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Rachel Millin, A. Cyrus Arman, Bosco S. Tjan; Reduced Neural Activity with Crowding is Independent of Attention and Task Difficulty. Journal of Vision 2010;10(7):1352. doi: https://doi.org/10.1167/10.7.1352.
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© ARVO (1962-2015); The Authors (2016-present)
Form vision in peripheral fields is limited by crowding, the impaired identification of a target object when surrounded by other items. Crowding is thought to reflect a failure in feature selection and integration, due to limited spatial resolution of attention or maladapted low-level receptive field properties related to lateral interactions. We investigated the top-down influences of task-difficulty and attention on the neuronal response to crowded and non-crowded letter triplets in the periphery using fMRI. In our first experiment, we changed the distance between target and flanking letters, and measured BOLD response while subjects identified the target letter. In a second experiment, we added conditions identical to the non-crowded stimuli but partially scrambled the target letter to increase task difficulty. We found that decrease in target-flanker separation was associated with decrease in BOLD in V2 through V4 for both experiments, but degrading the target in the non-crowded condition did not cause any such decrease. Therefore, the reduced BOLD signal observed in the crowded condition was not due to increased task difficulty per se or any related differences in attention demands. To further determine any interaction between attention and crowding, we measured BOLD signal induced by peripherally presented crowded and non-crowded letter triplets, while subjects’ attention was directed to an unrelated task at fixation. Two conditions were added where only the flankers were displayed, without the center letter. We found that in the non-crowded condition, adding the center letter increased BOLD signal, while in the crowded condition the center letter produced no signal increase, consistent with the interpretation that crowding suppressed signal from the center letter. Together, these results show that the low-level neural response correlated with crowding is independent of task difficulty and attention, indicative of a bottom-up input-driven cause of crowding.
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