Vision has become one of the most important senses in our modern world. The performance of visual functions can there- fore have a decisive impact in the interactions of individuals between themselves and with their environment. Vision implies both primary processes, such as the transformation of light waves into nerve impulses, and complex processes such as the interpretation of a visual scene. Some functions are well established at birth, others develop gradually over the years. In some cases, the effects of ageing can be felt relatively early and alter some visual functionalities.
During the first half of life, from birth to adulthood, the relevance of the various processes involved in vision may vary. The newborn draws visual information from his environment by detecting contrast roughly, which enables him to perceive shapes. Coordination of the movements of both eyes results in binocular vision from the sixth month of life. As the motor functions develop, near distances become increasingly important for the child. Consequently, spatial resolution in terms of acuity and the ability of the eye to focus in near vision, that is to say, accommodation become essential. Finally, at school age, acuity becomes a crucial factor, determining the efficiency of the child’s interaction with his environment. However, in the very first years at school, the time spent looking at close-up detail is limited. Later, near vision is increasingly frequent and for longer periods. Visual experience develops with age.
As a general rule, it is from the age of 30 that the physiological mechanisms of vision start to decline perceptibly. For example, due to a reduction in its power of transmission, the eye needs more light. Reduced accommodation also makes near vision increasingly difficult. Paradoxically, visual needs often increase considerably, whereas visual performance deteriorates. (Figure 1)
Fig. 1: Visual Stress evolution according to age (Weighed by different stress components: contrast, acuity, accommodation, binocularity, visual experience, far vision, sustained near vision, organic changes...)
Antinomic progress in visual performance and requirements with age often lead to an increase in ocular stress, leading to various visual complaints, also known as asthenopia. In general, asthenopic complaints can result either from simple.
factors or from a combination of several factors. The lack of correction or poor prescription can often be the cause, as well as more complex ocular problems. Other less obvious factors can also come into play, such a bad posture when working for long periods, for example. Scientific studies have shown a correlation between the performance of the eye and the prevalence of asthenopic complaints. Consequently, optimal correction would appear to be a good pre-requisite in the reduction of visual stress.
Other studies have shown that asthenopic complaints may also be linked to the intensity and duration of this ocular stress. Unfortunately, visual fatigue remains highly subjective, which makes the association of these complaints with specific causes difficult. For example, the feeling of «eyes stinging» is not necessarily due to a dry environment. This feeling can also be due to under-correction. Despite the difficulty in linking asthenopic symptoms to specific causes, at least two situations have been
established. Firstly, one can identify two varieties of symptoms known as external (such as eyes watering) and internal (such as blurred vision), which indicate two varieties of complaints clearly linked to induced conditions 1. Secondly, the visual load induced by prolonged accommodation in near vision is underestimated whereas it constitutes an important cause of asthenopia 2.
It is preferable to adopt a multifactor approach to reduce asthenopic complaints. Ergonomics is a science that takes account of physical factors, both cognitive and organizational, in the definition, establishment, and assessment of tasks, activities, products, environments, and systems in order to make them compatible with the requirements, abilities, and limitations of individuals.
In most cases, the ergonomics of concept is intended, using measurements taken previously, to improve the human environment (Figure 2). For example, the size and contrast of characters on a computer screen must meet criteria established by the visual performance of the normal eye. In other cases, ergonomics is used to highlight the fact that it is up to humans to adapt to their environment when measurements show that it is not realistic to seek to modify the latter. The prescription of optimal correction is the very example of an ergonomic adaptation of humans to their environment. 3
Fig. 2: Ergonomics (environment, field of vision, posture) of work at a desk: Group of elements making big demands on intermediate vision (com- puter) and near vision (telephone, notebook, etc.).
Many authors have concentrated, and continue to do so, on the understanding of visual fatigue. Their main difficulty is to link this subjective feeling to a measurable quantity. Since the link between asthenopic complaints and prolonged accommodation has been made, several studies have been undertaken on this subject, and more specifically on the microfluctuations of accommodation.
The first to show these oscillations of accommodation was Collins in 1937, using an electronic refractometer. When the eye fixes an immobile target, accommodation that comes into play is not constant. It varies around an average position: this is what is known as micro fluctuations of accommodation (Figure 3). Their amplitude is no more than 0.25D and they reduce with age 4. This phenomenon is not random: it is translated in the form of a Low Frequency Component signal (LFCs) and High-Frequency Component signal (HFCs). For some people, the micro fluctuations react like a loop to serve the accommodation.
Fig. 3: Demonstration of microfluctuations of accommodation – Measurements taken by modified auto-refractometer.
«For others, it is merely a background noise …» ….. The part played by each of these microfluctuation components in regulating accommodation remains an important question and many hypotheses have been made.
Some authors, such as P. Denieul and C. Miège 5, agree that low frequencies (< 0.6Hz) are considered as background noise and would signify poor accommodation only if they were large scale. On the contrary, high frequencies that are situated around 2Hz are linked to good accommodation. Indeed, when things go out of focus 5 or when the luminance or contrast of the object falls 6, one observes an increase in the amplitude of low frequencies at the same time as a reduction in that of the high frequencies.
The hypothesis that LFCs and HFCs intervene simultaneously in the control of accommodation, yet without being antagonistic, has been put forward 6. HFCs witness, for example, to a reduction in the depth of the field (increase in pupil size); LFCs for their part intervene in the case of an object at low spatial frequencies and during visual fatigue…
Indeed, measurement of micro fluctuations of accommodation would be an objective means by which to evaluate the level of the visual fatigue of the patient 7. Not being absolutely controllable, they constitute a totally unconscious phenomenon and would, therefore, be a factor that could betray, in spite of ourselves, visual fatigue.
This hypothesis, put forward by P. Denieul has been backed up by a study undertaken by T. Iwasaki and S. Kurimoto in 1987 8. They measured the variation of microfluctuations of accommodation before and after prolonged computer work and before and after prolonged work involving writing on paper. They coupled their results with the complaints expressed by the individuals. At the end of the experiment the measurements
show a clear and significant increase in low frequency compo- nents in the case of computer work. Moreover, it is after this task that people complained of blurred vision, tears, visual fatigue, general fatigue, headaches, etc. After working on paper, people complained much less and the amplitude of LFCs varies in this case less, with a spectrum similar to that found before the work was started. Visual fatigue therefore appears to be correlated with an accentuation of microfluctuations of accom- modation, more specifically that of low frequencies.
This same hypothesis is found in the articles of Gilmartin and Winn in 1992 9 and then in 1999 10. In the first article they concluded that there is a link between microfluctuations and intraocular pressure, the latter characterising a level of visual stress during prolonged near vision tasks. In the second article, they observed an increase in LFCs after 20 minutes of work on the computer display.
To conclude, it would appear that the level of microfluctuations, or more specifically the amplitude of LFCs, is indeed linked to the level of visual fatigue. Under normal conditions, only high frequencies are present, characterising a state of resting or equilibrium. Finally, although it has been possible to show microfluctuations of accommodation by different means of measurement, their study and evaluation still, unfortunately, remain experimental and not part of clinical practice.
In conclusion, visual fatigue is a very widespread phenomenon that can be manifested in several forms. There are solutions today to reduce it, but they depend on its cause. For example, it is unanimously acknowledged that good visual correction can in most cases relieve visual fatigue, but it is certain that in-depth research still needs to be undertaken into the understanding of the accommodation process and, more specifically, into microfluctuations of accommodation.