Visual Acuity Thresholds in Amblyopes.

THE use of the visuscope in an amblyopic eye helps to determine the part of the retina used during uniocular fixation. However, it is also important to know from a clinical standpoint whether the fovea is inhibited through a peripheral or a central mechanism, and whether in a particular case, the resolving power of the rods or the cones is affected. Various authors have studied the behaviour of the amblyopic eye in order to establish the nature of the inhibitory scotoma, especially under conditions of altered illumination. Thus, Ludvigh (1942) found that some cases of amblyopia ex anopsia showed a reduced light differential threshold, and Irvine (1948) reported changes in the "light differential sensitivity" in amblyopic eyes. These findings indicated that the retinal sensitivity might be altered because of a peripheral mechanism, but this observation has not been followed up until recently. Enoch (1957, 1959a, b), working on the directional sensitivity of the amblyopic retina, suggested the possibility that amblyopia in squint has a varied aetiology, and that in some cases the fovea may be pathological although appearing normal when examined with the ophthalmoscope. There are, however, cases with definite central suppression, in which fixation and visual acuity improve considerably with occlusion and pleoptic treatment. Wald and Burian (1944) found the light sense unimpaired in cases ofamblyopia accompanying strabismus, and they suggested that the inhibition was cortical rather than retinal. von Noorden and Burian (1959a, b) also found that the amblyopic eye with squint differed from the normal eye as well as from eyes showing retinal pathology. They compared the behaviour of the three groups using neutral density filters, and their findings confirmed the view that in squint-amblyopes the deterioration in vision was due mainly to central suppression. Oppel and Kranke (1958) and Oppel (1960) examined the dark-adaptation curves of a large series of amblyopes, and concluded that squint-amblyopes