Abstract
Purpose: :
To compare the effects of visual noise on the precision of heading judgments in low vision patients with central visual loss resulting from either age-related macular degeneration (AMD) or diabetic retinopathy (DR) to similarly aged normal subjects.
Methods: :
Four groups of observers made heading judgments using a relatively standard optic flow display. The groups were low vision patients either with AMD aged >60 years or with DR aged >60 years, older normals aged >60 years, and younger normals aged 21-50 years. Observers were seated 50 cm from a display that subtended 44.6 arc deg in the horizontal dimension. The observer saw a red line at the center of the screen. Pressing a mouse began a trial. During the 833 mS trial, an optic flow pattern of 100 white dots on a black background appeared at random locations and moved away from a focus of expansion (FOE). Dots had a lifetime of 467 mS or were replaced as they moved off of the screen. At the end of the trial the central red line reappeared. The observers’ task was to determine if the FOE was to the right or left of the red line. A double staircase was employed to determine a threshold for the FOEs to the right and to the left of the center. Precision was defined as ½ the difference of the thresholds. Observers repeated the judgments under 4 conditions: a no noise conditions and 3 conditions in which the individual flow vectors were perturbed by Gaussian directional noise along the x-axis. The standard deviation (SD) of the noise varied from 0.92 to 2.75 deg.
Results: :
As the SD of directional noise increased, the precision thresholds for the young normal, older normal, and DR groups remained constant at about 3.13, 4.24, and 5.58 deg., respectively. In the absence of noise, the precision of AMD patients was 4.78 deg and not different from that of older normals (p > 0.1). As noise increased precision thresholds of AMD patients increased to 8.76 arc deg. in a linear fashion (r = 0.33; p < 0.02).
Conclusions: :
Noise at the levels used in this experiment does not impair heading precision in normals or DR patients. Noise does impair the performance of AMD patients. The difference in the effects of noise on the performance of AMD and DR patients does not appear to result from age differences in the populations.
Keywords: low vision • age-related macular degeneration • clinical research methodology