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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please DARK-FOCUS: ANOMALOUS REFRACTIVE ERRORS AND ACCOMMODATION 14 CONCLUSIONS AND RECOMMENDATIONS The working group concludes that the contrast sensitivity function offers potential for characterizing individual differences not captured by conventional high contrast visual acuity measures. We further conclude that the contrast sensitivity, possibly combined with some measure of phase sensitivity, offers potential for predicting real-world performance on tasks involving complex visual stimuli. We therefore recommend that applied studies on the relationship between contrast sensitivity functions and visual task performance be carried out. We recommend at least two types of studies: (1) The value of the contrast sensitivity function in predicting performance on complex visual tasks and in discriminating among individuals in their ability on these tasks needs to be further explored and (2) The relationship between amplitude and phase information in the perception of simple and complex stimuli must be better understood. DARK-FOCUS: ANOMALOUS REFRACTIVE ERRORS AND ACCOMMODATION Predicting an individuals performance under adverse conditions poses an important challenge to vision research and assessment. Present techniques have proven to be invaluable for screening and optimizing visual performance under ideal stimulus conditions, such as reading high contrast text or distant signs in bright illumination. They often fall short, however, when visibility is reduced by low illumination or inclement weather. Individuals who have comparable visual capabilities under normal conditions can differ greatly under adverse viewing conditions. It is not uncommon, for example, to find two pilots who both have excellent visual acuity in the examining room, yet, at night or in a bright empty sky, one can consistently detect and identify targets faster than the other. Recent research on visual accommodation reveals one of the mechanisms responsible for individual differences in target detection and recognition, and it has led to the development of a new approach to predicting and to optimizing performance under low visibility conditions. BACKGROUND use the print version of this publication as the authoritative version for attribution. The major insight from this research can be summarized quite simply. Whenever visual stimulation is degraded, as it is at night or in bad weather, the eyes tend to adjust involuntarily for a distance determined by the individual's “resting” or “tonus” state. This resting focus, also referred to as the “dark-focus” and “tonic accommodation,” usually corresponds to an intermediate distance, but it varies widely across observers. Figure 8 shows the distribution of dark-focus values of 220 college students who were either emmetropic or were wearing their normal refractive corrections (Leibowitz and Owens, 1978). Individual dark-foci

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please DARK-FOCUS: ANOMALOUS REFRACTIVE ERRORS AND ACCOMMODATION 15 ranged from slight hyperopia (−0.25 diopters) to strong myopia (4.0 diopters), with an average value of 1.5 diopters, which corresponds to a focal distance of only 67 cm. use the print version of this publication as the authoritative version for attribution. FIGURE 8 Distribution of dark-focus values of 220 college students. All measures were taken with a laser optometer in total darkness with the subjects' normal refractive correction in place. SOURCE: Leibowitz and Owens, 1978. Reprinted with permission from H. W. Leibowitz. Copyright 1978 by Dr. W. Junk Publishers.

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please DARK-FOCUS: ANOMALOUS REFRACTIVE ERRORS AND ACCOMMODATION 16 Low Visibility Conditions As visibility of a stimulus is reduced, accommodation is progressively biased toward the individual's dark- focus. This phenomenon is illustrated in Figure 9 for a hypothetical subject whose dark-focus corresponds to 1.0 diopter. Note that the accuracy of accommodation depends on two factors: the quality of the stimulus and the subject's characteristic dark-focus. For strong stimuli, such as a bright acuity chart, accommodative responses are fairly accurate, producing a response function with a slope that approaches the ideal value of 1.0. With weaker stimuli, the eyes' focusing range gradually diminishes, producing response functions with progressively shallower slopes. With very weak stimulation, accommodation remains at the dark-focus regardless of stimulus distance (Johnson, 1976). Thus, the eyes become functionally “presbyopic” when stimulation is degraded; they become “myopic” for targets located beyond the dark-focus and “hyperopic” for targets nearer than the dark- focus, focusing accurately only for targets located at the same distance as their dark-focus. A similar bias toward the dark-focus is observed when the eye's depth of focus is increased by reducing pupillary diameter (Hennessy et al., 1976). In this case, the loss of accommodative responsiveness presumably results from “opening the loop” of the accommodation control system rather than from degrading the visual stimulus. Thus, whenever variations of the eye's focus have no appreciable effect on the quality of the use the print version of this publication as the authoritative version for attribution. FIGURE 9 Hypothetical response functions illustrating the effect of reduced stimulation on accommodative responsiveness. SOURCE: Owens, 1984. Reprinted with permission from D. A. Owens. Copyright 1984 by Sigma XI Scientific Research Society.

