Psychophysics of reading XX. Linking letter recognition to reading speed in central and peripheral vision

Psychophysics of reading: xviii. the effect of print size on reading speed in normal peripheral vision

People with macular scotoma tend to read and visually scan more slowly than others with equivalently reduced visual acuity but intact central fields We measured fixation eye movements and considered the contribution of fixation variability and centripetal eye drift to poor visual performance These factors might confound efforts to consistently use an optimum retinal locus outside of the macula We measured monocular horizontal and vertical eye movements using a search coil eyetracker while subjects with naturally occurring central scotomata or control subjects with simulated scotomata eccentrically fixated a single character that was sized to their visual acuity Motivated subjects with long-standing stable maculopathies were chosen to estimate attainable performance limits During attempts to eccentrically fixate, an ubiquitous foveal pursuit or centripetal drift tendency was not found; rather a pattern of drift was idiosyncratic from subject to subject This finding was confirmed by an analysis of eye drift of 32 eyes with long-standing bilateral macular scotomata Moreover, the eye drift speeds (15-200 minarc/sec) were too low to be of functional significance Drift speeds during eccentric fixation with a visible target were not significantly different than those after the target was extinguished; however, drift speeds were greater than during foveal fixation This suggests that the fovea has a specialized control of slow eye movements Fixation variability increased with scotoma size for both simulated and real scotomata, with an abrupt rise when scotomata diameters exceeded 20 degrees C A significant minority of subjects (39%) adopted two or more distinct preferred retinal loci (PRL) during fixation Multiple PRL were also more likely if scotoma size exceeded 20 degrees C Reasonably steady fixation is thus attainable when central scotoma sizes are smaller than approximately 20 degrees C

Eccentric fixation with macular scotoma

Low vision reading with sequential word presentation

Saccade control without a fovea.

A critical part of visual perceptions, our ability to orient to a visual environment and quickly identify its most interesting attributes, depends upon our control of eye movements. Within a few seconds of confronting a page of text or a city street our oculomotor control system organizes and executes a series of saccades. The eye darts about the scene, bringing aspects of interest into the narrow field of the fovea for the resolution of detail. In this manner, our oculomotor system paints a global perception of a visual scene or transmits a message from the printed page to the brain.

R. W. Cummings

Papers

Redevelopment of Fixation and Scanning Eye Movements Following the Loss of Foveal Function