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Paola Binda, M. Concetta Morrone, Geoffrey M. Boynton, Scott O. Murray; Spatial attention affects perceived stimulus position. Journal of Vision 2011;11(11):229. doi: 10.1167/11.11.229.
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Spatial attention can shift receptive field positions (Womelsdorf et al., 2006) and alter neural maps even in early visual areas (Fischer & Whitney, 2009). Here we report a shift in the perceived location of a peripheral stimulus when it is attended, but not when attention is continuously directed at fixation.
Two 3 × 0.5 deg vertical bars (100% contrast) were flashed (50 ms) in the right peripheral visual field (18 deg eccentricity), one above and one below the horizontal meridian, with variable Stimulus Onset Asynchrony (SOA). Subjects judged the horizontal location of the bottom bar relative to the top bar. When the bars were asynchronous, the second bar was perceived more to the right (more peripheral) than the first one by about 20% of the stimulus eccentricity. The size of perceived displacement increased with SOA up to about 250 ms and remained constant for all SOAs tested (up to 3.5 s). Crucially, the mislocalization effect vanished when spatial attention was drawn away from the stimuli. Attention was drawn away from the stimulus by adding a demanding primary task at fixation (reporting the sum of two digits flashed simultaneously with the peripheral stimuli). With attention drawn away, localization of the two peripheral stimuli appeared to be equal distances from fixation, regardless of their order of presentation. However, the localization bias reemerged when the primary task was moved to the location of the stimuli.
Our findings are consistent with the hypothesis that a covert shift of spatial attention causes distortions of visual space as a result of either shifting receptive fields of neurons that represent the attended location, or by a shift of perceived eye position.
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