Visual fatigue comprises all those symptoms that arise after excessive stress on any of the functions of the eye. Among the most important of these are straining the clarity muscle of accommodation by looking too closely at very small objects and effects of strong local contrasts on the retina. Visual fatigue manifests itself as: 1) painful irritation (burning) often accompanied by lachrymation (tearing watering), reddening of the eye and conjunctivitis 2) double vision 3) headaches 4) reduce powers of accommodation and convergence reducing visual acuity, sensitivity to contrast and speed of perception. These symptoms are brought about in particular by strenuous fine work, reading poorly printed texts or low-quality computer images, inadequate lighting, exposure to flickering light or optical aberrations of the viewer's eyes. Elderly people are, of course, more prone to visual fatigue.
Obviously, all types of visual work can contribute to the general fatigue discussed earlier since every job that calls for more rapid and precise eye movements will make heavier demands on perception, concentration and motor control. So whenever the eyes are overstressed for long periods the symptoms of eyestrain (sore eyes and headaches) will be added to those of general fatigue.
The effects of visual fatigue on a person's occupation may include: loss of productivity, lowering of quality, more mistakes, increased accident rate, visual complaints. (Fitting the Task to the Human, Taylor & Francis, 1997)
Age, through a "fatigue effect", influences the measured depth of the glaucomatous defect. (1)
Visual evoked potentials (VEPs) were recorded in a group of 20 operators aged 25-45 years during two sessions, before and after a 5-hour work at computer terminals, in order to assess visual system fatigue. Most of the examined operators suffered various complaints concerning not only the organ of vision but the central nervous system as well. (2)
The visual reaction-time test henceforth showed a mean reaction time of 0.189 seconds for the rested drivers, 0.223 seconds for the tired, and 0.309 seconds for the very tired nighttime drivers (p < .001). Mean reaction time for the early morning drivers was 0.190 seconds corresponding to 0.246 seconds in the equivalent nighttime group (p < .001).(3)
One-hundred-and-twenty cab drivers of vacant cabs were stopped by the police on a major highway just outside the city of Copenhagen. Eighty drivers declared themselves rested, 38 tired, and 2 very tired. The reaction test showed considerable individual variation, but subdivision of the drivers according to level of tiredness demonstrated a statistically significant and uniform difference throughout the night (p < 0.0001).(4)
The study examined the effect of continuous VDT work on 1) visual acuity, refraction and oculomotor functions (ZCSV: zone of clear, single vision) and 2) the effect of 15-min restitution time on the oculomotor functions (ZCSV). In both groups there were a significant reduction in visual acuity, refraction changes in myopic direction and reduced ciliar and vergence muscle capacity.(5)
A visual field test was performed on 100 volunteers to study learning or fatigue effects during one session. We noted no difference between trained and untrained subjects or between normal and pathological eyes. However, patients with refractive errors, especially myopes, revealed a larger learning effect than did emmetropes.(6)
The aim of this study was to analyse the effects of different types of metabolic fatigue, induced by anaerobic alactacid, anaerobic lactacid, sub-maximal aerobic, and maximal aerobic efforts, on the performance of a sensory task (peripheral threshold detection), a sensory-motor task (coincidence-anticipation), and a cognitive task (recall in central vision). Results show that performance in peripheral vision improves with all types of effort.(7)
The present study was designed to investigate the effects of different types of physical fatigue-past demand for exertion-on the performance of a visual detection task performed with and without masking. It appears therefore that a physically fit population (mean VO2 max = 62.39 ml X kg(-1) X min(-1)) shows no detectable decrement in a perceptual task performed after fatiguing exercises recruiting various types of energy sources.(8)
Representative groups of workers with various occupational activities, associated with considerable loading of visual system, were systematically examined by clinical and functional methods in the course of several years. The results reveal considerable alterations in visual system within one working day. The most general features of those alterations are as follows: elevation of the threshold of electric sensitivity towards the end of the working day, reduction of light-distinguishing and colour distinguishing sensitivity, the closest point of clear vision is removed father, unproper ergogram with an ascending course, reduction of fusion ability towards the end of the working day, etc.(9)
A study of the neurophysiological basis of reaction time change was undertaken as a means of exploring the physiological mechanisms of local muscular fatigue effects upon sensorimotor performance. The quality of total reaction time was found to deteriorate, particularly when responses were resisted.(10)
The present study was designed to investigate the effects of exercise to exhaustion on different components of visual capacity: visual field, and coincidence/anticipation capacity. Pre- and post-fatigue visual measurements were not significantly different. The results demonstrated the independence between metabolic fatigue and visual capacity.(11)