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Appendix A A Review of Methodology in Studies of Visual Funchons Dunng VDT Tasks John 0. Memo This appendix reviews field and laboratory studies of temporary changes in various oculomotor functions in subjects during the course of performing VDT tasks for a period of up to several hours. Only a few such studies have been published; this review discusses the four studies that constitute virtually the entire literature in this area as of mid-1982. The one known exception is a study by Murch (1982b) that measured changes in visual function in subjects performing tasks at two different types of VDTs; it is not included here because insufficient detail for a review was provided in the brief published report. One study reviewed here also compared changes in measures of visual function in subjects performing VDT and hardcopy tasks. Although the changes in visual function reported in these studies were interpreted by the investigators as evidence of "visual fatigue" associated with viewing VI)Ts, this interpretation is uncertain. Two of the studies reviewed here included questionnaires regarding symptoms of ocular discomfort. Although the investigators concluded that symptoms of ocular discomfort are associated with changes in visual function, they did not report whether the variables were statistically related. There are, of course, many practical difficulties in designing and conducting this kind of research. This review is intended to provide an illustration of the kinds of difficulties encountered. John O. Merritt, who is a senior scientist with Perceptronics, Inc., was a consultant to the panel. The author gratefully acknowl- edges the opportunity to review a prepublication copy of a critique of human factors research on VDTs (Helander et al., 1983~. 219

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220 Gunnarsson, E., and Soderberg, Ie 1980. Eyestrain Result- ing from VDT Work at the Swedish Telecommunications Administration. Eye Changes and Visual Strain During Various Working Procedurese Stockho m. National Board of Occupational Safety and Health. Gunnarsson and Soderberg measured changes in the near points of accommodation and convergence under normal and intensified conditions of VDT use. Questionnaires and interviews were also used to obtain information regarding symptoms of '~visual strain." The report argues that changes in the near point of convergence may be a useful objective measure of visual strain associated with VDT work. The measures of accommodative and convergence near points were based on subjective responses. The authors report that changes in both near points were greater under intensified than under normal conditions, but the data are given only in the form of graphs; the statistical analyses performed are not described, and the level of significance is not stated. The data are presented only as mean values of changes; absolute levels and variances in the data are not reported. Because of the recriprocal nature of optical power measures, a change in the near point of accom- modatiom-for example, from 10 to 13 cmrepresents a far greater loss in accommodation than a change from 20 to 23 cm. The subjects were self-selected samples of opportunity; no comparison or control group of subjects performing non-VDT work was used. The authors did not report whether normal and inten- sified conditions were counterbalanced. No attempt was made to determine if task variables and subjective responses regarding symptoms of visual strain were statistically related to the measured changes in optometric functions. Haider, M., Kundi, M., and Weissenbock, M. 1980. Worker strain related to VDUs with differently coloured charac- ters. Po. 53-64 in E. Grandjean and E. Vigliani, eds.. Ergonomic Aspects of Visual Display Terminals. London: Taylor & Francis. H aider and coworkers measured visual acuity in 13 VDT operators and a comparison group of 9 non-VDT workers before and after four 3-hour sessions. Subjective state, chromatic adaptation, degree of optical illusion, subjective visual acuity, subjective color vision, and heart rate were also measured. A questionnaire on asthenopia and other physical symptoms was administered following the test sessions. No details are presented, but appar- ently the relationship between reported physical symptoms and

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221 visual acuity was not assessed. Each subject in the VDT group was tested for two of the sessions using a VDT with green characters and for the other two sessions using a VDT with yellow characters. The VDT group performed a test protocol; the non-VDT group performed their normal office work (primarily typing). The authors reported that a statistically significant temporary reduc- tion in visual acuity occurred following the test sessions in the VDT group, while the comparison group showed almost no change in acuity. The reduction in acuity was greater when VDTs having green characters were used. The authors referred to the tem- porary reduction in acuity as "temporary myopization" and attributed it to "accommodation strain." Several aspects of the design of this study and the analysis of results make this interpretation uncertain. The data are pre- sented in the form of graphs; absolute values are not reported, the statistical analyses performed are not described, and although changes in acuity for the VDT group are reported to be statist tically significant, the level of significance is not reported. Visual acuity was measured using wall charts at a distance of 4 m; accommodation was not measured. This method is subject to influences resulting from factors other than changes in accom- modation caused by temporary myopic defocus blur. For example, general fatigue (which has been shown to be associated with pupil- lary constriction and a resulting reduction in the need for accom- modationsee the discussion in Chapter 7) following the 3-hour test session could have affected the measurements, either through its effect on pupil size or by influencing the motivation of the subjects. It is also possible that spatial frequency adaptation to the VDT and source documents could have reduced contrast sensitivity to the letters and numerals of the wall chart, reducing measured acuity. The comparison group, which showed almost no change in visual acuity following work, consisted of non-VDT workers per- forming traditional office work, mainly typing. Because the tasks performed by this group were quite different from the test pro- tocol performed by the VDT group and because subjects in the two groups were not appropriately matched in other respects, no conclusions can be drawn regarding the relative effects of VDT and hard-copy displays on visual acuity. The approach taken in the comparison of acuity changes between subjects using VDTs having green or yellow characters is promising, since it recognizes that the particular type of VDT display used may make a difference in the kinds of optometric effects that could be associated with prolonged or intensive use. The study also attempted to obtain test-retest measures by using two different green and two different yellow VDTs in a within- subjects design with an appropriately counterbalanced order.

