Click for next page ( 82


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 81
OC ULOMOTOR AND SPAT IAL ORI EN TAT I ON FACTORS .. ~

OCR for page 81

OCR for page 81
INTRODUCTION Herschel W. Leibowitz The importance of emphasizing oculomotor and spatial orientation derives in part from the history of research in vision. Because one of the most critical aspects of visual performance is the ability to read, visual research and tests have been directed toward evaluating and cor- recting reading ability. There are literally scores of studies in the literature describing the functional relation between visual acuity, contrast sensitivity, and luminance. However, the demands on the vis- ual system in the military context involve much more than reading or appreciation of contrast. Significant additional factors in the mili- tary environment include spatial orientation, prolonged exposure to low ambient illuminances, and concomitant stimulation of the vestibular system. A consequence of the multidimensional nature of visual demands in military operations is that our present understanding, as well as the tests available for evaluating visual performance, is incomplete. In particular, lowered illuminance results in significant changes in the oculomotor adjustments of accomodation and convergence. These changes, as well as their striking individual differences, have minimal conse- quences during daylight observations but become critical under low- luminance observation conditions. It is essential in studying human performance that our efforts be directed toward the "worst-case" con- ditions which include twilight and night observations, degraded visual environments, and observer motion. Not only is performance degraded at night but illusions, spatial disorientation, and distortions of distance and motion are also more likely. The papers in this section serve to define the problem, to document some encouraging progress that has been made during the past years, and to provide guidelines for future research. The important message under- lying these papers is that vision is multidimensional and that it in- volves different kinds of tasks, particularly recognition and spatial orientation, with different functional relationships to ambient illu- minance. 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. Reference has been mace to the fact that the present report repre- sents the first systematic attempt by the Committee on Vision to survey the night vision literature since the end of World War II. Even a 83

OCR for page 81
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.

OCR for page 81
NORMAL VARIATIONS OF VISUAL ACCOMMODATION AND BIN=UL~ WRGENCE: , SOME IMPLICATIONS FOR NIGHT VISION - D. Alfred Owens One of the axioms of vision science, as well as everyday experi- ence, is that visual performance generally deteriorates under low illu- mination. Since the earliest psychophysical research, results have been presented documenting the sensory changes that occur with reduced illumination, such as diminished color vision, acuity, and contrast sensitivity. More recently, behavioral research on eye movements has shown that these sensory changes are coupled with variations in the efficiency of oculomotor functions, particularly of accommodation and binocular vergence. Although the sensory processes that guide accommo- dation and vergence are not yet fully understood, both responses depend heavily on variables such as spatial contrast and retinal disparity for accurate and efficient control (e.g., Toates, 1972, 1974~. This infor- mation is gradually lost with reduced illumination, and, as a result, the accuracy and operating range of accommodation and vergence progres- sively decrease. These normal variations of oculomotor behavior con- tribute 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. Like the skeletal muscles, the eye muscles exhibit continued tonic activity in the absence of stimulation and active movement (Granit, 1970; Burian and van Noorden, 1974~. Contrary to popular traditional theories, however, the eyes' resting (or tonus) posture does not correspond to optical infinity. Rather, it typically falls at an intermediate distance, which varies widely among individuals with normal vision (Leibowitz and Owens, 1978; Owens 1984~. Although the intermediate resting state has been observed by several investigators during the past century (Cogan, 1937; Morgan, 1946; Schober, 1954; Weber, cited by Cornelius, 1861), it was not gen- erally accepted until recently when measurements of accommodation and vergence under open-loop conditions became available. 85

