical correction to compensate for the dark focus bias.
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From page 94... ...
Standard clinical refractions indicated that neither subject required corrective lenses. These studies support the hypothesis that anomalous refractive errors, such as night myopia and empty-field myopia, are due primarily to the dark focus bias of accommodation, and they demonstrate that optical corrections based on the dark focus can significantly enhance visual detection and recognition under degraded visibility conditions.
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From page 95... ...
The problem of prescribing night myopia corrections for moderately degraded conditions is illustrated by a Swedish study that used a laser optometer to compare anomalous refractive errors of 163 subjects, who wore their optimal daytime optical correction, at luminances of 120, Ge 15, and 0.001 cd/m2 (Epstein et al., 1981~. Consistent with the results illustrated in Figure 3, they found that the magnitude of night myopia increased from an average of -0.35 D at 0.15 cd/m2 to -1.01 D at 0.001 cd/m2.
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From page 96... ...
. No simple formula could be used to predict the best correction under the landscape condition from the laser optometer measures, although the optometer measures were useful for identifying subjects who might be susceptible to night myopia.
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From page 97... ...
This hypothesis was particularly attractive because both the specif ic distance tendency and the oculomotor resting postures exhibit large individual differences. Although this possibility has not been investigated as thoroughly as the relation of the resting state to anomalous refractive errors, the available evidence indicates that oculomotor biases do indeed contribute to spatial illusions in low illumination.
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From page 98... ...
Comparison of pre- and postadaptation measurements of distance perception and oculomotor resting states indicated that perceptual adaptation may be dependent on adaptive changes in the dark vergence posture. Viewing through the experimental spectacles for 20 min produced a sign recant increase in the perceived distance of the light point after the spectacles were removed (the typical aftereffect)
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From page 99... ...
Moreover, the wide individual differences of the resting states among normal observers imply that an individual's characteristic dark focus and dark vergence values may be valuable predictors of his/her night vision capabilities. Further information about several issues is needed, however, before routine assessment and utilization of resting state measures can be implemented.
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From page 100... ...
Over a series of presentations, the distance of the light source is var fed following a bracketing procedure which culminates when the red line and white point appear to be superimposed; at this point, the stimulus source is positioned at the subject's dark vergence distance. It is possible that this procedure affects the subject's dark vergence value (for example, the subject may change vergence when looking for the next stimulus flash)
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From page 101... ...
Population Norms While it is known that the dark focus and dark vergence postures exhibit large individual differences that are not detected in standard clinical exams, the parameters of these variables in the general (or military) populations are not yet known.
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From page 102... ...
It has been learned that whenever visual stimulation is degraded, as it is at night or in bad weather, both vergence and accommodation tend to shift involuntarily toward the individual's characteristic resting postures, which has been referred to as the dark focus and dark vergence. Although both resting states typically correspond to an intermediate distance, they exhibit wide individual differences, and the two resting postures of given individual often correspond to different distances.
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From page 103... ...
1974 On the position of the plane of stationarity in laser refraction. American Journal of Optometry and Physiological Optics .
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From page 104... ...
itz, H.W., and D.A. Owens 1975a Night myopia and the intermediate dark focus of accommodation.
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From page 105... ...
Ciufredda, eds., Vergence Eye Movements: Basic and Clinical Aspects. Boston: Butterworth Publishers.
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From page 106... ...
1974 Vergence eye movements. Documenta Ophthalmologica 37:153-214.
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From page 107... ...
This relative movement of the visual surround provides information for the organism that it is in motion. It also elicits postural adjustments which preserve spatial orientation ana reflexive eye movements which stabilize the retinal images of visual detail.
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From page 108... ...
Movement of the visual surround produces compensatory eye movements which minimize retinal image slip of the visual surround. These movements, termed optokinetic nYstagmus, consist of a slow or smooth movement in the direction opposite of head motion (in the same direction of any visual contour motion relative to the observer)
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From page 109... ...
Although the authors interpreted this finding as evidence that the retinal periphery dominates orientation responses, data on the effectiveness of central visual field detail on the stabilization of posture was not reported, thereby preventing quantitative comparison of central and peripheral vision. Preliminary measures have been obtained (R.B Post, unpublished data)
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From page 110... ...
A number of studies have indicated that stimulation of the central visual field contributes to a greater degree to OKN than peripheral field stimulation (e.g., Cheng and Outerbridge, 1975; Post et al., 1984~. Optokinetic stimulation of central vision in one direction and simultaneous stimulation of a larger area of the periphery by contours moving in the opposite direction produces an OKH that is appropriate for the central stimulus (Brands et al., 19731.
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From page 111... ...
report that the ability of subjects to fixate the center of a stimulus is not influenced, or is only slightly influenced, by variation of the luminance of the stimulus. Whereas the perceived direction of retinal stimulation seems little affected by changes in luminance, it is likely that information about eye position, that is, the direction of gaze, may be severely altered under conditions typical of night viewing, such as reduced visual fields or while viewing scenes containing moving detail.
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From page 112... ...
1967 The effect of change of target field luminance and colour on fixation eye movements. Optica Acta 14:213-217.
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From page 113... ...
