Work with Different Gaze Inclinations

[Citation: Lie, I. & Fostervold, K.I. (1995) VDT - Work with Different Gaze Inclinations, Work With Display Units '94, eds. Grieco, A., Molteni, G., Occhipinti, E. and Piccoli, B., (pp. 137-142). Amsterdam: Elsevier.]

I. Lie and K.I. Fostervold

Vision Laboratory Institute of Psychology University of Oslo
P.O. Box 1094, Blindern, N. - 0317 Oslo, Norway
E-mail: K.I.Fostervold@psykologi.uio.no

Introduction

Visual and oculomotor consequences of sustained excessive near-work have been research topics in visual perception and optometry for almost 50 years (1,2,3). These issues have received renewed interest in recent years with the massive introduction of VDTs into almost every sector of employment, indicating subjective complaints to be aggravated under prolonged interactive VDT work (c.f., 4,5,6,7).

From the point of view of visual ecology our eyes are primarily developed for prolonged scanning and fixation operations at longer distances and for intermittent focusing activity at near distances (8,9). Under natural scanning conditions of every-day life, eye movements are closely integrated with head and body movements. With small visual fields at close distances, like a VDT screen, this eye-head-body scanning program is "arrested" and the natural dynamic vergence and accommodation performance is changed into mainly static muscle activity (10).

When working at short distances, vision is usually engaged in eye-head coordination activities requiring a downward gaze. This is normally found to be the case in reading, sewing, handicraft work, etc. The recommended position of the VDT screen, making a gaze angle of 5 degrees to 20 degrees below the Frankfurter plane, represents, therefore, a very rare exception from what may be considered natural working conditions at near.

Investigations of the resting states of the oculomotor systems in different gaze directions suggest that work with downward gaze could serve to reduce visual fatigue (11,12,13). Corresponding results in favour of downward gaze are reported for accommodation (14) and for eyelid position, showing eye irritation to be reduced when the eyelid covers an increased part of the eyeball with downward gazing (15).

These results on visual fatiguing are supported by a recent study from our laboratory, showing a smaller increase of subjective symptoms during a period of proof-reading when the screen was lowered from 15 degrees to 45 degrees (16). Two follow-up studies are reported in the present paper.

Method

In both studies the VDT-task required visual search for meaningful words in an array of letter strings. The ratio of meaningful to meaningless words was 1 to 5. Different measures of visual fatigue were compared under two conditions of gaze inclination; 15 degrees and 45 degrees under the Frankfurter plane.

Study 1.

In Study 1, 12 female subjects were recruited from the office staff at the University of Oslo. Their mean age was 30.8 years, and the age span was from 21 to 38 years. All subjects were experienced VDT-users and had VDT-work, mostly data entry and routine data dialogue, as a major part of their working day. All of them had a history of problems related to VDT-work. Their symptoms varied, but could be summarized as astenopic symptoms and visual fatigue. The study was set up as a within-subject repeated measures design, with subjects run in a counterbalanced way (ABBA) (17). Under both conditions the VDT-task was presented as a single-spaced, WordPerfect 5.1, document on a 14 inch monochrome monitor (HR monitor 31.11, Ericsson Information systems AB, Sweden), with light (amber) characters on dark background. The monitor had a refresh rate of 50 Hz and the resolution was 720 x 350 pixels. The subjects were comfortably seated on a multi-adjustable chair and were given time and individual instructions on how to do necessary adjustments in order to prevent postural discomfort. The table and chair were then adjusted to obtain a viewing distance of 57 cm.

Each work session lasted for 1 hour, with an inter-session period of at least 1 day. The subjects were instructed to work as quickly and accurately as possible, without pauses. Work-related symptoms were measured after 5 minutes of work and immediately after each work session, using a paper and pencil test, the 14 item - 7 step Cantril scale, developed by the authors. Modifications of this symptom scale have been used in other studies (10,18). A measure of productivity was also computed. The number of meaningful words identified during each work session was registered and the percentage of words identified from the total number of meaningful words in the document calculated.

Study 2

In Study 2, 60 unselected female subjects participated from an introductory class in Psychology. The mean age was 22.9 years, and the age span was from 19 to 33 years, 35 of the subjects had some form of optical correction and nearly half of them (19 subjects) wore this correction daily. The subjects were randomly divided into two groups in a traditional parallel group design. The work session was increased from one to two hours and pre-post measures of contrast sensitivity and refraction were included together with a follow-up symptom questionnaire. Both reduced contrast sensitivity and myopization have been shown to indicate oculomotor fatigue (18,19,20) and the follow-up questionnaire was included because subjects in the first study often reported an increase in subjective symptoms after the work session was completed. Apart from the above mentioned differences in this study follows the same procedures as Study 1. It also includes the same productivity measure and symptom scale, with the addition of one item (Do you experience a general feeling of tiredness?).

