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Michael K. Chiu, Julian Wallace, Anirvan S. Nandy, Bosco S. Tjan; Peripheral crowding with unlimited viewing time. Journal of Vision 2011;11(11):1154. doi: 10.1167/11.11.1154.
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Studies of peripheral form vision are often limited to briefly presented stimuli to avoid eye movements. These duration-constrained results present a narrow perspective of peripheral form vision, and may not generalize to the visual conditions of people with central field loss. Here, we studied peripheral crowding with unlimited stimulus duration by using a gaze-contingent display. We chose crowding because it is a key limiting factor of peripheral form vision.
We measured contrast threshold elevation as a function of target-flanker spacing for identifying 26 lower case letters at 50% correct (corrected for guessing). In the FIXATION condition, subjects maintained fixation at a marker while a target letter, flanked above and below, was presented at 12 degrees eccentricity in the lower visual field for an unlimited duration. In the SCOTOMA condition an artificial central scotoma, 12 degrees in diameter, constrained target eccentricity.
Wallace and Tjan (2010 VSS), using the same stimuli with 250 ms duration, reported a spatial extent of crowding of between 3–4 degrees at 10 degrees eccentricity and a contrast threshold elevation of 10. The median stimulus duration (and response time) in the current study was 900 ms. The spatial extent of crowding was 1.5–2.3 degrees across subjects and conditions, which is half of the short-duration experiment after adjustment for the eccentricity difference. Threshold elevation remained unchanged. Compared to the FIXATION condition, the SCOTOMA condition was 80 ms longer in presentation duration, but slightly smaller in spatial extent of crowding.
Our results show that peripheral form vision is effortful: the preferred stimulus duration is many times the duration in a typical peripheral-vision experiment (900 ms vs. 100–200 ms). While duration significantly reduced the spatial extent of crowding, the effect of crowding (e.g. threshold elevation) remains the same. Theories of peripheral form vision should account for this dynamic aspect of crowding.
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