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MiYoung Kwon, Gordon E. Legge; Spatial-frequency Requirements For Reading Revisited. Invest. Ophthalmol. Vis. Sci. 2011;52(14):1906.
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Legge et al. (Vision Res., 1985) measured reading speed for text that was low-pass filtered with a range of cutoff spatial frequencies. Above 2 cycles per letter (CPL) reading speed was constant at its maximum level. For cutoff frequencies below 2 CPL, reading speed decreased rapidly. It remains unresolved why the critical cutoff frequency for reading speed is near 2 CPL. Previous research indicates that two important factors limiting reading speed are the spatial resolution of individual letters and the size of the visual span (the number of letters that can be recognized without moving the eyes). In this study, we asked whether the bandwidth requirement for rapid reading can be related to the effects of bandwidth on letter recognition and the size of the visual span.
Visual span profiles were measured by asking participants to recognize letters in trigrams (random strings of three letters) flashed for 150 ms at varying letter positions left and right of the fixation. Over a block of trials, a profile was built up showing letter recognition accuracy versus letter position. The area under this profile was defined to be the size of the visual span. Reading speed was measured with Rapid Serial Visual Presentation (RSVP). The size of the visual span and RSVP reading speed were measured using low-pass filtered letters with various cutoff frequencies (0.8~2.5 CPL). Recognition data for individual low-pass-filtered letters, obtained under similar testing conditions, were available from a previous study.
We found that the size of the visual span and RSVP reading speed showed similar dependence on spatial-frequency bandwidth. Both the size of the visual span and reading speed increased with cutoff frequency up to a critical cutoff of 1.4 CPL, which is lower than the previous estimate of 2 CPL. A regression analysis using the size of the visual span as a predictor indicated that 92% of the variability in RSVP reading could be explained by the size of the visual span.
These results are consistent with the hypothesis that the visual span plays a limiting role in reading speed. Comparison of these findings with our previous results on single letter recognition suggests that the bandwidth requirements for reading and the size of the visual span are closely linked to the bandwidth requirement for single letter recognition.
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