Visual span: A sensory bottleneck on reading speed

Purpose. Visual span profiles are plots of letter-recognition accuracy as a function of letter position left or right of the midline. We hypothesize that the size of the visual span, summarized as the area under the profile, limits reading speed in normal and low vision. We tested this hypothesis by measuring the correlation between size of the visual span and reading speed for variations in the contrast, size and retinal eccentricity of letters. Method. In one experiment, visual-span profiles and RSVP reading speeds were measured for 5 normally sighted subjects for letter contrasts ranging from threshold (1.5%) to 92%. In a second experiment, similar measurements were made for 3 subjects for letter sizes ranging from the acuity limit (4') to 4°. Finally, similar measurements were obtained from our previous published work for retinal eccentricities from 0° to 20°. Results. For all 3 data sets, reading speed covaried closely with the size of the visual span, with high correlations between log reading speed and area under the visual-span profiles. Both rose sharply at low contrast and leveled out at high contrast (individual correlations from .97 to .99). For character size, both peaked for middle values with a decline for small and large letters (individual correlations from .75 to .99). For eccentricity, both declined in peripheral vision (group correlation = .98). Conclusions. Our findings are consistent with the view that reduction of the size of the visual span limits normal reading speed near the acuity limit, near contrast threshold, and in peripheral vision. Because most reading deficits in low vision are associated with losses in acuity, contrast sensitivity or field, our findings suggest that a reduced visual span is a key factor in limiting low-vision reading.