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Seong Taek Jeon, Joshua Hamid, Daphne Maurer, Terri Lewis; The letter in the crowd: Developmental trajectory of single letter acuity and foveal crowding. Journal of Vision 2009;9(8):999. doi: 10.1167/9.8.999.
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Crowding (Stuart & Burian, 1962) refers to impaired target recognition caused by surrounding contours. Studies on developmental changes in crowding (Atkinson et al., 1988; Bondarko and Semenov, 2005; Semenov et al., 2000) fail to provide a clear picture. To investigate the developmental trajectory of foveal crowding, groups (N = 20/age group) of adults (mean age = 19.4 yrs, range 18 – 23 yrs) and children aged 5.5, 8.5 and 11.5 years (+/− 3 months) were asked to discriminate the orientation of a Sloan letter E. We first measured the single-letter threshold, defined as the stroke width discriminable at 79% correct performance. We then multiplied the single-letter threshold by 1.2 and surrounded it with flankers consisting of four sets of three bars randomly oriented horizontally or vertically. The crowding threshold was measured as the distance between the nearest edges of the flankers and the central letter yielding 79% correct performance.
Mean single-letter thresholds were 1.0, 0.8, 0.8 and 0.8 arcmin for 5-, 8-, 11-year-olds and adults, respectively. Single-letter thresholds for 5-year-olds were significantly worse than those for all older age groups, which did not differ significantly from each other. The crowding threshold did not differ significantly among children, (9.9, 9.7, and 7.8 times stroke width for 5-, 8-, and 11-year-olds, respectively) but decreased significantly to 3.5 times the threshold stroke width in adults. Thus, single letter acuity is mature by age 8 but even 11-year-olds need more space between adjacent contours than do adults to avoid the deleterious effects of crowding. According to current theories (Levi, 2007; Motter, 2002; Pelli et al., 2007), crowding occurs when features of stimuli are inappropriately combined. Therefore, the stronger influence of crowding on younger children might be caused by immaturities in the brain areas where early visual inputs are combined, such as V4.
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