Abstract
Purpose:
Reading is impacted negatively in most subjects with central field loss. These subjects typically fixate with a non-foveal preferred retinal locus, which can include areas of localized sensitivity loss, or micro-scotomas. In this study, we simulated micro-scotomas at the fovea and in the peripheral retina of normal subjects to assess their impact on rapid-serial-visual-presentation (RSVP) reading speed.
Methods:
A custom MATLAB program using Psychtoolbox-3 simulated 13 x 13 min micro-scotomas that were superimposed on text presented in RSVP format. Reading material included 60-character MNread sentences and 53 ± 8 character sentences, taken from novels. Five naïve subjects with normal vision (ages: 23-31 years) read words presented at or above the critical print size (CPS) at the fovea and at 5 and 10 deg in the inferior visual field. At each eccentricity, critical print size (CPS) was estimated as 0.4 + mean logMAR reading acuity, determined in a preliminary experiment. Text was presented monocularly to the preferred eye on an 85-Hz CRT display as black ‘Courier’ letters on a uniform white background. In different conditions, randomly distributed 13 x 13 min blocks corresponding to 0, 13, 26 and 39% of the text area were set to the background luminance to simulate micro-scotomas. The location of the micro-scotomas was jittered (SDx, SDy = 0.3 deg) across successive words to simulate fixational eye movements. A staircase algorithm was used to estimate the threshold exposure duration for each combination of retinal eccentricity and micro-scotoma density. Reading speed in words per minute (WPM) was determined for each subject from the average of two staircase estimates.
Results:
Average foveal reading speed with no micro-scotomas was 688 ± 269 WPM. Reading speeds at 5 and 10 deg eccentricity were similar and approximately half of the foveal value. With 39% micro-scotomas, average RSVP reading speeds decreased at the fovea, 5 and 10 deg locations by approximately 80, 65 and 25%, respectively, from the no-scotoma condition.
Conclusions:
The impact of simulated micro-scotomas on reading speed for words that scale in size with eccentricity is greater in the central compared to peripheral retina. This outcome can be accounted for by the probability that randomly positioned micro-scotomas “cluster” into groups that approximate individual letter features in size.