The eye lens’ curvature changes its dioptric ability to maintain a clear image of objects as the distance varies. This self-adjusting mechanism on optical system calls accommodation, which varies from 0.01 m (10 D) to infinity (0 D) for normal and young eyes. The ciliary body around the lens controls the curvature of the lens by contracting and relaxing, and has a large number of suspensor ligaments connected to the lens. Theoretically, the ciliary body should be in fixed state when looking at an object, but actually it adjusts repeatedly instead of adjusting immediately to a suitable curvature during the accommodation process. In time, it becomes unstable and fluctuates with changes lower than 1 D and a frequency up to a few Hz. These are called accommodative microfluctuations [1, 2, 23, 42-47]. As people with normal vision look at a close object, the activity of the ciliary body is relatively low when compared to viewing a far object. When people with visual fatigue look at a far object, the activity of the ciliary body is significantly higher compared to normal vision, and remains high while looking at short distances, but is not as obvious compared to people with normal vision [2, 43]. When reading at short distances, people with myopia exhibit a significant increase in the power of accommodative microfluctuations [48].
3.1 Phenomena of Accommodative Microfluctuations
Collins (1937) [49] used an infrared optometer to measure patients and found that high frequency fluctuations occurred during accommodation. Since then, there have been a series of studies on this phenomenon. Arnulf et al.
indicated that the fluctuations make images on the retina focus poorly. In the research, the double-pass ophthalmoscopic method was used to take pictures of retina images while subjects watched stable targets. The results showed that a series of adjustments let the ciliary body keep a balanced position while the amplitude is about 0.1D, which maintains an optimum response between the lens and the image but changes the contrast of the retina image further. As technology advanced, Campbell [50] developed a high-speed infrared optometer which has a high temporal resolution to record accommodative microfluctuations almost continuously, and can both quantify and establish a frequency domain for the microfluctuations [2, 23, 46, 51]. These microfluctuations act as the sinusoidal wave [2] and are defined according to two components: low frequency component (LFC) and high frequency
component (HFC). Because the fluctuation frequency of about 5Hz is stable, the fluctuations above 5Hz are not adopted. The root mean square (rms) of amplitudes below 5Hz ranges between 0.02D to 0.2D [44]. Some research has reported peaks in the HFC [49, 50, 52, 53]. In addition to the classical IR autorefractometer, a wave-front aberrometer and ultrasound have also been used in the research. Wave-front recording accommodation can compute high order aberrations, like Zernike polynomials. So beyond spherical defocus and astigmatism [54, 55], additional information is available on the eyes’ optics to analyze the microfluctuations and yield a total defocus error. The Ultrasonographic method, which is completely different from other optical methods, can track microfluctuations by measuring variations in the eye’s morphology [56, 57]. Finally, an IR autorefractometer will be introduced in section 3.5.
3.2 The Power Spectrum of Accommodative Microfluctuations
The behavior of accommodative microfluctuations is complex, without rules and nonlinear in time; however, regular patterns exist while transferring the waveform into the frequency domain. According to the waveform, accommodative microfluctuations consist of two components: low frequency component (LFC) and high frequency component (HFC); the former is defined below 0.6 Hz and the latter between 1.0 to 2.3 Hz [1, 2, 23, 44, 45].Accommodative microfluctuations may be affected by a distance of target [43, 58-60], pupil diameter [47, 61-63], the form and contras of target [64, 65], the luminance of target [1, 66, 67], the eye’s age, astigmatism [68],visual fatigue [2, 43, 69, 70] ,bi/monocular observation of the target [62, 69], and artifacts such as cardiopulmonary signals [52, 71, 72] or other rhythmical physiological systems.
Neurological control also affects the LFC’s wavelength, and arterial signals correlate highly to the HFC [1, 44].
