Video Displays, Work, and Vision (1983) / Chapter Skim
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7. Visual Tasks, Functions, and Symptoms
7. Visual Tasks, Functions, and Symptoms
Pages 143-172
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From page 143... ...
VISUAL ISSUES IN VDT STUDIES Field Surveys Visual Complaints Field surveys of the vision of VDT workers have received widespread attention in both popular literature (e.g., New York Committee for Occupational Safety and Health, 1980; Working Women, National Association of Office Workers, 1980; Dehiatteo et al., 1981; Working Women Education Fund, 1981) and technical literature (e.g., Grandjean and Vigliani, 1980)
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From page 144... ...
Unfortunately, however, studies have not established which factors in VDT work -- e.g., image quality, lighting conditions, workstation design, visual task requirements, visual status of workers, job design, and psychosocial variables -- are correlated with visual complaints, and no conclusions can be drawn as to whether reported symptoms can be attributed to use of VDTs per se, to other variables not unique to VDT work, or to some combination of these factors. The visual complaints of VDT workers described in the field studies appear to us to be qualitatively similar to complaints reported to clinicians by people of all ages and a wide variety of occupations.
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From page 145... ...
measured visual acuity, astigmatism, near phorias, amblyopia, and color blindness on four groups of VDT workers performing different tasks (input, editing, question~ndanswer, creative) and comparison groups of non-VDT workers: no differences between VDT and non-VDT workers were found on any of the measures except for acuity.
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From page 146... ...
and Ostberg and coworkers (1980) measured accommodative response and dark focus before and after work in three groups of VDT workers - group of air traffic controllers and two groups of telephone office VDT workers -- each of which performed different types of tasks.
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From page 147... ...
of the temporary changes in measured visual function reported to follow VDT work, as well as non-VDT visual tasks, finds no suggestion of damage (in the sense of long-term irreversible anatomical or physiological changes) to the visual system.
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From page 148... ...
Some problems with workstation features and visual task requirements are clearly not unique to VDT jobs: for example, workers in various non-VDT jobs—secretaries, editors, teachers, et= -- must read hard copy or handwriting of poor legibility. In some VDT jobs, workers must deal with both poor-quality video displays and poor-quality hard copy, and they may experience cumulative effects in task difficulty and the effort required.
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From page 149... ...
VDT workstation design can also present difficulties similar to but more pronounced than those encountered in traditional clerical workstations. For example, VDT workstations may impose additional postural constraints and requirements for head and eye movements (see Chapter 6~.
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From page 150... ...
do not suggest a problem. Kolers and coworkers found little difference in observed eye movements between reading a static display and scrolled text, especially when the scrolled display was rapidly paced.
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From page 151... ...
Muscular activity is therefore needed t maintain accommodation and to move the eye to new fixations (through eye movements called saccades, discussed below)
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From page 152... ...
The Carmichael and Dearborn study illustrates many of the issues encountered in research on effects of prolonged performance of visual tasks. They found no decrement in reading comprehension or eye movement performance (number of fixations and regressions)
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From page 153... ...
. Refractive Errors and Visual Difficulties There are a number of clinical conditions involving small uncorrected refractive errors and oculomotor imbalances that can cause visual difficulties with prolonged near work or critical detail work.
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From page 154... ...
OCULOMOTOR FACTORS AFFECTING VISUAL PERFORMANCE As discussed above, the concept of visual fatigue is one that has escaped canonical scientific definition, and there has been little progress in relating subjective measures of fatigue to performance of visual tasks or physiological functions. Several investigators have measured oculomotor functions during prolonged performance of visual tasks (including VDT work)
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From page 155... ...
We discuss the difficulties in relating these reported oculomotor changes to complaints of visual discomfort, and we review studies that have used visual tasks such as reading printed materials and microfilm, inspection tasks, tracking tasks, and, more recently, reading from VDTs. Eye Movements Eye-movement changes during sustained performance of visual tasks may be considered at several different levels of extraoculomotor function: saccadic trajectory, dual-mode tracking movements, higher-level eye movements, and gaze and the vestibuloocular reflex.
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From page 156... ...
Students of reading eye movements use tracking movements as a related but simpler form of sequential reading eye movements. Indeed, Pavlides (1981)
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From page 157... ...
