Indicators and Methods for Measuring Visual Fatigue

In visual fatigue research, there are various and extensive indicators and measuring methods, which can be divided into five types [10][[18-21]: (1) the measurement of the oculomotor systems, e.g., eye movement velocity, accommodation power, convergence, viewing distance, pupil diameter, er and blinking; (2) the measurement of visual acuity, e.g., visual acuity, critical fusion frequency (CFF); (3) measuring the performance of visual tasks, i.e., recognition speed and error detection rate; (4) the report of asthenopia symptoms, and (5) brain activity measurements like functional magnetic resonance imaging (fMRI), magneto encephalography (MEG) and electroencephalography (EEG) for observing neural activity affected by visual fatigue, both temporally (MEG and EEG) and spatially (fMRI).

Among the eight most common and obvious ways cited in numerous research papers, are seven objective or subjective indicators according to Chi and Lin [15] from Megaw(1995) such as accommodation power, pupil diameter, visual acuity, eye movement velocity, critical fusion frequency, the subjective rating of visual fatigue, visual task performance, and brain activity measurements which is applied to few researches. The subjective rating for visual fatigue is an indicator related to a judgment on the subjects’ level of visual fatigue; it will have more inaccuracies, while others have objective indicators according to the response of intraocular components or the visual cortex’ neural activity. The following sections give brief introductions to the eight indicators.

2.3.1 Accommodation Power

Accommodation refers to the ability of the crystalline lens to adjust the

curvature to an object as its distance varies. The principle is that the curvature is adjusted by the ciliary body so that objects can be clearly imprinted on the retina.

A diopter is the accommodation’s unit of measurement. W. Jaschinski-Kruza [22] reported that accommodation is connected to visual fatigue. Charman and Heron [23] claimed that microfluctuations exist in the ciliary body, which shows that accommodation can be evaluated whether or not the subjects suffer from visual fatigue. Sumio Yano et al.[11, 24] reported that there are apparent variations in the accommodative response after watching stereoscopic displays.

Instead of measuring the crystalline lens’ accommodation power by using specific stimuli, due to the lack of measurement methods early studies focused on measuring the accommodation time or the nearest point of accommodation.

Aside from using the VDT near-point tester to measure accommodation, laser, infrared and polarized vernier optometry were also included. Subjects’

accommodations should be measured before and after the experiment in order to record their response, which can then determine whether or not the fatigue occurs. Iwasaki et al. found that after an hour of viewing the test target, the accommodation time increases. Thereafter, subjects were tested on a visual experiment for fifteen min, and then their eye was stimulated by a distant refractometer target for two min. It was discovered that after comparing the before and after accommodative responses, that visual fatigue decreases as the ciliary body relaxes when looking at a distant object; this proves that accommodation is definitely related to visual fatigue [25].

2.3.2 Pupil Diameter

The iris must dilate or contract as a screen’s intensity changes, so that the dilator sphincter is forced to act frequently. The depth of field increases as the diameter of the pupil contracts, which affects the eye’s accommodation ability.

Consequently, some research assesses the degree of visual fatigue by the changes in the pupil’s diameter. By using records from infrared photography, Geacintov and Peavler (1974) observed that subjects’ pupils do contract after watching continuous displays. Murata (1997) claimed that it is appropriate to use the changes in pupil diameter to assess visual fatigue [26]. Backs and Walrath (1992) confirmed that the pupil’s diameter response is highly sensitive to processing information; however, Taptagaporn and Saito (1990) [27]

considered that it is easy for exterior stimuli to change the pupil, i.e., brightness, subjects’ emotions and difficulty in analyzing information. Every variable should be controlled. Whether the pupil’s diameter relates to the subjects’

fatigue is not clear; so the results are inconclusive.

2.3.3 Visual Acuity

Haider et al. [28] pointed out that after working uninterrupted with a VDT

for three hours, subjects have transient myopia when looking at distant targets.