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please DARK-FOCUS: ANOMALOUS REFRACTIVE ERRORS AND ACCOMMODATION 17 retinal image, as with small pupils or with images formed by optical interference in the plane of the retina, accommodation returns to the individual's dark-focus adjustment (Leibowitz and Owens, 1978). This phenomenon is most commonly encountered when using optical instruments such as binoculars or microscopes (Hennessy, 1975), and it may be related to problems of size and distance perception that occur with artificial display systems. This normal variation of the eyes' focusing behavior can greatly hinder the ability to detect weak stimuli and to resolve fine details, and it provides a basis for understanding and correcting a variety of anomalous refractive errors that have puzzled vision specialists for years. Nearly 200 years ago, the British astronomer Nevil Maskelyne reported that he became myopic in dim illumination (Levine, 1965). This problem, called night or twilight myopia, was rediscovered on several occasions and was a topic of great research interest during the 1940s and 1950s (e.g., Wald and Griffin, 1947). These studies found large individual differences in the magnitude of night myopia that were not predictable on the basis of standard clinical examinations. Furthermore, they revealed that similar anomalous myopias occur under bright-light conditions, including “empty-field” or “space myopia” (Whiteside, 1952, 1957; Westheimer, 1957) and “instrument myopia” (Schober, 1954; Hennessy, 1975). Later work showed that some individuals also become “myopic” when viewing distant objects through an intervening surface or screen, a phenomenon called the Mandelbaum effect (Mandelbaum, 1960; Owens, 1979). Optical Corrections We now know that these anomalous “myopias” are not refractive errors in the usual sense; that is, they are not due to structural characteristics of the eyes; rather they arise from normal variations of accommodative responsiveness. When stimulation is weak, the eye tends to stay at its resting focus (Figure 9). Since the resting focus is not closely correlated with refractive status, these focusing errors are not predictable on the basis of standard measurements of refraction (Maddock et al., 1981; Simonelli, 1983). They can be corrected, however, by simply providing a spectacle prescription based on the individual's dark-focus. In effect, this prescription optically repositions the individual's dark-focus so that it matches the distance of the visual task. The visual enhancement resulting from these special optical corrections depends on two factors: (1) the individual's characteristic dark-focus and (2) the quality of the available stimulation. In general, the greatest benefits for distance vision are obtained under the most degraded stimulus conditions and for subjects who have a relatively near dark-focus. Spectacle corrections based on the dark-focus were found to enhance visual resolution by as much as 25 percent under simulated night driving conditions (Owens and Leibowitz, 1976b). Even greater benefits have been obtained with corrections of empty-field myopia, a problem of special concern to pilots. One study found that the detection range of a small target (1 min arc) in a bright ganzfeld (an evenly illuminated use the print version of this publication as the authoritative version for attribution.

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please DARK-FOCUS: ANOMALOUS REFRACTIVE ERRORS AND ACCOMMODATION 18 field without focusable contours) improved from 26 to 315 percent, depending on the subject's dark-focus distance (Post et al., 1979). Another study found that spectacle corrections based on the dark-focus improved contrast sensitivity by as much as a factor of six for circular targets (diameter from 1.0 to 7.5 min arc) in a ganzfeld (Luria, 1980). Generally, the smaller the stimulus, the greater the effect of space myopia. In all cases, the greatest improvements were obtained for subjects with relatively near dark-focus values. It is interesting to note that the subjects exhibiting the greatest and the least improvement in Luria's (1980) study were both clinically emmetropic--i.e., standard clinical tests of refractive error indicated that neither subject required corrective lenses. We want to emphasize that the same dark-focus prescription is not appropriate for all low visibility conditions. Since accommodative responsiveness decreases gradually with reduced stimulation, optimal correction for moderate visibility conditions, such as night driving, is a compromise between the individual's usual daytime prescription and the full dark-focus correction (Owens and Leibowitz, 1976). In contrast, performance in empty-field conditions is best with the full dark-focus correction (Post et al., 1979). The Mandelbaum Effect and Dirty Windscreens Accommodative biases toward the resting focus are not confined to low visibility conditions. As mentioned earlier, despite effort to see a distant object, the eyes will involuntarily focus for an intervening screen that is positioned near the distance of the dark-focus (Owens, 1979). This phenomenon, the Mandelbaum effect, could be especially hazardous to pilots who are attempting to see distant aircraft or beacons through a dirty or scratched windscreen, and Roscoe (1982; Roscoe and Hull, 1982) has argued that this problem should be an important consideration for cockpit design. Other studies have shown that accommodation for cathode ray tube (CRT) displays is also biased toward the user's resting focus (Kintz and Bowker, 1982), suggesting that reading glasses prescribed on the basis of the resting focus may be of some benefit for near visual tasks (Weisz, 1980). Space Perception and Interactions With Binocular Vergence Accommodation and binocular vergence have been recognized for centuries as potentially important determinants of visual space perception, and modern theorists generally agree that these oculomotor processes affect the perception of distance, size, depth, and velocity. The efficiency of space perception deteriorates greatly under the reduced stimulus conditions encountered at night or in an empty sky. Gogel (1977) and others have argued that many of the spatial illusions found under reduced cues are related to systematic misperception of distance. In general, people tend to underestimate far distances and to overestimate near distances, thus exhibiting a perceptual bias that is qualitatively use the print version of this publication as the authoritative version for attribution.

