PSYCHOPHYSICS OF READING .16. THE VISUAL SPAN IN NORMAL AND LOW-VISION

The visual span in reading is the number of characters that can be rec ognized at a glance. The shrinking visual span hypothesis attributes r eading speed deficits in low vision, and slow reading in normal vision at low contrast, to a reduction in the visual span, This hypothesis p redicts that reading time (msec/word) becomes increasingly dependent o n word length as text contrast decreases. We tested and confirmed this prediction using the rapid serial visual presentation (RSVP) method, Estimates of the visual span ranged from about 10 characters for high- contrast text to less than two characters for low-contrast text. Eye-m ovement recordings showed that longer reading times at low contrast ar e partitioned about equally between prolonged fixation times and an in creased number of saccades (presumably related to a reduced visual spa n). RSVP measurements for six out of seven low-vision subjects reveale d a strong dependence of reading time on word length, as expected from reduced visual spans. (C) 1997 Elsevier Science Ltd.