OCR for page 81
86 Individual Differences Typical values of the resting positions of accommodation and ver- gence are illustrated in Figure 1. These distributions represent measurements taken in total darknesss, referred to as dark focus and dark vergence. Both sets of data were obtained from the same sample of 60 college students (Owens and Leibowitz, 1980~. These data reveal two important and unexpected variations of the resting state of the eyes. First, both resting states exhibit substantial between-subject varia- bility. All subjects had ostensibly normal vision by conventional cri- teria, yet individual dark focus values ranged from about 25 cm to low hyperopia, and dark vergence values ranged from about 60 cm to infinity. These large individual differences are not picked up by standard clini- cal assessment techniques. The second point, which is not immediately obvious from Figure 1, is that the two resting postures for a given individual are often quite different. Although the mean dark focus and mean dark vergence corres- pond to an intermediate distance (76 and 116 cm, respectively), these mean values are significantly different. Moreover, within-subject comparison of the dark vergence and dark focus values illustrated in Figure 1 showed that they are only weakly correlated (r = 0.32~. These findings indicate that an individual's vergence and refractive state in darkness often differ greatly from those obtained under high illumina- tion, and they imply that knowledge of the resting (dark) state of one system (accommodation or vergence) does not allow prediction of the resting state of the other system. 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. A third investigation reported the opposite relation, i.e., that relative to the far point, the dark focus of myopes tends to be nearer than that of hyperopes (Simonelli, 19831. The basis for this discrepancy is not clear, but one possibility is a difference in the def inition of ametropia. Maddock et al. (1981) and Epstein et al. ( 1981) used standard clinical ret raction techniques to determine their subjects' refractive status, while Simonelli (1983) used measures obtained with a polarized Vernier opto- meter of the subjects' accommodative response for a distant monocular target. Comparisons of the dark vergence to measurements of phoria have also yielded inconsistent results. One study reported that dark vergence measures were signif icantly correlated (r = 0.62) with near phoria but not significantly correlated with distance phoria (Francis

OCR for page 81
87 _ ~0 Cal , ~ - IX CY - CD ~ Dud n be JO E t' . -or - ~ l 1 1 1 I ~t _ ~O ~_ 8 Q.} _ C) > o o o ~ o ~. . . ~ no ~ flu a nbeJ ~ - 8 - o - - ~lu c Vat --I a 0 .o In s 0 s JJ In - In set a, ~5 :' tQ ~ Q 0 u O 0 ~ In u' tQ 0 QJ v ~ - ~ ~Q y ~ a, CQ v e o a' ~Q u] o ~ - o ~ o co v Q U] .^ . - . - o ~5 o o U] . - O (q ~ _I O en ~ ~: C~ . o 11 - ~5 o V 3 o a ~4 a a) ~5 U] V ~ O - ~: - U] a U] U] - {Q s" ~ ~ tn u] ~ ~ v s ~ o co - Y N n3 3 o a) Q JJ s ~n a) o .. o U]

OCR for page 81
88 and Owens, 1983~. These differences are probably related to differences in the stimuli for accommodation and vergence in particular test situa- tions and to complex synergistic interactions of accommodation and ver- gence that arise from stimulation of either system. THE RESTING STATE AND VARIATIONS OF OCULOMOTOR PERFORMANCE The intermediate resting state concept has helped to clarify nor- mal 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. Dark Focus Bias Johnson (1976) was among the first to report progressive biases of accommodation with reduced illumination. He used a laser optometer to measure accommodative responses for a high-contrast target, similar to a reticle, viewed monocularly in Maxwellian view at four luminance levels ranging from 0.051 to 51.4 cd/m . His results for four subjects are presented in Figure 2 as mean accommodative response functions under the four luminance conditions. Under the highest luminances, all sub- jects exhibited accurate focusing performance, with response functions approximating the ideal prediction indicated by the dashed line. As luminance was reduced, however, accommodation became increasingly myopic for stimuli nearer than the dark focus. At the lowest luminance, accom modation remained near the dark focus regardless of stimulus distance. These data show that the operating range of accommodation gradually collapses toward the dark focus, resulting in functional presbyopia, as illumination is reduced through low photopic to mesopic levels. This tendency is also found with more complex natural stimuli. In one study, Leibowitz and Owens (1975a) measured the refractive state of 30 subjects while they viewed total darkness and a distant outdoor scene at three luminances ( full daylight and with light reduced 1. 95 and 4.2 log units by neural-density filters). The results are illustrated in Figure 3 as scatter diagrams comparing individual dark focus values with the sub- jects' accom~r.odative responses for the outdoor scene. In this case, accurate focusing responses would yield data falling along the horizon- tal line at 0 diopter~ (D), and accommmodative responses equivalent to the sub jects' dark focus would yield data falling along the diagonal theoretical line. Consistent with Johnson's (1976) results, as lumi- nance was reduced from daylight to an approximation of nautical twi- light, accommodative responses shifted progressively toward the sub- jects' characteristic dark focus values. As a result, subjects with a relatively near dark focus became increasingly myopic for the distant scene, while these with a relatively distant dark focus showed little change in refraction. It is important to note that biases of accommodation toward the dark focus are not conf ined to conditions of low illumination. Indeed,