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please DARK-FOCUS: ANOMALOUS REFRACTIVE ERRORS AND ACCOMMODATION 19 similar to the oculomotor response biases found under reduced stimulus conditions. Other studies indicate that distance perception in the dark is unrelated to the resting focus but is significantly correlated with the resting or tonus state of binocular vergence (Owens and Leibowitz, 1980; 1983). Post and Leibowitz (1982) have found that illusions of motion under impoverished stimulus conditions can result from an inappropriate vestibulo-ocular reflex determined by the observer's tonic vergence state. A growing body of evidence indicates that adaptive variations of oculomotor tonus play an important role in the adaptation of space perception to optical displacement and near work. Reviews of this literature have been published by Ebenholtz (1981), Ebenholtz and Fisher (1982), Shebilske (1981), and Owens and Leibowitz (1983). IMPLICATIONS The evidence indicates that the resting state of the eyes may be a key factor for predicting and optimizing visual performance under a wide variety of conditions. It allows correction of anomalous refractive errors that can seriously limit detection and identification under low visibility conditions, and it may be useful for prescribing glasses to optimize performance on near visual tasks. These applications follow the principle of matching the individual's dark-focus to the visual task by optical means. An alternate approach might be to use the dark-focus as a criterion for selecting personnel for tasks that involve predominantly near or distant vision. In addition, the resting states of accommodation and binocular vergence appear to be an important factor for understanding and predicting individual differences in space perception (Owens, in press). The dark-focus should also be considered when developing clinical tests of contrast sensitivity (see the previous section). Research has shown that the spatial frequency response of accommodation is similar to that of the contrast sensitivity function. Focusing responses are most accurate for intermediate spatial frequencies (3-5 cpd) and are progressively less accurate for higher and lower spatial frequencies (Owens, 1980; Bour, 1981). Moreover, relatively high levels of contrast are required to stimulate accommodation (Raymond et al., 1984a, 1984b). Unless care is taken, therefore, the grating target will not be focused properly on the retina, resulting in an underestimation of sensitivity, particularly at high spatial frequencies at which focus is most important. This problem can be minimized either (1) by placing the test field at the optical distance of each individual's dark- focus or (2) by providing a high contrast accommodative stimulus on or around the test field. RECOMMENDATIONS The working group believes that decisions regarding the value of routine assessment and utilization of the dark-focus would benefit from further use the print version of this publication as the authoritative version for attribution.

