Dark Focus, Dark Vergence and Subjective Reports of Visual Fatigue during CRT Display Viewing
Authors: Karn Keith S. , Mershon Donald H. 
Published: January 2012

This study investigated the effects of two hours of CRT display viewing on dark focus, dark vengence, and subjective ratings of visual fatigue. Recently several studies have reported shifts of the dark focus of accommodation and dark vergence after short periods of display viewing at extreme distances. Many studies have attempted to link these and other changes in the oculomotor systems with subjective feelings of visual fatigue. Despite these research efforts and intense interest, causes and correlates of visual fatigue have not yet been adequately quantified. The research reported here is part of a larger study or changes in oculomotor functions during display viewing.

Methods used for measuring oculomotor functions included use of a laser-Badal optometer to measure dark focus and use of a dichoptic alignment task for dark vergence measurements. A five-point rating scale was used to collect subject assessments of visual fatigue. Thirty students (age 18–25) screened for normal visual function (corrections allowed) were divided into two groups. One group viewed a display at a standard viewing distance (0.4m), the other group viewed a display at a farther distance (2m). Alphanumeric and pictographic information was presented to the subjects on standard video monitors (white characters on a dark background). The sizes of the two displays were selected such that height of the characters subtended the same visual angle when viewed on each display. The subject's primary task was to read text and interpret pictographic displays on the CRT while playing a menu-based computer game. Dependent variables, including dark vergence, dark focus, and the subjective ratings of visual fatigue, were measured three times (before, midway, and after) the two-hour display viewing task, in order to investigate changes in these variables over time.

The split-plot factorial designs were subjected to analysis of variance tests. The results of the subjective ratings indicated that visual fatigue increased significantly with time-on-task (P < 0.005). The mean values for pretest measures of dark focus (1.63 diopters) and dark vergence (5.08 prism diopters) were in accord with results previously reported in the literature for similar populations and did not differ significantly for near versus tar display viewers. Also in accord with previous findings, the pre-test dark focus and dark vergence values showed little evidence of correlation (r = 0.11).

Although there was a slight inward shift in mean dark focus over time for the near display viewers, there were no significant main or interactive effects of time-on-task and display viewing distance on dark focus. Dark vergence, however, was significantly higher for those who viewed the near display than for the far display viewers (P< 0.05). There was also an interesting interaction between time-on-task and viewing distance on dark vergence (P< 0.05). As expected, the subjects who viewed the near display exhibited an inward shift in mean dark vergence after the first hour on the task, while the mean dark vergence for those who viewed the far display shifted outward during this time. Surprisingly, during the second hour, these trends reversed such that the mean dark vergence shifted back to approximately pre-test levels for both groups. This return of mean dark vergence toward initial values indicates that the vergence system may have a mechanism by which it can compensate for prolonged demands for convergence at fixed distances.

The fact that none of the oculomotor changes were significantly correlated with the subjective reports of visual fatigue suggests that visual fatigue is a complex phenomenon. Perhaps no single measure of oculomotor function will be adequate to explain the mechanisms of visual fatigue. The data also indicates that two-hour work sessions should have little effect on oculomotor variables, provided that the displays are within the normal range of display viewing distances.