OCR for page 81
89 3.0 J.G. ~ = 1 ~ 2.O Q o _ ~ i.0 D.J .~ ~ ,, ~ it' it' a' -A 30 o 2.0 l.0 ! ~! 1 1.0 2 C) 3.0 0 1.0 2 0 SO QGcominodative St,'m~l~> Distance (Dioptefs) t ~ S K. >=I ~D Am. =I .? 5142 cd,~rr2 ' 6.14 cd/m2 Q5i cdim2 O.~! C~,,~2 I',' in, ~ FIGURE 2 Accommodative response functions of four subjects for a mono- cular reticle stimulus viewed at luminance levels ranging from 51.4 to 0.051 cd/m2. Individual dark focus values are indicated by the black arrows. As luminance decreases, accommodative responses are increas- ingly iased toward the subject' s dark focus, producing anomalous ref. rac- tive errors for targets farther ~ night myopia) and nearer ~ night hyper- opia) than the resting posture. Source: Johnson ( 1976~ .

OCR for page 81
so 3 2 - DAYLIGHT / r=0.38 / 1 / cn rat _~ Q o ~ _ `_7 . ~ ~ 2 - . _ o o O o o c: Cl: 3 2 1 / 7 . . ~. ~ ,( - 's ~ o . i.95 fog units Coffer/ r=0.63 / ~_.~e _-'/ x,,, ~ . .s . a.2 log units town r = 0.70 /~ :,~' /,,' ~ ,, . , - ., i'. , / . . O // 1 2 3 Focus in Tofol [darkness (droplets) FIGURE 3 Scatter diagrams illustrating the relationship between indi- v~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. As luminance decreased from daylight to nautical twilight, focusing responses shifted toward the dark focus, indicated by the diagonal theoretical lines. Thus, subjects with a near dark focus exhibited correspondingly greater twilight and night myopia. Source: Le~bowitz and Owens ( 1975a) .

OCR for page 81
91 they appear under a wide variety of commonly occurring viewing condi- tions, for example, (1) when viewing a bright empty field (Whiteside, 1952; Westheimer, 1957; Leibowitz and Owens, 1975b); (2) when using optical instruments (Hennessy, 1975) or viewing through small pupils (Hennessy et al., 1976~; (3) when viewing distant objects through an intervening surface or screen positioned near the distance of the sub- ject's dark focus (i.e., the Mandelbaum effect) (Owens, 1979~; and (4) when reading text presented by projection devices (Kintz and Bowker, 1982) 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. This was first demonstrated by Ivanoff and Bourdy (1954), who found increasing fixation disparity (i.e., vergence errors) as the luminance of a peripheral binocular stimulus decreased to sco- topic levels. The pattern of fixation disparity varied with the sub- ject and the distance of the target. In general, convergent fixation disparities were found with a distant target, and divergent fixation disparities were found with a near target. The magnitude of these fixation errors varied among subjects, however, with some subjects showing a greater bias toward near fixation (i.e., larger convergent fixation disparity), while others showed a greater bias toward far fixation (lee., larger divergent fixation disparity!. This finding foreshadowed later evidence for individual differences in dark ver- gence (Figure 11. More recently, Francis and Owens (1983) found that changes in vergence accuracy are related to the subject's dark vergence posture. Subjects viewed dim binocular stimuli presented over a range of dis- tances, with retinal eccentricities ranging from 2 to 8 degrees. With increasing retinal eccentricity, vergence responses were progressively biased toward the individual's dark vergence distance, resulting in convergent errors for farther targets and divergent errors for nearer targets. Thus, as fusional stimulation is degraded, the vergence sys- tem is biased toward its resting state in much the same manner as the accommodative system. Summary Research conducted mostly during the past decade has led to a new conception of the behavior of accommodation and vergence. In opposi- tion to traditional views fundamental to modern clinical practice, vergence and accommodation do not relax at the far point of their operating range. Rather, they typically rest at an intermediate dis- tance, which averages about 1 m and varies widely among normal sub- jects. When visual stimulation is degraded, as with reduced illumi- nation, accommodation and vergence responses are progressively biased toward their respective resting postures. These biases produce