3.3 Relationship between Accommodative Microfluctuations and Visual Fatigue
Among the factors mentioned in section 3.2 that influence accommodative microfluctuations, the effect on the pupil’s diameter is the most obvious. The pupil changes with light, and when the diameter is smaller, the HFC fluctuations are imperceptible, although the LFC increases; when the diameter increases, the HFC fluctuations become obvious and the LFC decreases [44]. Geacintov and Peavler (1974), Goldwater (1974), and Ukai et al. (1997) reported that there is a connection between pupil instability and visual fatigue [1]. Some recent research has studied eye variations and visual fatigue after viewing VDTs; it indicates the close connection between the pupil’s variations and accommodative
microfluctuations [1]; it showed that patients with asthenopia can indeed be diagnosed from changes in the HFC [2]. Table 3-1 shows that the HFC range for people with normal vision is about 40 to 60 while viewing a stable target at about 0 to -0.75 D, and that with asthenopia, it is about 60 to 70; the difference is imperceptible if the target is about -1.0 to -3.0 D. Suzuki et al. (2001) [43]
tried using a color code to show the position of targets, the accommodative response amplitude and the HFC value as figures. While viewing a distant, stable target, Fig. 3-1 shows that a normal subject’s HFC is about 50 to 60, labeled in green, while Fig, 3-2 shows a subject with asthenopia is above 60, labeled in red. After combining these results, it is concluded that the ciliary body’s tension is low when viewing far objects, so its variation is large if visual fatigue occurs, and slight with short distances. Thus, the ciliary body’s tension is recognized by the HFC variations and subjects are assessed to see if they suffer visual fatigue.
Figure 3-1: The HFC of normal subjects [43].
Figure 3-2: The HFC of subjects with asthenopia [43].
Table 3-1: The HFC of subjects in different distances of target. The “-”
means no fatigue; the “+” means fatigue [2].
Subject HFC1
(0~-0.75D)
HFC2
(-1.0~-3.0D) fatigue
1 49.8 69.0
-2 56.6 71.2
-3 54.4 72.6
-4 64.8 77.0 +
5 65.2 70.4 +
3 62.4 71.1 +
7 60.7 69.7 +
3.4 Automatic Refractor-Keratometer
In section 3.3 Dr. Kajita used the HFC to evaluate the state of visual fatigue by measuring the accommodation of different subjects, and Suzuki proved that the HFC values reveal if subjects suffer visual fatigue. So in 2003 and 2005, Dr.
Kajita published two patents, for the auto refract-keratometer design (Speedy-K;
Nikon, Tokyo, Japan) developed from his research. Fig. 3-3 shows infrared illuminated optotype that the auto refract-keratometer measures and records the
eye’s accommodative microfluctuations with in real time. A stepper motor controls the target distance. The relative position between the target and the optical lens changes when the motor steps, so subjects are stimulated by multiple distances during the measurement process. The chopper and receiver in the device sample the target’s stimulation; the sample rate can be modified depending on the requirements. In order to acquire a large sample of accommodative responses and calculate HFC values, an auto refract-keratometer was applied in this research. There were 8 target distances in the device: +0.5D, 0D, -0.5D, -1.0D, -1.5D, -2.0D, -2.5D and -3.0D, from far to near. 0D means 6 meters in Optometry where the accommodative response is very low, and +0.5D is above 6 meters where the ciliary body is almost unadjusted so the image is blurred to the eye. The device used in this research features the Speedy-K Ver.
MF-1 software that communicates with RS-232 and calculates the HFC;
therefore, the subject’s HFC can be analyzed and shown on the software interface.
Figure 3-3: The target in the auto refract-keratometer.
3.5 Comparisons between the HFC and other Indicators
The indicators for evaluating visual fatigue, accommodation power, pupil diameter, visual acuity, eye movement velocity, critical fusion frequency, subjective rating of visual fatigue, visual task performances and brain activity
measurements, have their own limitations. For instance, accommodation power is best used to evaluate visual fatigue because the lens curvature becomes larger in close work and fatigue occurs over long periods of time. Work station illumination is different from that of a target or dynamic information, and the iris’ sphincter muscle, as a pupil constrictor, has to control the amount of light entering the eyes. Because it is easy to feel tired when working long hours, the pupil’s diameter is a suitable evaluator under these conditions. The disadvantage to a subjective rating, is that the method’s objectivity depends on the subjects’
psychological and physiological feelings, rating it inadequate. So it is generally only used to supplement other methods.
The HFC is part of the accommodative microfluctuations spectrum, and is calculated according to accommodation records. Section 3.2 shows the high correlation between the HFC and cardiopulmonary signals; therefore eyes suffering from visual fatigue can be evaluated by the HFC variations because physiological aspects influence cardiopulmonary signals. In addition to measuring accommodative microfluctuations and calculating the HFC, the device can be used to measure accommodation power, which is more functional than optometers in the past.
The HFC is a speedy and uncomplicated indicator to evaluate visual fatigue.
Although there is little research on visual fatigue with the HFC due to the expense involved in using the device, it can test a subject’s level of visual fatigue.