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Sang Chul Chong, Sabine Kastner, Anne Treisman; Effects of focused and distributed attention on neural competition. Journal of Vision 2004;4(8):9. doi: https://doi.org/10.1167/4.8.9.
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When multiple stimuli are simultaneously present in a scene, they may compete with each other for neural representation in visual cortex. Responses to an otherwise optimal stimulus presented within a neuron's receptive field (RF) were reduced when a second stimulus was simultaneously present within the same RF (Desimone & Duncan, 1995). Directing attention to one of the stimuli counteracted the suppressive influence of the second stimulus. Using fMRI, we investigated the suppressive interactions among multiple stimuli when attention was directed to either one of the stimuli or all of them. We hypothesized that only focused attention, but not distributed attention, would reduce the suppressive interactions among multiple stimuli. When all the stimuli receive the same attentional modulation, the suppressive interactions should not be reduced. We presented four circles of different sizes to the upper-right visual field and one reference circle around the fixation point. In an unattended condition, subjects counted randomly oriented Ts or Ls at the fixation point. In a focused attention condition, they counted occurrences of a circle at the nearest location from the fixation point that had the same size as the reference circle and ignored circles presented in the other locations. In a distributed attention condition, they counted how often the mean size of all four circles matched the size of the reference circle. We found that suppressive interactions among multiple circles were reduced in V4 when subjects paid attention to one of the four locations, as compared to the unattended condition, consistent with Kastner et al (1998)'s findings. However, suppressive interactions were not reduced when they paid attention to all four items as a set, in order to compute their mean size. These results suggest that whereas focused attention serves to filter out irrelevant information, distributed attention provides an average representation of multiple stimuli.
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