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Fang Fang, Sheng He; Modulation of V1 response to a target by adjacent distractors in attended and non-attended conditions. Journal of Vision 2007;7(9):596. doi: 10.1167/7.9.596.
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When a peripheral target is flanked by distractors, especially in a radial orientation, its visibility is significantly reduced. This phenomenon is known as crowding or surround suppression, which has been an active research topic for more than four decades. However, little is known about its underlying neural mechanism. It is known that crowding has a “radial [[gt]] tangential” asymmetry as well as an “outward [[gt]] inward” asymmetry. We conducted two fMRI experiments to study the modulation of V1 response to a target by spatially adjacent distractors when subjects performed a fixation task (non-attended condition) or a contrast-discrimination task on the target (attended condition). The target was always positioned on the right horizontal meridian. In the first experiment, the target was presented either alone, or with two flanking distractors positioned either tangentially (above and underneath the target) or radially (left and right of the target). In the second experiment, the target was presented either alone or with an inward distractor (left of the target) or an outward distractor (right of the target). The stimuli in the attended and non-attended conditions were exactly the same. We found that, in both attended and non-attended conditions, the pattern of V1 responses to the target were radial[[gt]]tangential[[gt]]single in the first experiment, and outward[[gt]]inward[[gt]]single in the second experiment. Attending to the target enhanced V1 responses, but attentional enhancement was larger when a distractor(s) was presented. Interestingly, attentional enhancements were stronger in the radial condition than in the tangential condition at the flanking locations where no physical stimuli were presented. These results suggest that crowding effects result from a combination of bottom-up lateral interaction and top-down attentional modulation, and reveal a neural correlate of unfocussed spatial attention in the crowding effect (Strasburger, 2005).
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