Studies On The Neural Mechanism Of The Crowding Effect In Visual System

In peripheral vision,when embedded in adjacent flanking distractors,a target becomes more difficult to perceive,a ubiquitous phenomenon in visual system known as crowding.Crowding imposes deleterious influences on target discrimination in the periphery and it has been reported to affect many types of visual tasks.Crowding represents an essential bottleneck for object recognition.Crowding also has important clinical implications for patients with amblyopia and macular degeneration.Although a lot of functional characteristics of the crowding effect have been studied,its neural mechanism remains unresolved.This study aims at investigating neurophysiological and functional mechanisms of the crowding effect;in particular,the study addresses two issues:1) where in the brain the crowding effect occurs and 2) how top-down attention modulates the crowding effect.The first part of the study is to localize the neural site of the crowding effect in the brain.We take advantage of the fact that visual space is represented retinotopically in visual cortical areas but with a small number of discontinuities.There is the division of the hemifield representation between the two hemispheres—the early visual cortices in each hemisphere represent the contralateral visual hemifield.Although each hemifield is represented continuously in V1,in V2 and V3 the upper and lower visual quadrants are further divided and separately represented in the ventral and dorsal projections.Thus,two visual stimuli presented next to each other on opposite sides of the vertical meridian project to the left and right primary and other early visual cortices,cortically far from each other; whereas,two stimuli presented next to each other on the same side of the vertical meridian project to the same contralateral visual cortex,cortically adjacent to each other.Two stimuli presented near the horizontal meridian are represented close to each other in V1 but could be represented far from each other in V2 and V3 if they are on opposite sides of the horizontal meridian.In the current study,we observed a strong crowding effect from spatially adjacent distractors with either Gabor or letter targets presented near the vertical or horizontal meridian.Interestingly,for a target presented near the vertical meridian,a distractor from the same side of the meridian(cortically near) had a significantly stronger crowding effect compared with an equidistant distractor presented on the opposite side (cortically remote).No such meridian modulation was observed across the horizontal meridian.These results constrain the cortical locus of the crowding effect to a stage in which left and right visual spaces are represented discontinuously but the upper and lower visual fields are represented continuously,likely beyond the early retinotopic areas.The second part of the study is to investigate the role of visual attention in visual crowding effect.Visual information processing is often considered to be a hierarchical process,with the early stages responsible for the detection and processing of simple features,while the late stages involved in integration and binding information.In addition, there are both bottom-up information processing and top-down feedback processing in the visual recognition process.Under this framework,a number of hypotheses about the mechanism of the crowding effect were proposed,such as 'integration field' hypothesis, suggesting that crowding occurs during the bottom-up integration process;it has also been suggested that crowding is due to the reduced resolution of the spatial attention.However, the role of spatial attention on crowding remains controversial.Here we examined this issue by manipulating the spatial distribution of visual attention through a pre-cueing paradigm and measuring the strength as well as the critical spacing of the crowding effect.The key novel point of this study is that instead of cuing the location of the target,the cue shaped the spatial profile of attention.Specifically,in an orientation discrimination task,a Gabor target was presented in the lower right visual field,and distractors(also Gabor patches) were presented either vertically(above and below the target) or horizontally(left and right of the target),with the distance between the target and distractors varied across trials and serving as one of the independent variables.Just before the appearance of the target and distractor stimuli,observers were asked to perform a Vernier discrimination task or a color discrimination task either in a vertical or horizontal axis,with the purpose of shaping observers' spatial attention to be along or perpendicular to the axis of the subsequent target-distractor configuration.Results consistently showed a reduced crowding effect as well as a reduction in critical spacing of crowding when observers' spatial attention was precued to be orthogonal to the target-distractor configuration,compared with when attention was pre-cued along the target-distractor configuration.These results provide strong support to the view that spatial attention plays a critical role in the crowding effect.