OCR for page 81
130 performance that frequently accompany changes in level of illumination is possible. In addition to the static aspects of stimulation discussed above, optokinetic stimulation has long been appreciated as a source of eye movement control in the service of gaze stability (ter Braak, 1936; Dichgans, 1977), with the capability of driving the eyes around all three axes (Crone, 1975~. Optokinetic patterns are highly functional in normal environments, serving to match eye and image velocity, but they also serve as an updating system for the vestibular ocular res- ponse (VOR). Since the VOR is a feed-forward system, error correction takes place via an external optokinetic loop, a pursuit system loop, or both (Miles and Lisberger, 1981; Ebenholtz, in press). Accordingly, the consequences of poor image quality for the several systems subser- ving gaze stabilization appear to be considerable. Furthermore, an interesting observation is that optokinetic expansion-contraction patterns may have specialized motion-detecting channels (Beverley and Regan, 1980) and also may drive accommodation and convergence (Ittelson and Ames, 1950; Kruger and Pola, 1985~. Although in the latter case the relation between image quality and stimulus adequacy remains to be established, the functional value of kinetic expansion patterns may lie in their capability to signal changes in distance over a range of distances well beyond the several meters usually assigned to accommo- dation and convergence. PRINC IPAL SOURCES OF OCULOMOTOR-ElASED ILLUSI ONS While it may be granted that static and kinetic sources of optical stimulation play a significant role in the control of eye movements, if the oculomotor control system did not contribute to perception, there would be few implications to be drawn for the understanding of visual illusions. In fact, the conscious registration of disjunctive and conjugate eye positions is capable of signaling distance, direc- tion, target orientation (Ebenholtz, in press) and, in general, the three-dimensional spatial path of a moving target. Consequently, the adequacy of registration of the position and trajectory of the eye determines the likelihood of oculomotor-based spatial illusions. In this respect, perhaps the single most important heuristic principle is that reflexive movements do not tend to be registered in consciousness, whereas voluntarily initiated eye movements do lead to an awareness of gaze direction and path. 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 move- ment of the surrounding environment. Table 1 lists relatively common modes of eye movement control according to the extent of the volitional component. Given these two categories of eye movement control, illusions of target position and movement path result from one of two sets of condi- tions. These are (1) maintaining fixation in the presence of perturba- tions of a reflexive nature (Post and Leibowitz, 1985; Whiteside et al., 1965y, and (2) maintaining fixation after set points in adaptive

OCR for page 81
131 TABLE 1 Eye Movement States with Dominant Reflexive and Volitional Components Reflexive Initiation Vestibuloocular and obolith driven Optokinetic and optostatic Vergence Volitional Initiation Fixation and pursuit oculomotor systems have been altered (Ebenholtz, in press). Since vol- untary movements may be associated with ,-efference (Merton, 1972) and since the latter is known to increase muscle spindle gain (~hitteridge, 1959), it seems reasonable to propose that increased sentience of the volitional states results from the competitive action of the reflexive eye movements. It is thus out of the need to balance voluntary against automatic modes of eye movement control that illusions are produced. On the other hand, competing patterns of control provided by stationary backgrounds (Yee et al., 1983; Raymond et al., 1984b) 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. Hence, lower sentience or awareness of eye position and eye trajectory (and, hence, of target movement path) is to be expected in darkness. IMPLICATIONS FOR RESEARCH, SELECTION, AND TWINING At dim illumination levels, the contrast between surface edges and backgrounds and between a figure and the ground diminish. At the same time accommodation to targets falls toward the resting level (Johnson, 1976~. The net effect of these processes is to bias the composition of retinal images in night vision in favor of low spatial frequencies, as if one were scanning an environment through low-band-pass filters. Since these images continue to play an important role in the control of oculomotor performance, however, it seems likely that important changes will occur in the efficiency of oculomotor control in night vision. There appears, however, to be a gap in knowledge of the potential inter- actions between a visual system in photopic, mesopic, and scotopic states of dark adaptation and various aspects of oculomotor control, such as pursuit tracking capability, peak optokinetic response, and the ability to maintain foveal fixation when in conflict with vestibular reflexes or peripheral optokinetic stimulation. On this last point, there is some recent evidence to indicate that a high-spatial-frequency, head-fixed target is critical to the suppression of optokinetic nystag- mus (Howard and Ohmi, 1984), but the role of levels of dark adaptation remains to be examined systematically.

