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Katharine Tillman, Denis Pelli, Jeremy Freeman, Michael Su, Tracey Berger, Najib Majaj; Reading is crowded. Journal of Vision 2007;7(9):341. doi: 10.1167/7.9.341.
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The old discovery that readers make several fixations per second, rather than a continuous sweep across the text, suggests that reading might be limited by the number of letters that can be acquired in one fixation. That span has been measured in various ways, but remains unexplained. Here we prove that the “visual span” is simply the number of characters that are not crowded. We measure RSVP reading rates for text, in both original and scrambled word order, as a function of size and spacing at central and peripheral locations. As text size increases, reading rate rises abruptly from zero to maximum rate. This classic reading-rate curve consists of a cliff and a plateau, characterized by two parameters, critical print size and maximum reading rate. Joining two ideas from the literature explains the whole curve. These ideas are Bouma's law of crowding and Legge's conjecture that reading rate is proportional to “visual span”. We show that Legge's visual span is the uncrowded span predicted by Bouma's law. This result joins Bouma and Legge to explain reading rate's dependence on letter size and spacing. This explanation is the uncrowded-span model of reading rate. Having explained the shape of the curve, we then predict its parameters. We use a “silent substitution” technique to measure the uncrowded span during reading. These substitutions spoil letter identification yet are undetectable when crowded. We find that the critical spacing for letter identification predicts both the critical spacing and the perceptual span for reading. Thus, crowding predicts the parameters that characterize both the cliff and the plateau of the reading rate curve. In all conditions tested — all sizes and spacings, central and peripheral, ordered and scrambled — reading is limited by crowding.
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