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DYNAMIC MEASURES 22 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. DYNAMIC MEASURES Most measures of vision and visual capacity have been static--that is, both the observer and the visual stimuli have been stationary. Yet many real-world tasks involve movement of the observer and/or the stimulus. The working group has identified two existing measures of visual ability that show promise of giving a better prediction of performance of real-world tasks: dynamic visual acuity and dynamic depth tracking. DYNAMIC VISUAL ACUITY Background Dynamic visual acuity is measured using acuity optotypes, such as Landolt C's, Sloan or Snellen letters, and checkerboard patterns, but under
DYNAMIC MEASURES 23 conditions in which these optotypes are moving and the observer must track them. During the past 30 years some 73 researchers have generated 81 reports on this topic, but, apart from the pioneering work by Ludvigh and Miller (1953, 1958; Miller and Ludvigh, 1953, 1962) and subsequent applications by Burg and coworkers (Burg, 1966, 1967, 1968; Burg and Hulbert, 1961; Henderson and Burg, 1973), there has been little sustained, programmatic effort to further develop measurement methodology or to understand the basis of dynamic visual acuity. There is general agreement that it decreases as a function of the acuity target's angular velocity with respect to the observer (Miller and Ludvigh, 1962; Morrison, 1980). This decrease in acuity is found for horizontal target movement (Ludvigh and Miller, 1953, 1958), vertical target movement (Miller and Ludvigh, 1953; Miller, 1958), and circular target movement (Ludvigh, 1949; Miller, 1956). Decrease in acuity is also found when a moving observer views a stationary target (Miller, 1958; Goodson and Miller, 1959). Other general findings are that dynamic visual acuity decreases with decreased exposure duration (Elkin, 1962; Miller, 1959; Mackworth and Kaplan, 1962; Crawford, 1960a, 1960b, 1960c) and increases with increased target contrast (Mayyasi et al., 1971). Dynamic visual acuity continues to improve with increasing luminance well above levels for which static acuity has reached an asymptote (Ludvigh, 1949; Miller, 1956, 1958). Males have slightly better dynamic visual acuity than females (Burg, 1966; Burg and Hulbert, 1961; Weissman and Freeburne, 1965). Dynamic visual acuity declines more rapidly with age than does static visual acuity (Burg, 1966; Reading, 1972a, 1972b). The correlation between dynamic and static visual acuity is generally low--i.e., there are large individual differences in dynamic visual acuity among subjects with similar static visual acuities (Ludvigh and Miller, 1954, 1958). The correlation between dynamic and static visual acuities is increased with lower target speeds, binocular viewing conditions, increased exposure durations, and free head movement (DeKlerk et al., 1964; Burg, 1966; Burg and Hulbert, 1961; Weissman and Freeburne, 1965). Most likely, dynamic visual acuity tends to be poorer than its static counterpart because, at high target velocities, the eyes fail to track the moving target accurately. This explanation was offered by Ludvigh and Miller (1958; Ludvigh, 1949), even though they did not have the benefit of direct measurements of tracking eye movements. Although this explanation has occasionally been questioned (Westheimer and McKee, 1975), it has enjoyed considerable experimental support (Murphy, 1978; Morgan et al., 1983). Of particular practical importance is the fact that Ludvigh's hypothesis seems to be able to account for individual differences in dynamic visual acuity as well as for the substantial changes in it that accompany practice (Murphy, 1978). Implications Although the relationship between dynamic and static visual acuities is not well understood and although there has not been an effective standardization of measures of dynamic visual acuity, the working group
DYNAMIC MEASURES 24 found much evidence that dynamic visual acuity is often more predictive of real-world task performance than are static acuity measures of vision. So it may be that combining measurement of contrast sensitivity function with dynamic, moving-target testing conditions can lead to more powerful measures of visual assessment that are predictive of visual task performance. DeKlerk et al. (1964) tested 30 pilots in their performance on in-flight measures of instrument, formation, and night flying ability. They reported that dynamic visual acuity correlated more highly than did static visual acuity with each of these performance measures. Burg (1967, 1968) investigated the relationships between a battery of seven vision tests (including dynamic visual acuity, static visual acuity, visual fields, and lateral phorias) and people's automobile driving records. He reported that dynamic visual acuity had the strongest relationship to automobile driving records of all the vision measures studied. Henderson and Burg (1973) studied the accident record of truck and bus drivers and found that there was a significant inverse relationship between dynamic visual acuity and accident record. Although the relationships between dynamic visual acuity and task performance discussed above are not particularly strong, they are stronger than those found with static measures. The working group concludes that these relationships found despite the coarseness of the measures, the large within-subject variability on dynamic visual acuity tests, and the lack of standards for measuring dynamic visual acuity warrant further investigation. The working group concludes that dynamic visual acuity has real potential for the assessment of vision. Recommendations The working group recommends that a study be conducted to investigate the relationship between dynamic visual acuity and some important real-world task such as flying ability. The working group further recommends that a dynamic contrast sensitivity function be measured using grating targets moving with a range of angular velocities, and that it be compared with measurements of static contrast sensitivity function in their relationship to flying ability. Sinusoidal grating targets having the general form of Gabor functions (see Appendix C) would seem useful in the dynamic contrast sensitivity task. It would be highly desirable to measure both eye movements and accommodative states in taking both static and dynamic contrast sensitivity measurements in order to better understand the relationship between them. DYNAMIC DEPTH TRACKING Background In many real-world visual motor tasks, the retinal image of an object expands or contracts, as a result of object motion toward or away from the observer, observer motion toward or away from the object, or a combination of both types of motion. Recent psychophysical (Beverly and
