Abstract
Purpose: Chung (IOVS, 43, 2002, 1270–1276) has shown that RSVP (Rapid Serial Visual Presentation) reading speed varies with letter spacing, peaking near the standard letter spacing and decreasing rapidly at smaller spacing. This pattern was linked to the crowding effect which refers to the interference in letter recognition from adjacent letters. It has been proposed that the size of the visual span — the number of letters recognized with high accuracy in a single fixation — is a visual factor limiting reading speed. We predict that the size of the visual span and reading speed will show a similar dependence on letter spacing. We tested this prediction for RSVP reading, and asked whether it generalizes to the reading of flashcards composed of blocks of text.
Methods: Five normally sighted adults participated. Visual span profiles, measured with trigrams (strings of 3 random letters with designated spacing), are plots of letter-recognition accuracy as a function of letter position left or right of the midline. Size of the visual span was defined as the area under this profile, converted to bits of information transmitted. Reading performance was measured using 2 presentation methods: RSVP and flashcard (a 56 character long sentence arranged in 4 lines). Four letter spacings (0.5x, 0.707x, 1x, 2x “standard spacing”) and two print sizes (0.08° and 0.15°) were used.
Results: Sizes of the visual span and reading speeds measured by the two presentation methods showed a qualitatively similar dependence on letter spacing for the two print sizes, and were highly correlated (correlation coefficient ranges from 0.710 to 0.999 and the median is 0.908) across 5 subjects.
Conclusions: The high correlation between reading speed and size of the visual span is consistent with the view that spacing effects on reading speed are due to changes in the size of the visual span.
Supported by NIH grant EY02934 and EY12810