Measures of contrast sensitivity were done, at a distance of 6 meters, with VCTS 6500 chart. (Vistech Consultants Inc., Dayton, Ohio, USA). Work related changes in contrast sensitivity may be a combined effect of spatial frequency adaption and myopization. To maximize the effect of myopization, the ordinary testing distance (3,3m - 10 feet) was extended to 6 m. The VCTS 6500 measurements were done in a room with stable artificial light conditions (30-70 ft.-L). Measurements of refraction were done with an autorefractometer (RM-A3000B, Topcon Corporation, Tokyo, Japan). The measurements were repeated ten times on each eye, to ensure representative values.

The monitor used in this study was a 14-inch low radiation colour monitor (TDV 5330, Tandberg Data A/S, Oslo, Norway) and the VDT-task were, under both conditions, presented as a single-spaced, Word for windows, 2.0, document with dark characters on light background. The monitor had a refresh rate of 72 Hz., non-interlaced, with a resolution of 800 x 600 pixels. The font used was 12 point, Times Roman.

Results

In Study 1, we observed an increase in subjective symptom strength, pre-post VDT work, in both conditions of gaze inclination. As shown in figure 1 this increase was larger under the 15 degree condition than under the 45 degree condition.

Figure 1.

1. Pain, tension behind/around the eyes; 2. Pain, tired/dryness of eyes; 3. Focusing difficulties; 4. Pain, tension in the neck/shoulders; 5. Headache; 6. Pain, tension in the back; 7. Dizziness; 8. Nausea; 9. Problems with linetracking; 10. Concentration problems; 11. "Foggy" letters, words or numbers; 12. "Doubling" letters, words or numbers; 13. "Jumping" letters, words or numbers; 14. Shivering text.

Figure 1. Study 1, Subjective Symptom Scores. Difference, Pre-Post, VDT - Work.

When computed as an aggregated symptom score this difference was statistically significant (t=2.48, df=11, p < .050).

In Study 2, when unselected subjects were used, pre-post work measurements of subjective symptoms still showed a significant increase in subjective symptoms. However, the results did not show any significant difference between the two experimental conditions. Interestingly enough, this picture is slightly different when we consider the subjects who wore optical corrections daily. Assuming these subjects to be more symptomatic than the other subjects, it is interesting to find that the aggregated symptom score for this subsample approaches statistical significance (t=1.97, df=16, p = .066). Analysis of single variables in this subsample showed two significant results "pain, tensionin the neck/shoulders" (t=2.58, df=16, p < .050) and "Pain, tired/dryness of eyes" (t=2.30, df=16, p < .050). These variables showed the largest increase in symptom strength under the 15 degree gaze inclination.

Analysis of the follow up questionnaire showed that as many as 93% of the sample experienced increased symptoms in the hours after the work session was completed. Nevertheless, only one symptom variable showed significant differences between the two gaze inclinations. This variable indicates that subjects working with the 15 degree gaze inclination have more "concentration problems" in the hours after the work session was completed than subjects working with the 45 degree gaze inclination (t=2.67, df=45, p=.011).

Figure 2, shows the pre-post measurements in contrast sensitivity for the two experimental groups of Study 2. Both groups show reduction of contrast sensitivity, the reduction being significantly smaller for 45 degree gaze inclination compared to 15 degree gaze inclination (F=4.28, df=57, p < 0.05).

Figure 2. Changes in contrast sensitivity. Pre/post VDT work, 15 degree and 45 degree gaze inclination.

Figure 3, presents the results from the pre-post refraction's done in Study 2. As this figure indicates, subjects working with the 15 degree gaze inclination experienced a change in the direction of myopa during the 2 hour VDT-work session. This is contrary to the group working with the downward gaze angle, whose change was in a hypermetropic direction. The difference of refraction change between the groups is statistically significant (t=.203, df=57, p < 0.50).

Figure 3. SPH - refraction: Pre/post-test difference. 15 degree inclination vs. 45 degree gaze inclination.

Measurements of productivity did not show any statistical significant results. However, there was a tendency toward higher productivity under the 45 degree gaze inclination in both studies.

Conclusion

The results from these studies are consistent with the common finding that even short periods of VDT-work have a detrimental influence upon both visual functions and subjective symptoms. The negative effects are, however reduced when gaze angle inclination is shifted from 15 degrees to 45 degrees below the Frankfurter plane, suggesting that the VDT-screen should be positioned according to a downward gaze of about 45 degrees. This suggestion is currently being tested in a 2-year field study in Norway.

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