Landholt rings were used to survey the vision changes in the test. The subjects’

average vision obviously decreased from 1.08 to 0.82, and the range became wider as the time was increased. Apart from the loss of vision, some researchers claimed that sensitivity also decreases. Perhaps fatigue in the oculomotor control systems is responsible for the loss of vision, e.g., accommodation or vergence systems relating to the lens’ accommodation ability, a lowering of sensitivity in the vision process, or a general reduction in arousal.

Therefore, the loss of vision, which is right for evaluating overall visual function, can also be used to evaluate visual fatigue. Besides using the Landholt rings to test eyesight, and asking and answering questions, some specific instruments are also available, such as the vision tester (OPTEC 2000, Stereo Optical) for example. The optotype inside the instrument is designed to employ the Landholt rings design concept. When doing research, eyesight should be tested before and after the experiment by the optotype. The test’s design can also stimulate eyes puts electrodes around the eyes and records potential changes in the corneoretina.

(2) A Video-Based Eye Tracker, which videotapes the subjects’ eye movements.

(3) A Scleral Search Coil, which assesses eye movement by using the electromagnetic induction theory. (4) The Dual Purkinje Image Tracker that uses different corneal refraction angles and lenses to shoot the pupil’s contour. (5) Infrared Oculography uses infrared rays to irradiate around the iris, and reflex back. The optic signals are transferred into electrical impulses so that the eye movements can be understood by the signals. Bahill and Stark [29] pointed out that eye movement parameters will be influenced by the saccadic eye movement control system. As the intraocular and extraocular muscles control eye movements, over time it would cause muscle fatigue. After studying the saccadic eye movement system, Saito et al. [30] found that, for VDT staffs, the eye movement amplitude and frequency are high after work as opposed to before work. Hallett [31] noted that eye movement velocity is the function of the saccadic angle (the amplitude of vibration) and movement frequency. In other words, eye movement velocity is equal to the saccadic angle shifts multiplied by the frequency. When the saccadic angle is large, it will cause excessive torsion, which presses on the optic nerve or leads to conjunctivitis, so the eyes will hurt or be hurt. Schmidt, Abel, DellOssen & Daroff (1979) and Stberg (1980) claimed that eye movements could be used to evaluate visual fatigue because the

reaction is obvious, especially for the ciliary body, so a visual fatigue study using eye movement velocity would be reliable.

2.3.5 Critical Fusion Frequency

Critical fusion frequency is a kind of temporal measure that enhances the flash frequency little by little. When subjects are looking at a flash of light, they feel it is continuous. In other words, the subjects fail to identify whether or not the light flashes. This flash of light reaches a critical point, which is called a critical fusion frequency. Analyzing the change in CFF is highly sensitive and suitable for assessing whether or not visual fatigue occurs. The CFF can be tested by using an instrument, (e.g., Handy Flicker, NEITZ), containing an optotype with different colors. The flash frequency can be gradually increased until the subjects feel that the optotype does not flash anymore; then the frequency can be recorded. This action can be repeated with decreasing frequency to observe whether or not the subjects’ CFF also tends to decrease t [32]. Osaka [33] noted that when a VDU task shows any large red or blue words, both the foveal and peripheral CFF test show deterioration. Iwasaki et al. [34]

studied the reactions of different CFF colors, and found that yellows and greens decreased after thirty min, and the red after fifteen. The distinction between color and light are provided by the cone cells and the rod cells, respectively, on the retina. Therefore, a decrease in the CFF represents a decrease in the retina’s function. For the work place, the brightness contrast is best from between 7:1 to 11:1. The lower the light, the more fatigue people feel. In low brightness contrast, the decrease in the degree of CFF is fairly obvious.