Saccadic smooth pursuit may also occur as a compensatory adaptation when the gain of smooth pursuit decreases.5 Visual feedback is a crucial element in the control of eye movements at this level. Higher-Level Eye Movement Patterns Reading A third level of eye movement performance comprises reading and looking, which are composed of sequential patterns of saccadic eye movements.
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From page 158... ...
Large saccades, such as occur when a user looks from a dark VDT to illuminated hard copy, may provide a light stimulus to the pupil and may also quickly change focal distance and stimulate an accommodative response. Vergence and accommodative response have not been thoroughly studied during reading.
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From page 159... ...
It is possible that some of these workers examine diagrams with sequences of eye movements that have been dubbed scanpaths. The scanpath, which is observed under certain laboratory conditions, is a nonrandom sequence of saccadic eye movements that scans a picture in an idiosyncratic pattern for each subject.
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From page 160... ...
Some visual tasks requiring concentrated attention reduce the rate of blinking (which may lead to dry eyes and irritation) , but excessive blinking has been associated with reading difficult text.
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From page 161... ...
The active ciliary muscle applies tension and stretches the peripheral portion of the zonule of Zinn, which causes the lenticular portion of the system to assume a particular
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From page 162... ...
characters (Sakrison, 1977; Kajiya and Ullner, 1981~. The accommodative mechanism of a user's eye might therefore be robbed of the error signal it needs in order to achieve good visual acuity.
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From page 163... ...
Because these static conditions occur most frequently in persons aged 30-40 years, they may be related to the development of presbyopia. Accommodative spasm and accommodative asthenopia are examples of minor ocular deficits that may be related to the incidence of complaints of ocular discomfort in VDT users.
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From page 164... ...
Fatigue and Accommodation The earliest work on fatigue and accommodation consisted of clinical impressions only, and there continues to be clinical literature (e.g., Weber, 1950~. Ergographic studies have been performed for the last 50 years (Lancaster and Williams, 1914; Howe, 1916; Blatt, 1934 Berens and Stark, 1932; Kurtz, 1937, 1938; Hofstetter, 1943; Ostberg, 1980; and Kintz and Bowker, 1982~.~7 Some of these studies have found deficits in accommodation that Methods of experimentally and clinically analyzing these dynamics include ergographic recordings and, more recently, the laser optometer.
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From page 165... ...
of changes in near points of accommodation and convergence did not include a control group of non-VDT workers performing similar visual tasks. Ostberg t6 Krueger (1980)
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From page 166... ...
reported decreases in accommodative response and shifts in dark focus point among air traffic controllers after two hours of VDT work. No significant changes were found among two groups of telephone operators performing work involving VDTs.
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From page 167... ...
Unlike saccadic motions, the smooth changes of vergence do not suppress visu al input or remap space into a stable perceptual space: that is, ~ he world may appear to move during changes of vergence. Of ale eye movements, ver
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From page 168... ...
Changes in the vergence system can often be demonstrated during prolonged performance of a visual task. Luckiesh and Moss (1935a, 1935b)
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From page 169... ...
Pupillary Constriction Associated with Prolonged Visual Work Pupillary constriction following prolonged visual work within an institutional setting has been studied by Geacintov and Peavler (1974) , who showed a significant pupillary~iameter decrease over the day's work in telephone operators using microfiche or telephone books.
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From page 170... ...
They used very bright light flashes well above sensory threshold and defined a ''border between comfort and discomfort"; and because they also believed that papillary constriction was the source of discomfort, they used atropine to block both the sphincter and ciliary muscles. This approach substantially reduced the reported discomfort in their subjects.
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From page 171... ...
These studies have done little to increase understanding of problems associated with VDT use and the differences between VDT tasks and other visual tasks that require prolonged near work. "Visual fatigue" remains a nebulous concept after many years of research and discussion (National Research Council, 1939; Carmichael and Dearborn, 1947~.
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From page 172... ...
The pupillary constriction experiments are the only studies that point to a specific phenomenon associated with discomfort. Two independent, established investigative groups suggested that drugs that stop papillary constriction also minimize subjective feelings of discomfort.
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