The differential threshold of retinal sensitivity, as contrasted with the absolute threshold, has also been investigated in amblyopic eyes. In testing the differential thresho
…ld, one asks how much brighter than its surroundings a test field must be so that it may be just perceived. A number of workers studied this question, and most found a slight elevation of the differential threshold of amblyopic eyes, provided the test field was small enough, on the order of magnitude of a few minutes. In other words, a small, shallow relative scotoma of the amblyopic eyes could be shown to exist. Various methods have been used to establish this fact, among them monocular static perimetry. Aulhorn 19 reported on 70 amblyopes tested with this method, 35 of whom had foveolar or extrafoveolar depression, 26 had only flattening of the curve, and 9 had the normal foveolar peak in their sensitivity curve. Donahue and coworkers 120 recently revisited this problem, using automated perimetry. They found that all types of amblyopia are associated with a generalized depression of light sensitivity, which was proportionately greatest at the fovea and highly correlated with visual acuity loss. A higher-than-normal central differential threshold was also found by EF Miller 309 under photopic conditions using rectangular test objects in the shape of luminous bars or slits of different widths. Grosvenor 174 confirmed the work of Miller but added important amplifications by testing normal and amblyopic eyes over a wide range of exposure times and background luminance levels. He found that "the normal and amblyopic eyes have essentially equal thresholds at the absolute threshold level, which substantiates the work of Wald and Burian." In addition, the fact that the thresholds for the two eyes are not the same at higher luminance levels "bears out their prediction that amblyopia has its effects mainly at higher levels of luminance." 174 Because of the impaired differential threshold of the light-adapted amblyopic eyes, Harms 184 questioned the conclusion by Wald and Burian 457 that the entire apparatus of simple light perception was normal in amblyopic eyes. However, Wald 456 pointed out that every brightness discrimination involved some degree of form discrimination, which indeed is impaired in amblyopia. Jung 234 explained the apparent contradiction between the findings of Wald and Burian and those of Harms by the following neurophysiologic considerations. The lateral inhibition, essential to contrast vision and visual acuity, disappears in dark adaptation, and spatial summation prevails. For scotopic vision there is no reduction in darklight perception, and diffuse light perception is retained by the amblyopic eye. This favors a separate neuronal mechanism for brightness perception. Herzau and coworkers 201 measured the differential light thresholds in the nasal and temporal visual field periphery in patients with strabismic amblyopia and found consistent threshold elevation in the nasal field. These data may be the psychophysical correlate of our finding that in human strabismic amblyopia the LGN has the most pronounced cell shrinkage in layers connected with the temporal retina, which corresponds to the nasal visual field. 348 Bowering and coworkers 53 evaluated the visual field with manual Goldmann perimetry in patients with deprivation amblyopia from congenital cataracts. They found in all children a restricted field, especially temporally, even when the deprivation began as late as 6 years of age and lasted less than 6 months. The restrictions were larger than those shown by the adults who developed cataracts. The restrictions were also larger after longer deprivation and monocular deprivation than after binocular deprivation. However, children who regularly patched the fellow nondeprived eye and, therefore, experienced less interocular competition, exhibited smaller restrictions temporally. Neither visual acuity nor optical factors could account for all of the restrictions in the deprived children. (MORE) 