OCR for page 81
132 As more is learned about the relation between the dark-adapted eye and oculomotor control, it may be advantageous to select aircrew on the basis of characteristics of their contrast sensitivity function. If detection and tracking of low-spatial-frequency images is regarded as an important performance characteristic, then screening for high sens- itivity to low-spatial-frequency targets would be appropriate. It is worth emphasizing, perhaps, that in a moving observer, target fixation, i.e., acquisition, and tracking occur frequently under condi- tions in which the VOR has been triggered. Under these conditions, the suppression of VOR by the foveal pursuit system is the indispensable mechanism underlying both target acquisition and localization. It is reasonable to suppose, therefore, that the larger the bandwidth of one's pursuit system frequency-response function (prior to cutoff), the more efficient the pursuit system will be in canceling perturbations emanat- ing from vestibular stimulation. 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 imme- diately after). Such training should take place with gravitoinertial force vectors approaching those encountered in actual flight. Analogous selection and training procedures should also be explored to examine the foveal pursuit system when it is perturbed by competing optokinetic patterns. Oculomotor performance data also are needed to know whether individuals without adaptive control systems (vergence, accommodation, VOR) suffer a higher than normal frequency of asthenopic and motion sickness signs and symptoms. Individuals should be tested for adaptive effects in dark focus, vergence, and VOR. Finally, and most importantly, there is the implication from this analysis of oculomotor function that full advantage should be taken of the current fund of knowledge to maximize night vision performance among aircrew members. To do so would entail as a long-term objective the establishment of a computerized visual function data base to make possible retrospective studies relating visual and oculomotor function to actual performance measures. Such a data base should cover a broad array of visual and oculomotor parameters thought to be relevant to various aspects of flight performance. The following is a sample of such a set of parameters: 1. dark focus, 2. resting vergence, and 3. temporal dark adaptation functions. The following parameters should be measured at mesopic and scotopic levels: 4. peak OKN for a target of known spatial frequency and contrast, 5. pursuit cutoff frequency for a foveal target of known spatial frequency and contrast, contrast sensitivity function,

OCR for page 81
133 7. pursuit suppression of the VOR, 8. pursuit suppression of OKN, 9. adaptability of the gain of the VOR, and 10. adaptability of the resting focus and resting vergence. REFERENCES Beverley, K.I., and D. Regan 1980 Temporal selectivity of changing-size channels. Journal of the Optical Society of America 70:1375-1377. Bielschowsky, A. 1938 Lectures on motor anomalies. 1. m e physiology of ocular movements. American Journal of Ophthalmology 21:843-854. Burian, H.M. 1939 Fusional movements: Role of peripheral retinal stimuli. Archives of Ophthalmology 21:486-491. Crone, R.A. 1975 Optically induced eye torsion. II. Optostatic and optokinetic cyc~oversion. Albrecht v. Graefes Archives fur klinisches und Experimentelle Ophthalmologie 196:1-7. Dichgans, J. 19?7 Optokinetic nystagmus as dependent on the retinal periphery via the vest~bular nucleus. Pp. 261-267 in R. Baker and A. Berthoz, eds., Control of Gaze by Brain Stem Neurons. Amsterdam: E1sevier. Ebenholtz, S.M. 1981 Hysteresis effects in the vergence control system: Perceptual implications. In D.F. Fisher, R.A. Monty, and J.W. Senders, eds., Eye Movements: Cognition and Visual Perception. Hillsdale, N.J.: Lawrence Erlbaum Associates. 1983 Accommodative hysteresis: A precursor for induced myopia? Investigative Ophthalmology and Visual Science 24:513-515. 1985 Accommodative hysteresis: Relation to resting focus. American Journal of Optometry and Physiological Optics 62:755-762. In Properties of adaptive oculomotor control systems and press perception. Acta Psychological In press. Ebenholtz, S.M., and S.K. Fisher 1982 Distance adaptation depends upon plastic-~y in the oculomotor control system. Perception and Psychophysics 31:551-560. Frisby, J., and J. Mayhew 1982 m e role of spatial frequency tuned channels in vergence control. Vision Research 20:727-732. - Howard, I.P., and M. Ohmi 1984 The efficiency of the central and peripheral retina in driving human optokinetic nystagmus. Vision Research 24:969-976. Ittelson, W.W., and A. Ames, Jr. 1950 Accommodation, convergence and their relation to apparent distance. Journal of Psychology 30:43-62.

