Predicting optimal accommodative performance from measures of the dark focus of accommodation.
Historically, visual accommodation has been described as resting at the far point of the eye's focusing range (Helmholtz, 1909). However, scattered empirical evidence from as early as the mid-19th century suggested an intermediate resting position (Weber, as cited in Cornelius, 1861; Morgan, 1957; Otero, 1951; Schober, 1954; Wald & Griffin, 1947). Using the newly developed laser optometer (Hennessy & Leibowitz, 1970, 1972), Leibowitz and his colleagues found new evidence for the intermediate resting-state theory and discovered wide individual differences in resting-state measures (Leibowitz & Owens, 1975b, 1978). Because the resting focus was first measured in darkness, it was operationally defined as the dark focus (Leibowitz & Owens, 1975a). Tests of 220 college-aged individuals yielded an average dark focus of 1.52 diopters (D; [approximately equal to] 66 cm), with individual values ranging from 0 to 4 D (Leibowitz & Owens, 1978). Similar findings were subsequently obtained by other laboratories using a laser optometer (Epstein, Ingelstam, Jansson, & Tengroth, 1981; Heron, Smith, & Winn, 1981; Mershon & Amerson, 1980). A literature review of the resting state of accommodation and the related concept of tonic accommodation has recently been published by Rosenfield, Ciuffreda, Hung, and Gilmartin (1993).
Since the mid-1970s, researchers have investigated the usefulness of the dark focus for predicting and optimizing performance under difficult visual conditions. In a study investigating "anomalous myopias" (which have no anatomical basis and are situation specific), Leibowitz and Owens (1975a) found strong positive correlations between individual dark focus values and participants' manifestations of night myopia, empty field myopia, and instrument myopia (r = .84, .81, and .68, respectively). Hennessy (1975) also reported a strong correlation between instrument myopia and the resting state of accommodation; individuals with closer dark focus values had higher instrument myopia (r = .78). These results suggested that dark-focus-based refractive corrections could be used to optimize visual performance at night and in high-altitude flight (see, e.g., Owens & Leibowitz, 1976; Post, Owens, Owens, & Leibowitz, 1979).
Another potential application of dark focus involves difficulties with near visual tasks. Ostberg (1980) reported that after a 2-h work shift, air traffic controllers exhibited a significant inward ("myopic") shift of their dark focus and an increased bias of accommodative responses toward their post-task dark focus value. He proposed that this bias was a symptom of visual fatigue and that comparison of the pre- and post- measurements of dark focus was one way to quantify such fatigue. Ebenholtz (1992) and Owens and Wolf-Kelly (1987) also found an inward shift of the dark focus and the far point following near work. This shift of the dark focus resulted in "transient myopia," which is often clinically attributed to accommodative "spasm" or "hysteresis." Moreover, Owens and Wolf-Kelly reported that the magnitude of adaptation was related to the observer's initial dark focus posture, and found that individuals with far dark focus values exhibited greater adaptation and greater symptoms of visual fatigue following a near task. These findings are consistent with several studies showing that accommodation is most accurate (Johnson, 1976) and oculomotor adaptation is minimal (Ebenholtz, 1992; Miller, Pigion, Wesner, & Patterson, 1983) for visual tasks located at the optical distance of the dark focus.
The practical utility of the dark focus came under serious doubt, however, when different measurement techniques - particularly those using objective, infrared optometers - yielded discrepant dark focus values. Although some studies found good correlations between dark focus values measured with laser optometers and those measured with infrared optometers (Bullimore, Gilmartin, & Hogan, 1986), others did not (Post, Johnson, & Owens, 1985; Post, Johnson, & Tsuetaki, 1984). In general, infrared optometers yield less myopic dark focus values than values obtained with a laser optometer. For example, Rosenfield (1989) reported that laser optometer dark focus values averaged [approximately equal to] 2 D, but dark focus values of the same individuals when measured with an infrared optometer had a mean of 1.28 D.
One possible interpretation of this discrepancy is that measures from infrared optometers are more valid because they are not contaminated by subjective factors (such as perception of nearness; Rosenfield & Ciuffreda, 1991) that may be idiosyncratic to the laser optometer. Another possibility is that the dark focus values vary as a consequence of differing levels of visual attention or "effort to see" (Francis, Jiang, Owens, Tyrrell, & Leibowitz, 1989). The laser optometer task requires active attention because observers must watch for intermittent test patterns, which appear at unpredictable intervals; when the stimulus appears, the observers must report apparent motion within the pattern, which is often a somewhat difficult task. On the other hand, infrared optometers typically measure dark focus when observers are looking passively into darkness. Some researchers have associated this task distinction with the effects of cognitive or mental load on measurements of the dark focus (Bullimore & Gilmartin, 1987; Malmstrom, Randle, Bendix, & Weber, 1980). However, active watching or looking does not necessarily involve mental activity or a cognitive demand, but, rather, can be considered a basic part of normal visual attention.
