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.

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.

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 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 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 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).

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 greater than 3 diopters. 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 interactions of accommodation and vergence eye movements and measurements of dark vergence as well as measurements of the dark-focus.

The intermediate resting states of accommodation and vergence, and their substantial intersubject variation, may provide new insights for predicting individual differences in the ability to localize objects under adverse visual conditions. It may also be fruitful to examine whether these basic individual differences in oculomotor behavior are related to performance on tests of dynamic acuity and motion in depth. Of more immediate concern for applications of the dark-focus, further research should be devoted to interactions of accommodation and vergence eye movements. Under most conditions, accommodation and vergence exhibit strong synergism; stimulation of either response initiates a correlated change in the other. Clinically, these interactions are most familiar in terms of the AC/A and CA/C ratios. In contrast to the strong synergy of their active responses, the resting states of accommodation and vergence appear to be relatively independent. Individual differences in the dark-focus and tonic vergence are not highly correlated; their average values are significantly different; and adaptive changes induced by optical displacements or near visual tasks can selectively affect either the resting state of accommodation or that of vergence (Owens and Leibowitz, 1983; Owens and Wolf, 1983). Although the resting states per se may be independent, adaptive or stress-related changes of either resting state are likely to influence the interactions of accommodation and vergence. Their synergism might also be affected by visual aids (e.g., night glasses) prescribed on the basis of the dark-focus, particularly if the aid were used in the presence of adequate stimulation. Furthermore, a target that may be inadequate for accommodation may nevertheless stimulate fusional (i.e., disparity) vergence, thereby producing a certain degree of vergence accommodation away from the expected dark-focus position (Miller, 1980). Measurements of interaction between accommodation and vergence could therefore play an important role in systematic investigations of dark-focus relationships to visual performance.