Crowding during restricted and free viewing

Abstract

Crowding impairs the perception of form in peripheral vision. It is likely to be a key limiting factor of form vision in patients without central vision. Crowding has been extensively studied in normally sighted individuals, typically with a stimulus duration of a few hundred milliseconds to avoid eye movements. These restricted testing conditions do not reflect the natural behavior of a patient with central field loss. Could unlimited stimulus duration and unrestricted eye movements change the properties of crowding in any fundamental way? We studied letter identification in the peripheral vision of normally sighted observers in three conditions: (i) a fixation condition with a brief stimulus presentation of 250 ms, (ii) another fixation condition but with an unlimited viewing time, and (iii) an unrestricted eye movement condition with an artificial central scotoma and an unlimited viewing time. In all conditions, contrast thresholds were measured as a function of target-to-flanker spacing, from which we estimated the spatial extent of crowding in terms of critical spacing. We found that presentation duration beyond 250 ms had little effect on critical spacing with stable gaze. With unrestricted eye movements and a simulated central scotoma, we found a large variability in critical spacing across observers, but more importantly, the variability in critical spacing was well correlated with the variability in target eccentricity. Our results assure that the large body of findings on crowding made with briefly presented stimuli remains relevant to conditions where viewing time is unconstrained. Our results further suggest that impaired oculomotor control associated with central vision loss can confound peripheral form vision beyond the limits imposed by crowding.

Appearance changes and error characteristics in crowding revealed by drawings

“…For example, it was shown that substitution and error rates were strongly reduced with increasing presentation times (Styles & Allport, 1986). Other studies, however, showed that unlimited viewing time only marginally changed crowding (Wallace, Chiu, Nandy, & Tjan, 2013), and high substitution rates also occurred with relatively long presentation times (2.4 s, Estes, Allmeyer, Reder, 1976;1.6 s, Zhang et al, 2012). An alternative explanation is that multiple presentations, i.e., multiple (peripheral) views of a single stimulus during one trial, caused the observed low substitution rates.…”

Training improves visual processing speed and generalizes to untrained functions

“…Thus, masking and crowding may be determined by multiple sources of interference operating at several levels of cortical processing 51 86 and each of them might affect the task. Among these factors are a) the proximity between the target and the flankers, which depends on the eccentricity 3 4 85 , b) the duration for which the target is visible [in the fovea longer presentation times reduce crowding and masking such that at presentation times longer than 120 ms there are no crowding effects 49 51 ; in the periphery, presentation times longer than 250 ms do not affect crowding 88 even though such elongated presentation times can involve eye movements that potentially increase crowding 87 , and whether presentation times shorter than 250 ms affect peripheral crowding is still unclear, c) the temporal order (dynamics) of the presentation (backward, simultaneous, or forward masking 49 58 89 ), d) the global configuration and grouping between the mask and the target elements, where collinear configuration seems to produce the maximal effect 57 86 90 , e) contrast – where higher crowding is found with a higher contrast threshold, and f) attention 91 . Thus, crowding and masking may or may not be correlated, depending on the particular spatial-temporal parameters chosen in the study.…”

Spatio-temporal properties of letter crowding

“…One of the better-known temporal properties of crowding is the effect of stimulus duration. In general, the critical spacing is larger for stimulus presented for a shorter duration than for a longer one (Chung & Mansfield, 2009 ; Kooi et al, 1994 ; Tripathy & Cavanagh, 2002 ; Wallace et al, 2013 ). Chung and Mansfield ( 2009 ) reported a reduction in the critical spacing by approximately half when the stimulus duration increased from 53 to 1000 ms (for targets and flankers with the same contrast polarity).…”