The purpose of the present study is not to address this cognitive issue but to examine whether dark focus measures, obtained either with or without active (attentive) viewing, are differentially correlated with accommodative performance when one is viewing a simple visual display.
The distinction between an individual's level of visual attention - and the dark focus measurement associated with it - raises two questions: (a) are there multiple dark focus postures based on visual attentiveness, and (b) if so, which posture predicts performance better under difficult conditions? The present study addressed these questions by measuring active dark focus (aDF) and passive dark focus (pDF) and by comparing those measures with optimal accommodation distance when reading in bright and in dim light. Dark focus values were recorded with an infrared optometer passively in total darkness (pDF) and while individuals actively performed a laser optometer task (aDF). Optimal accommodation distance was defined as the stimulus distance at which accommodation was most accurate in weak and in strong stimulus conditions. Accommodative response functions were obtained while observers fixated on newsprint under bright and dim conditions. After linear regression equations were fitted to each accommodative response function, a "pivot point" (where the two regression functions crossed) was calculated. This pivot point, first demonstrated by Johnson (1976), was defined as the optimal accommodation distance, regardless of stimulus quality. The primary objective was to determine whether the optimal accommodation distance for each participant could be predicted better by either the aDF or the pDF.
There were 7 women and 3 men who were emmetropic or had contact-lens corrected vision participating in the experiment (mean age = 20.4 years; average acuity = 20/19 or 0.95 minimum angle of resolution). Of the participants, 5 wore contact lenses; none reported any visual pathologies.
Accommodation was measured using a Canon R1 infrared autorefractor. A laser optometer mounted atop the Canon R1 was positioned so that participants could perform the laser task (the active viewing condition) while their accommodation was assessed with the Canon R1. The laser optometer task required directional judgments of the motion of laser speckle patterns presented intermittently at eye level. Pattern presentation distances were determined using a bracketing technique with starting distances of approximately -1 and 10 D. A complete discussion of the laser optometer apparatus and task can be found in Hennessy & Leibowitz (1970, 1972) and Owens (1984). Accommodative response functions (ARFs) were measured from the left eye with the Canon R1 while the right eye viewed newsprint stimuli presented at optical distances of 0, 0.5, 1, 2, 3, 4, and 5 D under luminances of 43 and 0.06 cd/[m.sup.2]; letter height was held constant at 8.4 arc-minutes by means of a Badal optical system (Ogle, 1971).
Following a 5-min dark adaptation period, initial dark focus measurements were made. Active and passive dark focus were measured with the autorefractor before and after the intervening accommodative response task (newsprint viewing) during a single experimental session that lasted for approximately 30 min. During the laser optometer task, observers were instructed to specify whether the laser speckles were moving up, down, or in random directions ("boiling") for each pattern presentation. The optical distance of the boiling response was recorded as the position of the dark focus on that trial.
The order of aDF and pDF measurements and the order of luminance conditions for the accommodative response task were counterbalanced across participants. For each participant, stimulus distances for the accommodative response task were also counterbalanced such that the three nearest (3, 4, and 5 D) and three farthest (0, 0.5, and 1 D) distances alternated randomly, and the middle distance (2 D) was randomly placed within the order. For example, one possible order for stimulus distances was 0.5, 4, 1, 2, 5, 0, and 3 D. Five measurements were taken at each distance and averaged. All participants signed an informed consent before the beginning of the experiment, and all were fully debriefed following the last set of measurements.
Pre- and post-dark-focus measurements were recorded to examine whether the intervening accommodative response task caused any change in the dark focus. There were no significant differences between pre- and post-measurements for aDF (t = 0.02, two-tailed, p = .99), or pDF (t = 1.53, two-tailed, p = .15). Hence, pre- and post-dark-focus values for each participant were combined (averaged) before further analysis. Active dark focus measures were found to be significantly higher ("nearer") than pDF measures (1.86 vs. 0.74 D, respectively; t = 2.41, two-tailed, p = .015, [Beta] = .34). Note that the power of the statistic is equal to 1-[Beta] (see Cohen, 1988). Power ratings were obtained using the GPOWER computer program (Buchner, Faul, & Erdfelder, 1996).
The group mean ARFs for newsprint at both luminances are presented in Figure 1. Group mean values of aDF and pDF are also plotted on the x axis. Participants' high luminance ARF had a significantly higher slope than their low luminance ARF (0.97 vs. 0.10, respectively; t = 10.24, two-tailed, p [less than] .0001, [Beta] [less than] .0001). As shown in Figure 1, the mean optimal accommodation distance - defined as the intersection of the two ARF functions at which accommodative accuracy was equivalent under both luminance conditions - is closer to the mean aDF than the mean pDF.
