Shape discrimination in age-related macular degeneration

Recent studies suggest that a global shape-discrimination task is sensitive to neural undersampling and/or irregular sampling, but is not affected by normal aging. In this study, the ability of patients with age-related macular degeneration (AMD) to perform the shape-discrimination task was examined.

Twenty patients with AMD (age range, 66-81 years) were selected on the basis of Snellen visual acuity of 20/50 or better in at least one eye and prior clinical documentation. A control group consisted of 10 older subjects (age range, 61-93 years) with normal findings in a fundus examination. Radial frequency (RF) patterns were used as stimuli. A spatial paradigm and a temporal two-alternative, forced-choice (2AFC) staircase paradigm were used. In each trial, two RF patterns (one deformed and one undeformed) were presented, and patients were asked to identify the deformed pattern. The peak spatial frequency of RF patterns was 5 cyc/deg; the radial modulation frequency was 8 cyc/360 degrees; mean radii were 0.5 degrees, 1 degrees, 2.0 degrees, or 2.5 degrees; and stimulus contrast was 80%. Thresholds for detecting the deformation were estimated by a maximum-likelihood fitting procedure.

Thirty-five of 40 eyes with AMD had 20/50 or better acuity. Among them, 29 eyes had early AMD (drusen, hyperpigmentation, hypopigmentation), 5 had extrafoveal geographic atrophy, and 1 had exudative AMD. With the spatial 2AFC, 91% (32/35) of eyes with AMD showed significant elevation of the threshold for detecting radial deformation of RF patterns when compared with normal control eyes. With the temporal 2AFC, 97% (31/32) of eyes with AMD showed significant threshold elevations, and the degree of the deficit in the shape discrimination did not correlate significantly with visual acuity loss (r = 0.3, P = 0.094). Comparison of the severity of AMD with shape-discrimination performance revealed that the average detection threshold of the eyes with extrafoveal geographic atrophy was significantly higher than that of the eyes with drusen only (P < 0.01), even though average acuity showed no significant difference.

Patients with AMD had significant deficits in performing the global shape-discrimination task. The dissociation of shape discrimination with visual acuity suggests that the shape-discrimination task may provide distinguishable information about the integrity of the photoreceptor mosaic in AMD.