OCR for page 81
134 Johnson, C .A. 1976 Ef f ects of luminance and stimulus d istance on accommodation and visual resolution. Journal of the Optical Society of America 66:138-142. Krishman, V.V., and L. Stark 1977 A heur ist ic model for the human vergence eye movement system. IEEE Transactions on Biomedical Engineering, BME-24, 44-49. P. B ., and J. Pola What drives accommodation? Changing size and blur in oppo sition. Amer ican Journal of Optometry and Physiological Optics (Abstract) 62:7P. Merton, P.A. 1972 How we control the contraction of our muscles. American 226:30-37. Miles, F.A., and S.G. Lisberger 1981 Plasticity in the vestibulo-ocular reflex: A new hypothesis. Annual Review of Neuroscience 4:273-299. - Owens, D.A. 1980 A comparison of accommodative responsiveness and contrast sensitivity for sinusoidal gratings. Vision Research 20:159-167. Post, R.B., and H.W. Leibowitz 1985 A revised analysis of the role of efference in motion pe rception. Perception 14 :6 31-643 . Post, R.B., C.L. Shuperb' and H.W. Leibowitz Implications of OKN suppression by smooth pursu it for induced motion. Perception and Psychophysics 36: 493-498. Raymond, J.E., IBM. Lindblad, and H.W. Leibowitz 1984a The effect of contrast on sustained detection. Vision Research 24 :183-188. Raymond, J.E., K.L. Shapiro, and D. Rose 1984b Optokinetic backgrounds affect perceived velocity during ocular tracking. Perception and Psychophysics 36: 221-224. Schor, C. 1980 Fixation disparity: A steady state error of disparity-induced vergence . American Journal of Optomet ry and Physiological Optics 57: 618-631. Shebilske, W.L., C.M. Karmichl, and D.R. Proffitt 1983 Induced esophoric shifts in eye convergence and illusory distance in reduced and structured viewing conditions. Journal of Exper imental Psychology: Human Perception and Performance 9: 270-277. ter Brask, J.W .G. 1936 Untersuohungen uber optokinetischen Nystagmus. Archives Neer- landaises de Physiolog ie de 1' homme et des Animaux 21: 309-370. (Translation in H. Colleweijn, Oculomotor System of the Rabbit and its Plasticity. New York: Springer-Verlag, 1981. Toates, F.~. 1970 A model of accommodation. Vision Research 10:1069-1076.

OCR for page 81
135 Tredici, T .J., and R.E . Miller 198S Night vision manual for the flight surgeon. Special Report 85-3, U.S. Air Force School of Aerospace Medicine, Aerospace Medical D ivision {AFSC ~ Brooks Air Force Base, Texas. Westheime r, G ., and D .E . Mitchell 1969 The sensory stimulus for disjunctive eye movements. Vision Research 9: 749-755. Whiteside, T.D.M., A. Graybiel, and J.I. Niven 1965 Visual illusions of movement. Brain 88 :193-210. Whitter idge, D. 1959 The ef feet of stimulation of intrafusal muscle f ibres on sens itivity to stretch of extraocular muscle spindles. Quarterly Journal of Experimental Physiology 44 :385-393. Yee, R. D., S .A. Daniels, D.W. Jones, R.W. Baloh, and V. Honrubia 1983 Effects of an optokinetic background on pursuit eye movements. Investigative Ophthalmology and Visual Science 24 :1115-1122. ..( .