Correlation analysis was used to examine the accommodative performance of individual participants. Optimal accommodation distances were calculated for each participant by fitting linear regression functions to his or her ARFs for both luminances. The two regression equations were set equal to each other and were solved for x (the position on the abscissa where functions intersected). Pearson's correlations were used to compare these optimal accommodation distances for individual observers with measurements of dark focus. The correlation coefficients are shown in Table 1. As illustrated in Figure 2, a significant positive correlation was found between aDF and optimal accommodation distance. There was no significant correlation between pDF and optimal accommodation distance.
The present experiment sought to determine whether differences of the dark focus obtained by different measurement techniques affect the utility of the dark focus in predicting visual performance. Like previous studies (e.g., Post, Johnson, & Owens, 1985; Post, Johnson, & Tsuetaki, 1984; Rosenfield, 1989), the present findings confirmed that aDF was significantly more myopic than pDF. Given that aDF and pDF were measured using the same instrument, we concluded that the difference between the two dark focus values was caused by task differences associated with the laser optometer. It seems plausible that the key difference was the observers' state of visual attentiveness or "effort to see" (Francis et al., 1989).
After confirming that a difference exists between active and passive measures of dark focus, we proceeded to determine which dark focus measure, if either, is related to optimal accommodative performance. Correlational analyses showed that aDF was closely related to the optimal accommodation distance, defined as the distance at which accommodation was accurate in dim as well as in bright conditions. No such relationship was found with passive dark focus measures. Given that the aDF is correlated with the pivot point of active accommodative functions, the aDF can be characterized theoretically as a "primary point of action" for motor control of accommodation (e.g., Ebenholtz, 1992; Reed, 1982). Therefore, active dark focus may be more useful in predicting and optimizing active accommodative performance under other difficult task conditions, such as the correction of anomalous refractive errors (e.g., night myopia; Owens & Leibowitz, 1976). Passive dark focus measures were not correlated with the pivot point of the accommodative response functions and, hence, would not serve as useful predictors of accommodative performance in challenging viewing conditions.
TABLE 1: Correlation Coefficients for aDF, pDF, and Optical Accommodation Distance Variables r [r.sup.2] p aDF/pDF .29 .084 .407 aDF/optical .81 .656 .004 accommodation distance pDF/optical .19 .036 .602 accommodation distance
Further research is needed to clarify the basis for differences between aDF and pDF and to explore the practical consequences for task performance. For example, it would be interesting to determine the extent to which variables like acuity, contrast sensitivity, and visual search are affected by locating the task at the aDF distance. Johnson (1976) found that grating acuity viewed through a Maxwellian optical system was best at this point. Extending this research to a wider variety of operational tasks would seem worthwhile.
Research on the distinction between aDF and pDF is also needed to clarify fundamental processes of accommodative behavior. The difference between the two could be related to the perception of nearness (Rosenfield & Ciuffreda, 1991). Another explanation (which we favor) is that any task requiring visual attention must involve some cognitive or autonomic nervous system activity (see, e.g., Tyrrell, Thayer, Friedman, Leibowitz, & Francis, 1995) that could be related to the disparity of dark focus measures. Given that aDF and pDF were measured with the same instrument, the present results also suggest that the dark focus difference is not an instrumental artifact; rather, it is related to the visual activity of the observer. Whatever the mechanism of this difference, active dark-focus-based correction may be more useful because of its relationship to the optimal accommodation distance.
This research was completed while Jeffrey T. Andre was a National Institute of Health (NIH) postdoctoral research fellow and visiting assistant professor at Franklin & Marshall College, Lancaster, Pennsylvania. This research was supported by NIH grant EY06673-02 and by research grants from Franklin & Marshall College. Portions of this research were presented at the 1997 meeting of the Association for Research in Vision and Ophthalmology, Fort Lauderdale, Florida, and the 1998 meeting of the Optical Society of America, Santa Fe, New Mexico.
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Jeffrey T. Andre is an assistant professor of psychology at Texas Tech University, Lubbock, Texas. He received his Ph.D. in experimental psychology in 1995 from Pennsylvania State University.
D. Alfred Owens is professor of psychology and the Chair of the Program for the Scientific & Philosophical Studies of Mind at Franklin & Marshall College. He received his Ph.D. in experimental psychology in 1976 from Pennsylvania State University.
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|Author:||Andre, Jeffrey T.; Owens, D. Alfred|
|Date:||Mar 1, 1999|
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