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222 Unfortunately, the authors did not provide sufficient detail for evaluation of their measurement apparatus and methods, the distribution of acuity values for each of their 13 VDT subjects for each of the four test sessions, and the inherent variability of their measures over time. Although absolute values are not reported, the graphs indicate that the mean acuity decrement associated with VDTs having yellow characters was approximately 0.14 diopter; the decrement for VDTs having green characters was approximtely 0.24 diopter. The postwork acuities correspond to approximately 20/21 for the yellow and 20/24 for the green. All else being equal, yellow char- acters would require more accommodative effort than green characters; the finding that yellow character displays produced less "temporary myopization" suggests that accommodative effort may not be related in a simple way to decrements in acuity. The small decrements in acuity reported in this study are typically found following a wide variety of visually demanding tasks that do not involve VDTs (see Chapter 7) and thus would not represent a deterioration of visual function specific to VDTs. Mourant, R. R., Lakshmanan, R., and Chantadisal, R. 1981. Visual fatigue and cathode ray tube display terminals. Human Factors 23~5~:529-540. Mourant and coworkers measured the length of time taken during a visual search and reading task to focus from a near point to a far point (termed outfocus time) and back again on the near point (termed infocus time) as a function of display type (CRT or hard copy) at the near point, subjects' age, and time on task. (Only the authors' Study II is considered here; Study I, which used only two subjects, is not discussed for that reason.) The study reports that outfocus time and infocus time were significantly higher for the CRT display and that both times increased as a function of time on task for both the CRT display and the hardcopy display. Neither outfocus nor infocus times differed significantly as a function of age. These results are interpreted as evidence of fatigue in eye movement, or accommodative mechanisms, or both. Several aspects of the design of this study are either unspeci- fied, or if specified, are problematic in the analysis of the results. For example, the viewing distances from the subjects to the CRT or hard copy are not specified, and whether these distances were controlled is not reported. The visual angles subtended by the near and far targets are also not reported, despite the importance of these factors in accommodative response. Information on relative characteristics of the CRT, hard-copy, and distant target displays--for example, polarity and character sizes--is not pre-

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223 sensed, even though these characteristics affect the accommoda- tive response required for the tasks given. The method used to determine outfocus and infocus times was subjective analysis of videotapes of the subjects as they alter- nately searched a near target (distance from the subject not specified) and performed a reading task on a far target (6 m from the subject). The indicators used to delineate outfocus and infocus times are not clearly described. Thus it is not possible to determine, for example, how accurately the outfocus time was discriminated in the analysis of the videotapes from the read time on the distant target or how accurately the infocus and search times were discriminated on the near target. Data on the perfor- mance of the subjects, which might be expected to be related to outfocus and infocus times, are not reported. In view of the subjective nature and inherent variability and imprecision of this type of measure, there may be no practical significance in the small differences in mean outfocus and infocus times for the CRT display compared with the hardcopy display (mean outfocus time of 0.013 seconds longer and mean infocus time of 0.012 seconds longer for the CRT), and the increase in outfocus and infocus times for both displays as a function of time on task may have no practical significance. An additional limitation on this type of measurement is that the time taken to perform the viewing sequence is dependent on the motivation and level of general fatigue of the subject, independent of the study's hypothesized fatigue of the ocular muscles. It is known that the latency of accommodation and the rate at which it can be changed vary between subjects and also for the same subject under different conditions (see Chapter 7~; however, no attempt is reported to obtain data on the normal variability of accommodative response in the subjects. It is possible that small changes such as those reported may be due to this normal variabil- ity. In addition, although the distance to the near target was not specified, it is assumed to be approximately 20 inches (normal reading distance); thus, both near and far targets could have simultaneously been within the subjects' depth of focus, requiring no difference in accommodation. Although the findings are reported as statistically significant, the statistical analyses performed are not clearly described. Outfocus and infocus times are likely to be highly correlated, but it appears that a multivariate analysis was not performed and that various possible interactions among variables were not examined. For example, although the purpose of this study was to compare accommodative response to CRT and paper displays, the data on relative increases during performance of the task in outfocus and infocus times for the two types of display are not reported. Thus it is not possible to determine whether the small differences in

