Conservation of crowding distance in human V4
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Crowding — the inability to recognize objects in clutter — severely limits object recognition and reading. In crowding, a simple target (e.g. a letter) that is recognizable alone cannot be recognized when surrounded by clutter that is less than the psychophysical crowding distance away (deg). Prior work shows that crowding distance scales linearly with target eccentricity and varies with the direction of crowding: crowding distance is approximately double for flankers placed radially rather than tangentially. Multiplying the psychophysical crowding distance by the cortical magnification factor yields the cortical crowding distance (mm of cortex). In V1, radial cortical crowding distance is a fixed number of mm and conserved across eccentricity, but not across orientation (Pelli, 2008). Since crowding distance in V1 is conserved radially across eccentricity, we imagined that there might be some downstream area, more involved in crowding, where the crowding distance is isotropic, conserved across both eccentricity and orientation. METHOD: We measured psychophysical crowding distances on 4 observers at eccentricities of ±2.5°, ±5°, and ±10°, radially and tangentially, for letter targets on the horizontal meridian. Results confirmed the well-known dependence on eccentricity and orientation. Using anatomical and functional MRI, we also measured each observer's retinotopic maps, and quantified tangential and radial cortical magnification in areas V1-hV4. RESULTS & CONCLUSION: We find that all four areas conserve cortical crowding distance across eccentricity, but only hV4 conserves crowding distance across both eccentricity and orientation. After averaging measurements across observers (n=4), we find that the V4 crowding distance is 3.0±0.2 mm (mean±rms error across orientation and eccentricity). Across both dimensions, conservation fails in V1-V3, with rms error exceeding 0.7 mm. The conservation of crowding distance in hV4 suggests that it mediates the receptive field of crowding, i.e. the integration of features to recognize a simple object. Meeting abstract presented at VSS 2018Keywords:
Crowding
Eccentricity (behavior)
Peripheral vision
Visual crowding is the disruptive effect of clutter on object recognition. Although most prominent in adult peripheral vision, crowding also disrupts foveal vision in typically developing children and those with strabismic amblyopia. Do these crowding effects share the same mechanism? Here we exploit observations that crowded errors in peripheral vision are not random: Target objects appear either averaged with the flankers (assimilation) or replaced by them (substitution). If amblyopic and developmental crowding share the same mechanism, then their errors should be similarly systematic. We tested foveal vision in children aged 3 to 8 years with typical vision or strabismic amblyopia and peripheral vision in typical adults. The perceptual effects of crowding were measured by requiring observers to adjust a reference stimulus to match the perceived orientation of a target “Vac-Man” element. When the target was surrounded by flankers that differed by ± 30°, all three groups (adults and children with typical or amblyopic vision) reported orientations between the target and flankers (assimilation). Errors were reduced with ± 90° differences but primarily matched the flanker orientation (substitution) when they did occur. A population pooling model of crowding successfully simulated this pattern of errors in all three groups. We conclude that the perceptual effects of amblyopic and developing crowding are systematic and resemble the near periphery in adults, suggesting a common underlying mechanism.
Crowding
Peripheral vision
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Crowding
Peripheral vision
Stimulus (psychology)
Salience (neuroscience)
Surround suppression
Psychometric function
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Visual crowding is the inability to recognize a target object in clutter. Previous studies have shown an increase in crowding in both parafoveal and peripheral vision in normal aging and glaucoma. Here, we ask whether there is any increase in foveal crowding in both normal aging and glaucomatous vision. Twenty-four patients with glaucoma and 24 age-matched normally sighted controls (mean age = 65 ± 7 vs. 60 ± 8 years old) participated in this study. For each subject, we measured the extent of foveal crowding using Pelli's foveal crowding paradigm (2016). We found that the average crowding zone was 0.061 degrees for glaucoma and 0.056 degrees for age-matched normal vision, respectively. These values fall into the range of foveal crowding zones (0.0125 degrees to 0.1 degrees) observed in young normal vision. We, however, did not find any evidence supporting increased foveal crowding in glaucoma (p = 0.375), at least in the early to moderate stages of glaucoma. In the light of previous studies on foveal crowding in normal young vision, we did not find any evidence supporting age-related changes in foveal crowding. Even if there is any, the effect appears to be rather inconsequential. Taken together, our findings suggest unlike parafoveal or peripheral crowding (2 degrees, 4 degrees, 8 degrees, and 10 degrees eccentricities), foveal crowding (<0.25 degrees eccentricity) appears to be less vulnerable to normal aging or moderate glaucomatous damage.
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Crowding
Peripheral vision
Eccentricity (behavior)
Vernier acuity
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Amblyopia is a developmental visual disorder of cortical origin, characterized by crowding and poor acuity in central vision of the affected eye. Crowding refers to the adverse effects of surrounding items on object identification, common only in normal peripheral but not central vision. We trained a group of adult human amblyopes on a crowded letter identification task to assess whether the crowding problem can be ameliorated. Letter size was fixed well above the acuity limit, and letter spacing was varied to obtain spacing thresholds for central target identification. Normally sighted observers practiced the same task in their lower peripheral visual field. Independent measures of acuity were taken in flanked and unflanked conditions before and after training to measure crowding ratios at three fixed letter separations. Practice improved the letter spacing thresholds of both groups on the training task, and crowding ratios were reduced after posttest. The reductions in crowding in amblyopes were associated with improvements in standard measures of visual acuity. Thus, perceptual learning reduced the deleterious effects of crowding in amblyopia and in the normal periphery. The results support the effectiveness of plasticity-based approaches for improving vision in adult amblyopes and suggest experience-dependent effects on the cortical substrates of crowding.
