Visual acuityi.e. the ability to discern letters, numbers and objects at a distance, is essential for all daily activities. To date, scholars have assumed that visual acuity is mainly determined by the optical system of the eye and the health of the retina.
Researchers at the University of Rochester (New York, USA) recently published a study in Nature Communicationsin which they analysed the influence of microscopic eye movements on visual acuity, evaluated using the Snellen table.
The human eyes are constantly movingeven when the gaze is apparently fixed, changing the perceived visual field and continuously changing the input of visual stimuli reaching the retina.
These imperceptible eye movementswhich were considered irrelevant in the past precisely because they were minor, are large at the microscopic level, compared to the size of retinal cells and move the image through many receptors. In order to determine whether or not these microscopic eye movements influence visual function, the study investigated their influence on the individual visual performance assessed using the Snellen table.
La Snellen table consists of 11 lines of capital letters; in each line an increasing number of letters in decreasing size are displayed. During the eye examination the patient is asked to read the letters in the lines: if visual acuity is normal (20/20 vision - which corresponds to 10/10 on the Italian scale) the patient will be able to read up to the eighth line from a distance of 20 feet.
Scholars have found that the microscopic eye movements contribute significantly to vision 20/20. Although human beings are not aware of making them, these movements are precisely controlled and can enable people to read at least two more lines on Snellen's table than when eye micromovements are absent or impaired.
Visual acuity, measured in the absence of ocular micro-movements, showed a drastic reductionsubjects who normally had 20/20 vision were on average only able to read up to the sixth line of the Snellen table, i.e. they showed a visual acuity of 20/30.
The study shows, therefore, that the achievement of the 20/20 vision is not only the result of a good eye system and a healthy retina, but also of a excellent motor controlon a level that escapes awareness.
The authors conclude that, given the important influence of microscopic fixational eye movements on visual acuity, specialists should consider them when analysing patients with impaired visual acuity (myopia and hyperopia).
The study also showed a correlation between eye movements and functional properties of the eye, suggesting that therapies based on oculomotor training can help improve visual acuity.
Source
Dr. Carmelo Chines
Direttore responsabile
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