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224 outfocus and infocus times between the CRT display condition and the hard-copy condition were constant from the beginning of the experiment (i.e., no interaction between type of display and time on the task) or whether, as the study concludes, "a larger increase occurred during the CRT task than in the hardcopy search." The small number of subjects (six) used in this study makes it difficult to generalize the results. In addition, four of the sub- jects were older than 50 years and thus presumably had little or no accommodation. Consequently, it is unlikely that accommoda- tion played a significant role in the results; the fact that neither outfocus nor infocus times varied as a function of age supports the idea that accommodation was problably not a significant factor. The attempt in this study to compare CRT and hard-copy tasks is a potentially valuable approach; however, because of problems in methodology and analysis, the conclusion that use of a CRT for 2 to 3 hours has a "measurable fatigue impact on the visual mech- anism and that impact is greater for CRT viewing than for hard- copy viewing" does not seem warranted. In addition, the conclu- sion seems to be based on the assumption that changes in measures of optometric functions signify changes in the level of fatigue of ocular muscles; this assumption has not been scientifically estab- lished as fact. . Ostberg, O. 1980. Accommodation and visual fatigue in display work. Pp. 41-52 in E. Grandjean and E. Vigliani, eds., Ergonomic Aspects of Visual Display Terminals. London: Taylor & Francis. ~ Ostberg compared dark focus (measured with a target at 6 m) and accommodative response to targets at distances varying from 0.25 m to 1 m for three groups of subjects: air traffic controllers, telephone sales clerks performing a mixture of VDT and tradi- tional office work, and telephone directory operators performing continuous VDT work. No control groups of workers performing comparable non-VDT work were included. Measurements were made before and after VDT work, using a laser optometer. Ostberg reported a statistically significant shift in the mean dark focus of the air traffic controllers from 0.94 diopter before work to 1.62 diopters following work; the air traffic controllers were also reported to show statistically significant reduced accom- modative responses following work, becoming more myopic for distant targets and more hyperopic for near targets. Absolute values of changes in accommodative response were not reported; however, examination of the graphs indicates that the values were fairly small. Changes in dark focus and accommodative response

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225 also occurred in the other two groups, but these changes were reported as not statistically significant. Although Ostberg attributed the findings for the air traffic controllers to their visually demanding VDT work, and refers to the changes in optometric function as evidence of visual fatigue associated with VDT work, several aspects of the design of the study make these interpretations uncertain. (the term visual fatigue apparently was used in this study to refer to accommo- dative state rather than to visual discomfort, which apparently was not assessed.) ~ The display screen used by the air traffic controllers was located at a distance of 65 cm from the subjects, requiring only 1.5 diopters of accommodation. This value approximates the dark focus for many individuals (Leibowitz and Owens, 1978) and thus should not require unusual accommodative effort; it is also similar to the reported mean dark focus of the air traffic controllers measured following work (1.62 diopters). The ambient working environment for the air traffic control- lers was not described, but it can be assumed that the level of ambient illumination was low, which is typical of air traffic control workplaces. The reduction in accommodative response following work may have been caused by papillary constriction resulting from the shift from subdued lighting conditions during work to the 250 cd/m luminance (reported in an earlier pub- lication on the results for the air traffic controllers by bstberg et al. t19803) of the test cards used in the vision tests following work. Although pupil diameter was not measured in this study, it is known that papillary constriction increases the depth of focus of the eye and thus reduces the need for accommodation; in addition, papillary constriction influences accommodation directly (see Chapter 7~. Pupillary constriction and thus a reduction in the amplitude of accommodation is also known to occur in subjects who are fatigued. Accommodative response and dark focus are known to be influenced by stress (Westheimer 1957; I-eibowitz, 1977), mood (Miller, 1978), refractive error (Maddock et al., 1981), and age (Bentivegna et al., 1981~. No data were reported on the normal individual variability in accommodative response or dark focus. The latency of the accommodative response and the rate at which it can be changed are known to vary between subjects and for the same subject under different conditions (see Chapter 7~. It is thought that dark focus, which increased in this study by approximately 0.68 diopter following work, may exhibit a normal variability of 0.25 to 1 diopter in individual subjects over periods ranging from hours to days (Miller, 1978; Mershon and Amerson, 1980~. Thus, the small changes in accommodative response or dark focus may not have been due solely to the visual stimulus.

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226 The differences in the findings between the air traffic con- trollers and the other two groups are attributed in the report primarily to the more visually demanding work performed by the air traffic controllers. There are several problematic aspects to this interpretation. The data for the two telephone group~-each of which performed a different type of task, with a different amount of actual VDT work, presumably in a different working environment (~e working environments were not described), on a different type of video display of unreported designwere pooled, and the average values were compared with average values obtained for the air traffic controllers, who performed tasks of a different nature for a specified period of time (2 hours), used video displays that are not typical of most VDTs, and worked in different environments than either of the two telephone groups. Because the three groups were not matched on any of these variables, the effects of differences in visual tasks are con- founded with the effects of differences in many other variable s; thus, it is not possible to attribute the findings to the more visually demanding work of the air traffic controllers. Even though the findings for the two telephone groups were reported as not statistically significant, Ostberg concludes that "distance myopia" and "near hyperopia" occurred in these groups and appears to attribute this to the use of VDTs. Even if the f indings had been statistically significant, it would not be possible to assess the relationship between the reported changes and use of VDTs because no control groups of non-VDT workers were included in this study.