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Lynn Olzak, Mingliang Gong; Masking, crowding or something else?. Journal of Vision 2013;13(15):P17. doi: https://doi.org/10.1167/13.15.52.
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© ARVO (1962-2015); The Authors (2016-present)
Fine spatial discriminations between two highly similar grating patches are poorer in the presence of an annular “mask”. A meta-analysis of previous experiments suggested that underlying mechanisms are in some ways similar to masking, but in other ways share properties characteristic of crowding. These studies, however, were performed with free-viewing, foveal fixation, where crowding is rarely observed. In the current experiments, we performed a “critical test” (Petrov, Poppel& McKee, 2007) to distinguish between crowding and masking. Crowding effects are asymmetric, with an outward half-annulus “mask” yielding poorer performance than an inward-facing half-annulus, whereas masking shows no difference between the two. Observers were six undergraduate students, all well practiced. Stimuli were 40-minute, sharp-edged sinusoidal, 4 cpd patches presented 9 degrees peripherally at 10% contrast. Both left and right fixation points were used. Stimuli were presented for 500 milliseconds with abrupt onset and offset. Each session consisted of 80 trials, and each observer replicated each session 10 times. Viewing was monocular. The stimuli differed very slightly in orientation, varying symmetrically around vertical. In control conditions, the stimulus patch was presented alone. In two separate masking conditions, a half annulus (cut vertically) was presented, facing either inward (away from fixation) or outward. Results for all observers showed significant masking effects relative to control. A comparison between the two masking conditions revealed individual differences. Three observers showed no difference between the masking conditions. The three who did show a difference showed worse performance in the inward condition, the opposite direction as crowding.
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