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John Greenwood, Joseph Danter, Rhiannon Finnie; Cortical distance determines the perceptual outcomes of crowding. Journal of Vision 2017;17(10):398. doi: 10.1167/17.10.398.
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© ARVO (1962-2015); The Authors (2016-present)
In peripheral vision, object recognition is disrupted by clutter. This crowding effect typically causes target and flanker objects to appear more alike (assimilation). However, tilt contrast effects also increase in peripheral vision, causing target and flanker objects to appear more dissimilar (repulsion). Although repulsion dominates in the parafovea with large target-flanker separations, assimilation increases with higher eccentricities and/or smaller separations (Mareschal, Morgan & Solomon, 2010). The common factor has been argued to be cortical distance: assimilation occurs when flankers are close to the target within retinotopic maps, while flankers at greater distances induce repulsion. Here we test this proposal with two psychophysical manipulations that dissociate cortical and physical distance. Observers (n=8) judged the orientation of a target Gabor (clockwise/counter-clockwise of vertical), flanked by two Gabors oriented either clockwise or counter-clockwise of vertical. We first manipulate cortical distance via the arrangement of target-flanker elements: because cortical magnification is higher along the radial dimension (extending from fixation), radially-positioned flankers will be cortically closer to the target than tangential/iso-eccentric flankers. Accordingly, we observe far more assimilation errors with radial flankers, while tangential flankers predominantly induce repulsion errors. We next manipulate cortical distance by presenting stimuli in the upper and lower visual fields. Because the upper-field representation is compressed (Fortenbaugh, Silver & Robertson, 2015), target-flanker separations will be effectively reduced relative to the lower field. Accordingly, for stimuli with the same physical eccentricity and target-flanker separation, we observe far greater assimilation in the upper than the lower visual field. Individual differences in visual-field size are also correlated with these assimilation rates. Our results suggest that cortical distance is a key determinant of the perceptual outcomes of crowding. By combining models of crowding and tilt contrast, we suggest that the compulsory pooling of orientation-selective population responses can provide a common mechanism for these effects.
Meeting abstract presented at VSS 2017
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