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Deyue Yu, Gunther Wagoner, Gordon E. Legge, Susana T. L. Chung; Sensory factors limiting horizontal and vertical reading speed. Journal of Vision 2009;9(8):820. doi: 10.1167/9.8.820.
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Reading speed is slower for text oriented vertically than horizontally. Yu et al. (VSS 2008) showed that slower reading of vertical text is associated with a smaller visual-span (the number of letters recognized with high accuracy without moving the eyes). Three low-level determinants of the size of the visual span are resolution (letter acuity at the tested letter position), mislocation (uncertainty about relative position of letters in strings) and crowding (interfering effects of flanking letters). In the present study, we asked which of these factors is most important in determining the size of the visual span, and in determining the horizontal/vertical difference. We measured visual-span profiles—plots of letter-recognition accuracy as a function of distance from fixation in letter positions—with trigrams (strings of three random letters) and also with isolated letters. The effect of resolution was estimated from isolated-letter profiles. Mislocation errors were extracted from trigram profiles. Errors due to crowding referred to the differences between isolated-letter and trigram profiles, taking into account mislocation errors. Consistent with our prior findings, we found that visual-span size (area under the profile) is smaller for vertical than horizontal text. When compared with a profile of perfect identification, the overall size reduction for vertically oriented text was 16.6 bits—1.1 bits were due to declining resolution from fixation, 2.9 bits to mislocations and 12.6 bits to crowding. For horizontal text, the overall size reduction was 5.3 bits—0.3 bits due to declining resolution, 1.3 bits to mislocations and 3.7 bits to crowding. We conclude that crowding is the major factor limiting the size of the visual span, and that the horizontal/vertical difference in the size of the visual span is associated with stronger crowding along the vertical midline.
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