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Denis Pelli, Hormet Yiltiz; Development of crowding: A new chart to measure crowding without requiring good fixation. Journal of Vision 2016;16(12):1115. doi: 10.1167/16.12.1115.
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© ARVO (1962-2015); The Authors (2016-present)
Evidence from the periphery, strabismic amblyopia, apperceptive agnosia, and posterior cortical atrophy show an association between the critical spacing of crowding and neural density, the number of cortical neurons per deg^2 responding to a given eye (see Strappini, et al., submitted to Cortex). Evidence for reduction in the critical spacing of crowding in development could be misleading if it's merely a result of improved fixation with age. David Regan introduced a repeated-letter acuity test, to measure acuity without requiring good fixation. We use a variant of that test here to measure acuity, and we introduce a new repeated-letter crowding test to measure crowding without requiring good fixation. Regan's repeated-letter acuity test repeats the target letter AAAAA. Our repeated-letter crowding test alternates two targets ABABABABAB and the observer is asked to report both targets. For a balanced design, we create a dual-target repeated-letter acuity test AAAAABBBBB, and ask the observer to report both targets. We also use conventional single-target tests for acuity A and crowding XAX. Center-to-center letter spacing was always 1.4 times the letter size, and we used QUEST to adjust size (or spacing) to threshold, allowing the other parameter to co-vary. Pilot data on three normally sighted students shows excellent agreement between threshold size & spacing measured with single vs. repeated targets: size 0.054±1 vs. 0.050±4 deg & spacing 0.083±6 vs. 0.081±10 deg (M±SE). Conclusions: 1. Repeating the targets does not change threshold size and spacing in normally sighted adult observers with good fixation. 2. Repeating a single letter does not produce crowding, presumably because jumbling the target and flanker features yields the same features as a target alone. 3. This chart is a promising way to measure the development of foveal crowding even in patients or children with poor fixation.
Meeting abstract presented at VSS 2016
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