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Denis G. Pelli; Towards an easier way to measure the visual span. Journal of Vision 2009;9(8):1002. doi: https://doi.org/10.1167/9.8.1002.
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While reading, people advance their eyes by a distance equal to the “visual span,” four times per second. Thus, the tenfold increase in reading speed during childhood implies a proportional increase in the visual span, and there is some data to support this (reviewed in Pelli & Tillman, 2008, Nature Neuroscience). The visual span is defined as the number of letters, in a line of text, that one can identify without moving one's eyes. That is hard to apply with children. In normal adults, the visual span is limited by crowding. It is not clear what determines the visual span of children. We are developing a paper test to measure visual span. Large visual gaps are introduced, isolating words below a certain length, and this impairs reading speed if the words are shorter than the visual span. We will describe several different designs and their merits. All require only a stopwatch and an observer who can read.
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