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About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please DARK-FOCUS: ANOMALOUS REFRACTIVE ERRORS AND ACCOMMODATION 20 information about four issues: (1) population norms; (2) stability and variation of the dark-focus; (3) development and comparison of different techniques for measuring the dark-focus; and (4) the relationship between the dark-focus and performance on detecting, recognizing, and localizing targets. Our recommendations are summarized below. Population Norms While it is clear that the dark-focus exhibits great individual differences that are not detected by standard clinical refraction, we do not yet know the parameters of such variation in the general population. Studies of college students indicate that the average dark-focus corresponds to about 1.5 diopters of myopia, with a standard deviation of 0.77 diopters. Studies of subjects of comparable age from outside the college population, however, report dark-focus values that are somewhat less myopic (Epstein et al., 1981; Owens et al., 1982). There is also some evidence that the distance of the dark-focus gradually recedes with age (Bentivegna et al., 1981; Simonelli, 1983) and that there is a weak negative correlation between the dark-focus and refractive status (Maddock et al., 1981). These findings suggest that the parameters of the target population must be assessed in order to predict the probable impact of spectacle prescriptions or personnelselection based on measurements of the dark-focus. At present, it appears that the primary target populations among Navy and Air Force personnel are pilots, who should benefit from correction of empty-field and night myopia, and personnel such as operators of radar and video display terminals, who have critical near vision tasks and may benefit from optical corrections for near visual tasks based on the dark-focus. The working group therefore recommends that initial screen studies and field evaluations concentrate on pilots and radar operators and others engaged in demanding visual tasks. Stability and Variation of the Dark-Focus Several studies have shown that the dark-focus of a given individual is relatively stable over time periods ranging up to a year (Miller, 1978a; Mershon and Amerson, 1980; Owens and Higgins, 1983). Other research, however, indicates that the momentary value of the dark-focus can be influenced by a number of situational variables, including near visual tasks (Ostberg, 1980; Ebenholtz, 1983, 1984; Owens and Wolf, 1983; Schor et al., 1984), emotional arousal (Westheimer, 1957; Leibowitz, 1976), mood (Miller, 1978b), and anxiety (Miller and LeBeau, 1982). Fixation on distant targets also is capable of shifting the dark-focus toward the far point (Ebenholtz, 1983, 1984). It appears that some individuals are more susceptible than others to such transient changes. Owens and Wolf (1983), for example, found that near reading induces a myopic shift of the dark-focus in subjects whose initial resting state was less than 1.5 diopters while inducing no change in those whose initial resting state was use the print version of this publication as the authoritative version for attribution. greater than 3 diopters.

OCR for page 14
About this PDF file: This new digital representation of the original work has been recomposed from XML files created from the original paper book, not from the original typesetting files. Page breaks are true to the original; line lengths, word breaks, heading styles, and other typesetting-specific formatting, however, cannot be retained, and some typographic errors may have been accidentally inserted. Please DARK-FOCUS: ANOMALOUS REFRACTIVE ERRORS AND ACCOMMODATION 21 Research by Miller and his associates indicates that the effects of mood and psychological stress on the dark- focus depend on specific personality traits (Miller, 1978b; Miller and LeBeau, 1982). Although the mechanisms for intraindividual variations of the dark-focus are still obscure, they probably reflect changes in the tonus of the ciliary muscle due to prolonged focusing effort or to systemic variations in autonomic arousal. In any event, the presence of such variations could have an impact on the prescription of optical corrections or selection of personnel on the basis of dark-focus, and further research should be directed toward clarification of their prevalence and underlying causes. Clinical Measurement of the Dark-Focus Most of the basic research on the dark-focus has utilized the laser optometer (Hennessy and Leibowitz, 1972). While this instrument has several advantages for laboratory applications, its limited reliability and demanding measurement task make it less appropriate for clinical use. “Dark retinoscopy” represents a promising alternative to the laser optometer for clinical assessment of the dark-focus. With this technique the examiner uses conventional static retinoscopy to measure refraction in total darkness. The retinoscope beam is not an adequate stimulus for monocular accommodation, and the patient's eye therefore remains at the dark-focus during the dark retinoscopy procedure (Owens et al., 1980). For dark retinoscopy to be successful, it is critically important that: (1) the test room be entirely dark with no visible stimuli other than the retinoscope beam, (2) the patient must view the retinoscope beam monocularly to avoid convergent accommodation, and (3) the examiner must take care to maintain a fixed working distance so that the dark refraction can be derived by subtracting the dioptric value of the working distance from the power of the neutralization lens. Preliminary investigations have shown this technique to be an effective means for prescribing corrections for night myopia (Owens et al., 1982). To our knowledge, no one has yet applied dark retinoscopy to correction of empty-field myopia or other anomalous refractive errors. Extensions of this sort may be a useful step toward evaluating and implementing dark retinoscopy for clinical assessment of the dark-focus. In addition, evaluation of other refractive techniques, modified to use low illumination conditions, may be worthwhile. The Relationship Between Dark-Focus and Pilot Performance Although the dark-focus has been linked to pilot performance (e.g., Post et al., 1979; Roscoe, 1982), further studies are needed to increase our understanding of the ways in which the dark-focus influences performance under operational conditions. These studies should include not only measurements of target detection and recognition but also measurements of target localization. They should also include evaluations of use the print version of this publication as the authoritative version for attribution.