Purpose:
Patients with advanced age-related macular degeneration (AMD) are slow in finding a target in a feature search task. This study examined whether the deficiency was the result of a global visual sensitivity/oculomotor impairment or a more localized lesion.
Methods:
44 patients with AMD and 25 age-matched normal controls (mean acuities 20/154 and 20/19) were asked to search for a 2ox2o square target among 1ox1o square distracters. Only reaction times (RT) of correct responses were analyzed. In each search session, the target appeared once at each of the 36 locations of a 6x6 grid, thus producing a 6x6 RT map. Three set sizes (8, 16 and 32 items) were tested at each of three field sizes (10o, 20o and 40o). Sufficient practice was given before testing.
Results:
Hit rates of both subject groups were high (>95%). The mean RT map of normals had the shape of a shallow smooth basin, with peripheral target locations elevated (longer RTs). A few AMD RT maps either had a similar shape and elevation as normal maps or were much elevated overall. In contrast, the majority of AMD RT maps had two features, a large area that was pointwise comparable with normal maps and a small area of high peaks that represented RTs several times longer than corresponding normal values. The graph shows the difference between an AMD map and the mean normal map. On average, 19 and 25 locations out of 36 (55% and 71%) on AMD maps fell within 1 and 2 standard deviations (SD) of mean normal values, respectively. The 20o field had the best RT map agreement (22 and 28 locations within 1 and 2 SD) and the 40o field had the worst (16 and 22 locations within 1 and 2 SD). Set size had little effect on map agreement. On average, there were 1.7 locations in each AMD map that were 2 SD higher than the normal mean. Because RT is usually averaged over all locations, a few very slow locations can determine the overall slowness of AMD search.
Conclusions:
Patients with advanced AMD could search as fast as age-matched normals at many target locations, but were very slow at others. In most AMD patients, slow search could be attributed to localized factors, presumably retinal lesions.
Keywords: visual search • age-related macular degeneration