2.3.6 Visual Task Performance

Many researchers prefer to observe if subjects suffer from visual fatigue by using direct or indirect visual performance measures. Aside from the reading [35]

and simulated inspection tasks [36], there are some experiments that have been designed specifically for this purpose. This kind of indicator evaluates the subjects’ visual response under different working conditions by comparing the before and after variation, so the degree of visual fatigue can be better understood. Nonetheless, there are three restrictions: (1) the drop in performance may not be caused by visual fatigue, e.g., it may be caused by boredom, low arousal or low spirits. Therefore, anything that can lower the visual performance should be avoided in a test so that there is nothing to connect the visual fatigue with the visual performance. (2) Perhaps performance is hindered by the subjects’ special effort or learning, so using something that can be learned or achieved through effort should also be avoided. (3) Do not use methods that are difficult to attain, e.g., analyze the data before and after reading to determine the occurrence of visual fatigue from the reading speed and comparison of the error

detection rate. Generally speaking, researchers like to observe whether or not the subjects with visual fatigue maintain the same performance levels after the experiment. Studies have shown that people who read monitors have a higher level of visual fatigue than those who read paper. As the visual fatigue increases, the reading efficiency is affected, either directly or indirectly. The research on multimedia dynamic information displays [37] showed that information load and speed of movement evidently influence visual performance measures and visual fatigue.

2.3.7 Subjective Rating of Visual Fatigue

Numerous researchers adopt the subjective rating method, which Sinclair [38] divided into five areas: rankings, questionnaires, interviews, ratings and checklists. Bullimore et al. [39] claimed that the advantage to adopting a subjective rating method for evaluating visual fatigue and visual performance measures is that it is easy to manipulate, involves little cost and is quick to evaluate, even if there are too many variables. The subjective rating method possesses higher sensitivity, but because it acts upon the subjects’ subjective senses, its diagnostic, validity and reliability are still up for discussion. The cause of fatigue is hard to find. Other objective evaluation indicators should be involved when evaluating visual fatigue in order to improve the diagnosis. The most general method to determine the degree of visual fatigue is through a questionnaire. Researchers have to design a form where relevant visual fatigue symptoms can be recorded. Subjects rank their symptoms on the form in five to seven points [11, 26, 32, 40], according to their situation before and after the experiment. Afterwards the researchers use the total for after the experiment minus the one before, and then the occurrence of visual fatigue can be analyzed.

There is another method that uses points for individual symptoms which occur, after watching the display minus the ones from before so that the strongest and the weakest symptoms can be compared.

2.3.8 Brain Activity Measurements

All perceptions and high-level recognition are processed in the brain.

Therefore, any visual fatigue occurring after the experiment is reflected by new activity in the brain. Apart from providing variations in brain activity, brain activity measurements also provide understandable perceptual and cognitive processes to characterize variations of pathology, such as specific visual deficiencies. In order to achieve high sensitivity and specificity, high-quality spatial information, such as functional magnetic resonance imaging (fMRI) can be combined with high-quality temporal resolution, such as magneto encephalography (MEG) and electroencephalography (EEG). Most of the relevant brain activity studies on depth perception focus on basic issues like

establishing the binocular vision’s real path. Some visual fatigue studies have used this method to discuss the relationship between perception and the causes of visual fatigue. Emoto et al. [12] measured visually evoked cortical potential with an EEG, which reflects fatigue of the relevant intraocular muscles, extraocular muscles and nerves in the brain, and used a P100 latency (positive component at approximately 100ms latency) as a fatigue index. The delays for temporally changing parallax were significant, and a high correlation was found between the relative vergence limits and P100 latencies. In preliminary research Hagura et al. [41] use an fMRI combined with an MEG as a tool to measure visual fatigue, which allows the dipole data obtained by the MEG to be superimposed onto a 3D model set up by the fMRI. Although the results showed that the back left side of the brain was active while viewing stereogram and isocontour maps, the dipole activity varied with different watching periods, and the maps were not clear enough to identify and locate those exact areas;

therefore, this method’s applications need further investigation.