OCR for page 81
GENERAL DI SCUSSI ON _ HARVEY: This is one for Bob Post. It has to do with flow patterns in movement. Is there a controversy today about the effects of fixa- tion when you're not f ixating in the direction of movement, where the flow pattern originates from? Or as I understand it, the focus of a flow pattern of motion shifts when you look at a direction different from the motion, and that then can give rise to very strange signals of which way your body is actually moving in space. POST: I don't know if there's a big controversy. In a paper by Regan and Beverley, there is a demonstration that the focus of expansion of a flow pattern corresponds to where fixation is rather than the point of collision, which you're headed toward. This contradicts the previous position of Gibson. BEDELL: Dr. Ebenholtz, I understood you to say that there was not an internal signal for reflex eye movements, whereas there was one for voluntary eye movements. 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? EBENHOLT2: I'd like to add a third hypothesis as well. I have been wondering along with others, for a long time, what in the world muscle spindles are doing in ocular muscles? And so it seems to me that I would like to pose a hypothesis that under reflexive control you simply are lacking the gamma efferent stimulation. And as a consequence, gain is low but is probably registered nonetheless. I am sure that some part of the brain has that recorded but that the gain is so low that conscious centers are not aware of reflexive eye movements. So I think that we have then the possibility of several nice hypotheses that now need to be tested. COPENHAGEN: I had a question about plasticity in the system. It almost seems as though we're approaching Heisenberg's uncertainty prin- ciple where you can get changes in the vestibular-ocular reflex in 10 minutes, so by testing it you're automatically changing it. We can see reorganization in the somatosensory cortex in a period of a few minutes. Would you like to comment on that? 136

OCR for page 81
137 EBENHOLTZ: I have no doubt that, depending on the starting point of the VOR, certain test procedures may indeed alter the very gain that you want to measure. The problem is complicated because the changes in gain, and also changes in phase that have been induced in VOR, decay at differential rates. Now if you imagine that these gain changes are varying from day to day and from time to time, depending upon the tasks that we have been involved in before getting to the laboratory, then you have differential decays of changes in VOR and phase gain back perhaps to some normal level. So that if you really want to measure VOR with some certainty you probably have to take the measurements after a period of isolation to begin with. At least we have a handle on the variance that one finds in these measures. It's in the very plasticity itself. MACLEOD: I'm curious about this plasticity you're discussing. Specifically, I'm wondering if anything is known about whether this adaptation can be situation contingent. For example, when a spectacle wearer adapts to an inappropriate perturbation of his VOR input, can he eventually learn to deal with the situation in such a way that when he takes them off, the system instantly readjusts? It sounds like a trivial question, but I think it has significant implications for how sophisticated an adaptation process is: whether it simply accumulates vestibular and retinal information and makes a blind correlation or whether it makes use of every sort of cue that is available. EBENHOLTZ: I think it is a very important question. People inter- ested in development of simulators--where anyone in the simulator will make certain head movements at the time when his eye is on the target-- are also very interested in whether these things transfer. Now I know that you can induce changes simultaneously in the VOR and in different directions. For example, from the VOR, changes that I've investigated in my laboratory, one obtains the illusory movement when the head moves in the horizontal direction. It is a very specific adaptive shift. If you move your head up and down (nodding) you do not get illusory move- ment unless you adapted initially also in the vertical plane. So we know that the nature of the plasticity, especially in the VOR, is very highly specific. Can you control the changes in VOR, or perhaps other ocular-motor systems, through cognitive links? There are one or two studies in the literature of prism adaptation that seem to suggest that simply the prism frames were enough to trigger the effect, but I remain rather skeptical. 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. But it is cer- tainly an open question as to whether we can train multiple adaptations simultaneously and simply trigger them by using the top-gown paradigm. We need some more research on this question. JOHNSON: I have a question for either Hersch or Jim [Sheehy] about trying to come up with rapid, simple, cark-focus corrections for low- luminance situations. I '11 just play devil's advocate here for a second. Why worry about the dark focus at all? Why worry about the theoretical implications? Why not just go out and define a performance task, fit the observer with a series of lenses, and find the corrections

