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Vineela Varikuti, John Siderov, Ebi Osuobeni; Foveal letter crowding: Is it due to contour interaction or gaze instability? . Journal of Vision 2014;14(10):786. doi: https://doi.org/10.1167/14.10.786.
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© ARVO (1962-2015); The Authors (2016-present)
Aim: Foveal crowding has been suggested to comprise elements of contour interaction, gaze instability and attention (Flom, 1991). The aim of this study was to investigate if the decreased visual thresholds with crowded vision charts are due to contour interaction or gaze instability by comparing visual acuity for repeat and complex (non-repeat) letter chart designs with equal inter-letter separations. Methods: Visual acuity for 3 normal (NE) and 4 amblyopic observers (NAE and AE) was determined using a method of constant stimuli for high contrast Sheridan Gardiner (SG) letters (black on white) in repeat letter chart format (SGR). Similar measurements were made for SG letters arranged in a complex letter format (SGC) similar to the Flom 'S' Chart. Inter-letter separations for both the charts ranged from abutting, 0.2, 0.4, 0.6, 1.0 letter width separations. For comparison, single letter, unflanked acuity was also measured. For some observers, inter-letter separation for the SGC chart was extended beyond 1.0 letter widths until acuity equaled SGR acuity at 1.0 letter width. Results: Isolated SG letter thresholds equaled SGR thresholds at 1.0 letter width and SGC thresholds for ≥ 1.0 letter width separations. SGC thresholds were significantly higher than the SGR thresholds indicating a crowding effect (ANOVA, P<0.05). However, the difference in thresholds between the two charts remained constant between the abutting and 1.0 letter separation conditions and averaged 0.08, 0.13 and 0.23 LogMAR for the NE's, NAE's and AE's respectively. Conclusions: The constant difference between repeat and complex (non-repeat) letter chart acuity for close inter-letter separations suggests that imprecise or inaccurate fixation contributes to foveal crowding. The difference between the two chart formats was more exaggerated for amblyopic eyes. This result confirms earlier reports showing the importance of relative gaze in assessing visual acuity and evaluating foveal crowding in both normal and amblyopic vision.
Meeting abstract presented at VSS 2014
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