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Maria Lev, Yehezkel Oren, Anna Sterkin, Uri Polat; Near visual acuity is reduced for brief presentation time and improves after practice. Invest. Ophthalmol. Vis. Sci. 2014;55(13):774.
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© ARVO (1962-2015); The Authors (2016-present)
Visual crowding, the inability to recognize objects in clutter, sets a fundamental limit on visual acuity (VA). It is assumed that, in the normal fovea, crowding does not exist or it only occurs over very small distances. We showed that in amblyopia an increased crowding effect was correlated with reduced temporal resolution (temporal crowding), suggesting a tradeoff between spatial and temporal processing of crowding. We also showed that limiting stimulus availability, using backward masking and 1-letter spacing, results in increased crowding.
Here we tested the crowding effect for over 100 participants (young and presbyopic) without backward masking, using a smaller letter spacing. We used the “contour interactions” method to measure the crowding effect on visual acuity (VA) of a single E letter size compared to VA in situation of target-flanker spacing of either 0.4 letter size which is assumed to be at the border of the critical window of crowding in terms of letter spacing. The results were compared to 1-letter spacing, which is assumed to produce no crowding effect and is widely used in clinical measurements, such as in ETDRS charts. The crowding effect was measured as a function of presentation time (30, 60, 120 and 240 msec). In addition, we explored how training affects crowding. Training consisted of detecting Gabor stimuli under spatial and temporal masking (Polat et al., 2012). It was carried out on iOS devices at a distance of 40 cm (GlassesOff application) and was shown to improve near VA by ~80% with persistence of more than 6 years.
Robust crowding effect exists in the fovea for short presentation times of 30 and 60 msec; however, participants overcome crowding for longer presentations times. Training resulted in significant reduction of the crowding effect for all presentation times, but more pronouncedly for shorter presentation times. The improvement was transferred to near VA and other visual functions.
While crowding limits VA, efficient temporal processing is very important in overcoming the crowding effect. Training improved processing speed, thereby overcoming this limit and enabling generalization of the improvement to other visual functions.
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