Red filter treatment in eccentric fixation.

RED filter treatment was devised by Brinker and Katz (1963) to overcome some of the disadvantages inherent in conventional pleoptic methods-the necessity for regular attendance at hospital, lengthy sessions with the ophthalmologist, expensive equipment, and the difficulty of obtaining full co-operation and attention from the younger patients. The principle of the red filter is related to the fact that the cones are more sensitive to the red end of the visual spectrum. In the central fovea, which measures little less than one degree, the cones which are the only light receptors, number approximately 147,000 per sq. mm. Their density decreases towards the periphery, and at 1I5 degrees from the centre there are more rods than cones per unit area. It thus seems logical to try to stimulate the cones by a light with a wavelength above the rod threshold. If this is possible, it may be expected that the area where the cones are most dense, i.e. the fovea, will be selected as the point of fixation in preference to an eccentric point, provided these receptors are free from any organic lesion. Recent work by von Noorden (1965) indicates that amblyopia may be regarded as a suppression of form or photopic vision (cone vision). Since the red filter inhibits scotopic vision, the amblyopic patient is forced to use the suppressed photopic cone vision. It is because of this mechanism that vision and fixation may improve in an amblyopic.