Crowding
Peripheral vision
Perceptual Learning
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How do effects of crowding manifest themselves when viewing elements of natural scenes? We studied the effects of crowding in central and peripheral vision using suprathreshold discrimination experiments. Observers rated the differences between two 5.2-deg patches of natural images that were presented alone or amongst four flankers. In the central condition the targets were located at fixation, and in the peripheral condition the targets were displayed at 16 degs eccentricity in the lower right visual field. In Experiment 1, the flankers were identical to one another - either the same as one of the target images (SAME) or completely different (DIFF) - and were located at 5.2, 6.6 or 8.2 degs (center-to-center) away from the target. In central vision, small crowding effects were found only at very small spacing distances in the SAME condition. In the periphery, crowding effects were evident in both SAME and DIFF conditions, although they were significantly higher in the SAME condition. Spacing distance between target and flankers did not (or barely) had an effect. In Experiment 2, the DIFF distractors were different to the targets and to each other, and were located at 5.2, 8.2 and 11.2 degs away from the targets. In central vision, there were a very small crowding effect for the DIFF condition at the nearest spacing but none for the SAME condition. In the periphery, crowding remains significant for both SAME and DIFF conditions, but the effects for SAME were only marginally larger than those for DIFF at the smallest spacing distance. These results are consistent with previous crowding research demonstrating: (1) weaker crowding in central vision and (2) stronger crowding when target and flankers are similar. We postulate that the increased crowding in the periphery with similar flankers and small distances is primarily caused by an increased likelihood for mismatched feature comparisons.
Peripheral vision
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Abstract Visual crowding is the disruptive effect of clutter on object recognition. Although most prominent in adult peripheral vision, crowding also disrupts foveal vision in typically-developing children and those with strabismic amblyopia. Do these crowding effects share the same mechanism? Here we exploit observations that crowded errors in peripheral vision are not random: target objects appear either averaged with the flankers (assimilation), or replaced by them (substitution). If amblyopic and developmental crowding share the same mechanism then their errors should be similarly systematic. We tested foveal vision in children aged 3-8 years with typical vision or strabismic amblyopia, and peripheral vision in typical adults. The perceptual effects of crowding were measured by requiring observers to adjust a reference stimulus to match the perceived orientation of a target ‘Vac-Man’ element. When the target was surrounded by flankers that differed by ±30°, all three groups (adults and children with typical or amblyopic vision) reported orientations between the target and flankers (assimilation). Errors were reduced with ±90° differences, but primarily matched the flanker orientation (substitution) when they did occur. A population pooling model of crowding successfully simulated this pattern of errors in all three groups. We conclude that the perceptual effects of amblyopic and developing crowding are systematic and resemble the near periphery in adults, suggesting a common underlying mechanism. Precis Crowding strongly disrupts peripheral vision, as well as the foveal vision of children with typical vision and amblyopia. We show that typically developing and amblyopic children make the same crowded errors as adults in the visual periphery, consistent with a common mechanism in all three cases.
Peripheral vision
Crowding
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Crowding
Peripheral vision
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Crowding increases with eccentricity and is most readily observed in the periphery. During natural, active vision, however, central vision plays an important role. Measures of critical distance to estimate crowding are difficult in central vision, as these distances are small. Any overlap of flankers with the target may create an overlay masking confound. The crowding factor method avoids this issue by simultaneously modulating target size and flanker distance and using a ratio to compare crowded to uncrowded conditions. This method was developed and applied in the periphery (Petrov & Meleshkevich, 2011b). In this work, we apply the method to characterize crowding in parafoveal vision (<3.5 visual degrees) with spatial uncertainty. We find that eccentricity and hemifield have less impact on crowding than in the periphery, yet radial/tangential asymmetries are clearly preserved. There are considerable idiosyncratic differences observed between participants. The crowding factor method provides a powerful tool for examining crowding in central and peripheral vision, which will be useful in future studies that seek to understand visual processing under natural, active viewing conditions.
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Peripheral vision
Eccentricity (behavior)
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Peripheral vision is characterized by reduced spatial resolution and inhibitory spatial interactions that extend over long distances. This work had three goals. (1) We considered whether the extensive crowding in peripheral vision is a consequence of a shift in the spatial scale of analysis. To test this, using a large range of target sizes and spatial frequencies, we measured the extent of crowding for targets that were limited in their spatial frequency content. (2) We considered whether crowding in peripheral vision can be explained on the basis of contrast masking by remote flanks. To test this hypothesis, we measured and compared crowding in a direction-identification experiment with masking by remote flanks in a detection experiment. In each of the experiments, our targets and flanks were composed of Gabor features, thus allowing us to control the feature contrast, spatial frequency, and orientation. (3) We examined the relationship between the suppressive and facilitatory interactions in peripheral contrast detection and crowding. Our results show that unlike the normal fovea (Levi, Klein, & Hariharan, 2002), peripheral crowding is not scale invariant nor is it attributable to simple contrast masking. Rather, our results suggest that inhibitory spatial interactions in peripheral crowding extend over larger distances than in the fovea for targets of the same size. In peripheral vision, the critical distance for crowding is approximately 0.1 times the target eccentricity. Observers can easily detect the features that compose our targets (Gabor patches) under conditions where crowding is strong. Thus, we speculate that peripheral crowding occurs because the target and flanks are combined or pooled at a second stage, following the stage of feature extraction. In peripheral vision, this pooling takes place over a large distance.
Peripheral vision
Crowding
Spatial frequency
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