Uncovering foveal crowding?

Abstract

Visual crowding, as context modulation, reduce the ability to recognize objects in clutter, sets a fundamental limit on visual perception and object recognition. It's considered that crowding does not exist in the fovea and extensive efforts explored crowding in the periphery revealed various models that consider several aspects of spatial processing. Studies showed that spatial and temporal crowding are correlated, suggesting a tradeoff between spatial and temporal processing of crowding. We hypothesized that limiting stimulus availability should decrease object recognition in clutter. Here we show, for the first time, that robust contour interactions exist in the fovea for much larger target-flanker spacing than reported previously: participants overcome crowded conditions for long presentations times but exhibit contour interaction effects for short presentation times. Thus, by enabling enough processing time in the fovea, contour interactions can be overcome, enabling object recognition. Our results suggest that contemporary models of context modulation should include both time and spatial processing.

Figure 1. Contour interaction effect at the fovea – one letter spacing: Data is presented for 11 participants.

a) The target was letter E (single) and the subject's task was to report the open direction of the E. In crowded cases, the target E was embedded in an array of other randomly oriented Es, separated by spacing of one letter. The task of the subject was to report the opening direction of the letter E under both conditions. b) The percentage correct (y axis) for the single (uncrowded, red line) and crowded (blue line) conditions are plotted against the stimulus duration (x axis). The letter size was 0.18 deg. and the target-spacing was one letter size c) The reaction time in msec (y axis) is plotted against the stimulus duration (x axis). There was no significant effect of contour interaction for all stimulus durations. The reaction time for the crowded conditions was always significantly slower by about 50 msec.

Figure 2. Comparison of the contour interaction effect for different letter spacing: Data are presented for 11 new participants.

a, c) The percentage correct (y axis) for the single (uncrowded, red line) and crowded (blue line) conditions are plotted against the stimulus duration (x axis). The letter size is 0.12 deg. and the letter spacing is one letter size (a) and 0.4 letters (c). There is a robust and significant effect of reduction in the percent correct at the short presentation times. c, d) The reaction time in msec (y axis) is plotted against the stimulus duration (x axis). The reaction time for the crowded conditions was always significantly and robustly slower for all presentation times.

Figure 3. Contour interaction effect relative to visual acuity for young and presbyopic participants.

a) The contour interaction effect (y axis) for the single (uncrowded, red line) and the crowded (blue line) conditions are plotted against the stimulus duration (x axis).The letter size is determined at the threshold of each participant and letter spacing is one letter size. For the presbyopic participants (N = 97 average age = 51 ± 0.64; mean ± se) there was no contour interaction effect for one letter spacing. b) The contour interaction effect (y axis) for one letter spacing (dark blue line, triangles) and 0.4 letter spacing (light blue line, circles) are plotted against the stimulus duration (x axis). The letter size is determined at the threshold for each participant. For new presbyopic participants (N = 41 average age = 50.3 ± 0.13; mean ± se) there was almost a constant contour interaction effect of about 0.5 ETDRS lines (~12%) for 0.4 letter spacing for all time presentation. c) As for b but for young participants (N = 18 average age = 25.4 ± 0.77; mean ± se). There was a robust, and almost constant, contour interaction effect of about 1.5 ETDRS lines (~41%) for 0.4 letter spacing for all presentation times.

Figure 4. Backward masking reveals a crowding effect at the fovea.

a) Stimuli-target and crowded conditions were the same as in Figure 1.After the presentation time, a mask consisting of random Es, separated by one letter spacing, appeared after inter stimulus intervals from 30 to 120 msec. The task of the subject was to report the opening direction of the target E under the single and crowded conditions. The dashed lines indicate the response level for no backward masking conditions (baseline inter stimulus interval = 0). b, d) The percentage correct (y axis) for the single (uncrowded, red line) and crowded (blue line) conditions are plotted against the inter stimulus interval (x axis) for presentation times of 60 (b) and 30 (d) msec. c,e) The reaction time in msec (reaction time, y axis) is plotted against inter stimulus interval (x axis) for presentation times of 60 (c) and 30 (e) msec. There was a significant effect of contour interaction for both stimulus durations in both the percent correct and the reaction time.