Festroger, L., and A.M. Easton 1974 Inferences about the efference system baseo on a perceptual illusion produced by eye movements.
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From page 114... ...
Perception and Psychophysics 12:471-473. 1978 The loss of position constancy during pursuit eye movements.
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From page 115... ...
10:193-203. Vision Research Steinman, R.M.
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From page 116... ...
In the mid-1950s, Schober (1954) suggested that night myopia is a consequence of the intermediate resting position or dark focus of accommodation.
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From page 117... ...
FIGURE 1 Dioptric distance of accommodation as a function of individual resting position at three ambient illuminance levels. Source: Leibowitz and Owens (1975b)
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From page 118... ...
Identification of the mechanism of night myopia suggested a possible basis for amelioration by wearing a special nighttime correction which compensates, on an individual basis, for the tendency of accost modation to return passively to the individual's resting position. Subsequent research indicated that a negative correction equal to approximately one half of the individual's dark resting position increased resolution in a simulated laboratory task and improved clarity and ameliorated symptoms of visual fatigue during prolonged night driving (Owens and Leibowitz, 1976~.
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From page 119... ...
Accommodation was measured with a Badal laser optometer. The speckle pattern was superimposed in the visual field after reflection from a moving drum.
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From page 120... ...
Three estimates of threshold were obtained per condit ion (F igu re 4~. The results indicate a monotonic increase in accommodative distance from the monocular to the full dark focus viewing conditions.
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From page 121... ...
and those from the present study suggest that the presence of a clearly visible stationary fixation point or contours can reduce or obviate night myopia so that there is no basis for considering special corrections based on the individual's dark focus of accommodation. The most obvious basis for this improvement is vergence-induced accommodation which counteracts the tendency of the eye to return to its intermediate resting position.
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From page 122... ...
Owens, D.A., and H.W. Leibowitz 1976 Night myopia: Cause and possible basis for amelioration.
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From page 123... ...
Vision Research 25:1655-1659. Schober, H.A.W.
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From page 124... ...
. Thus it is becoming incr eas ing ly clear that the quality of the retinal image plays a critical role in the control of eye movements.
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From page 125... ...
. Large individual differences exist in the degree of adaptability of these systems (Ebenholtz, 1985; Schor, 1980~.
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From page 127... ...
o ~ c ~ o - ~ ~ · o ~ Q 0 ~ _ ._ i_ Q ° o in .0 o c x - 1 .0 127 1 1 ' 111 20° s.o 4.0 3.0 2.0 1.0 o 60° ~ 1 _ , _ 1 _ F+P F F+P F+P F , 111 \ 111 11111 \ 111 60~ rr 1 111 / T n: 16 tS 7 t (min)
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From page 128... ...
128 ~' ::: ~''W''-'`~ o.o Cal , , ,+_ _1 _ _ ~ , . 1 ' r34.-l ~ CAT LICE__ ~ ~ .1 FIGURE 3 View of apparatus.
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From page 129... ...
If this is true then it is reasonable to expect that the quality of these images, i.e., their contrast and spatial frequency, plays a modulating role in eye movement control. An additional implication may be drawn for the difference so~ietimes found when phenomena are compared during darkness and full illumination.
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From page 130... ...
A typical example in the first instance is found when unregistered after nystagmus causes the movement of retinal images to be interpreted, not as to eye movements but to actual movement of the surrounding environment. Table 1 lists relatively common modes of eye movement control according to the extent of the volitional component.
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From page 131... ...
are either not present or are represented with poor image quality during eye movements in darkness. Consequently, muscle spindle gain can be assumed to be lower during darkness than under full illumination conditions because of reduced y-efferent stimulation.
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From page 132... ...
Since individuals differ in their pursuit system cutoff frequency, the possibility of screening for high cutoff frequencies should be considered. Similar considerations should be given to the development of training procedures to enhance the efficiency of pursuit eye movements made in the context of vestibular stimulation (i.e., during and immediately after)
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From page 133... ...
Monty, and J.W. Senders, eds., Eye Movements: Cognition and Visual Perception.
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From page 134... ...
American Journal of Optomet ry and Physiological Optics 57: 618-631. Shebilske, W.L., C.M.
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From page 135... ...
Honrubia 1983 Effects of an optokinetic background on pursuit eye movements. Investigative Ophthalmology and Visual Science 24 :1115-1122.
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From page 136... ...
An alternative possibility seems to be that there is an internal signal for both kinds of eye movements, but for an involuntary eye movement, say a vestibular-ocular movement, there's also simultaneously a signal of the head movement, which would tend to cancel out the signal of eye movement. Can you distinguish between those two things?
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From page 137... ...
I think rather what is triggering it are head movements. I think you have to get into the system with a particular phase and frequency of head movement before you can trigger it.
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From page 138... ...
OWENS: To my knowledge, there are a couple of optometrists in private practice who use dark retinoscopy to prescribe for night myopia corrections for those patients who complain. Dark retinoscopy is just standard static retinoscopy without a fixation target.
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From page 139... ...
We have seen evidence that the dark focus predicts the best correction for empty-field myopia, and it is probably the best predictor for very dark conditions. Other data indicate that visual problems associated with the use of optical instruments and with fatigue from near work depend on the individual's resting state.
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