OCR for page 81
138 for which performance is optimal? You circumvent all the problems of trying to make measurements of oculomotor responses, developing instru- mentation, and then translating the results into something that again gets back to performance. LEIBOWITZ: That is what we were trying to do with the field studies. We did not use any instruments, but rather had subjects sit in the car and rotate the square-wave grating (by means of a motor) illuminated by the automobile headlights until it could be resolved. What was surprising was that in almost every case a correction ba see on the dark focus of accommodation degraded acuity, which, in turn, led us to question our night myopia correction procedure. In practice, there is another consideration because during driving, the problem of seeing small stimuli, which are most affected by refractive errors, is not very great. Although contrast is certainly critical during night driving, there are very few stimuli subtending small visual angles that are cri- tical during nighttime driving. The implication of these data is to question the basis for night vision corrections based simply on the dark focus. Regarding your suggestion, one could follow the procedure practiced by some people when they buy glasses. They just go into a store and try them on. I am confident we can do better than this, but in any event, as you suggest, the final criterion must be performance in the relevant real-world situation. KINNEY: I had another question along the same lines. Did I under- stand you to say that the test in the field was 16 cycles/degree? And did you use any ether one? LEIBOW~TZ: The target, when oriented vertically, presented a 15-cycle/degree square wave. The procedure was to start at a random set point and rotate until the observer could just discern the stripes. he then measured the angle subtending by the target and calculated the threshold in cycles per degree. Contrast of the target, which was i~lu- minated by standard automobile headlights, was 77 percent. KINNEY: Have you tried lower spatial frequencies? I would think that woula be more applicable to the laboratory studies, that this would be a major difference between the two, if the dark focus does not work so well for the higher ones. SHEEHY: Bell, in the laboratory we tried to number different spa- tial frequencies. The f irst one we started with was a 4-cycle/degree pattern, and we found absolutely no effect except that people were less sensitive. So with that we switched to ~ 16- and then 24-cycle/degree pattern, both sinusoids. Again, they were adjusting the contrast of that pattern while wearing their corrections. The results were iden- tical. MONACO: Fred [Owens!, Her sch had mentioned something about the dark retinoscopy. I was wondering whether you might want to elaborate a little on that in terms of its being an effective tool for defining the dark focus value. Is there ongoing work with that? OWENS: To my knowledge, there are a couple of optometrists in pri- vate 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. Now, theory has always held that it is essential to control accommodation; the standard

OCR for page 81
139 techniques are either by cycloplegia or by having the patient fixate a distant acuity chart during the retinoscopic exam. We were surprised that a retinoscope beam is not an effective stimulus for accommodation, that when a patient is fixating the retinoscope beam without any other visible contours, their accommodation returns to the individual's dark- focus state. So it is possible to measure with a clinical instrument that's readily available. That's the advantage of dark retinoscopy. I don't think we have sufficient data to indicate how effective this technique would be for wide-scale screening. I'd like to take the opportunity to come back to a point that Dr. Johnson raised a few moments ago about "why measure the resting state anyway?" There may be a couple of good reasons. One is that for individuals who recognize their problems with night vision, we found measurement of their dark focus to be quite effective in coming up with a correction that helps them--and without going out in the field and doing a sort of empirical "trial-by-error" determination. Perhaps a better argument, though, is the general importance of ocu- lomotor bonus for visual performance under a wide range of conditions. 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. For example, individuals who have a near-dark focus and near-aark vergence are less susceptible to fatigue from near work. I suspect this is related to the sort of plas- t~city that Dr. Ebenholtz was talking about. Symptoms of visual fatigue appear to be related to changes of oculomotor tonus of the kind that Dr. Ebenholtz discussed; the problems might be avoided by setting the working distance to correspond to the individual's resting posture. My point is that we may be able to predict and correct a number of different problems on the basis of the resting state, which would be preferable to separate attempts to solve each problem empirically. The night-driving situation, in particular, may be problematic--because it involves a number of interacting factors, such as variations in low- level response gain and vergence-accommodation synergy.

OCR for page 81