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Katsumi Minakata, Yamaya Sosa, Mark E. McCourt; The Effect of Visual Scanning in Line Bisection. Journal of Vision 2011;11(11):213. doi: 10.1167/11.11.213.
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Pseudoneglect refers to a systematic leftward bias in the perceived midpoint of horizontal lines, and reflects the specialization of the right hemisphere for the deployment of visuospatial attention. Line scanning direction modulates this tonic bias such that leftward error increases with rightward scanning and decreases with leftward scanning. The origin of this scanning effect is unknown. Using eye-tracking and tachistoscopic line bisection task (McCourt & Olafson, 1997), the type (saccadic vs. smooth pursuit) and direction (leftward vs. rightward) of attentional scanning, executed with or without eye movements (overt vs. covert), were manipulated. Observers overtly or covertly attended a smoothly (11.5o/s) or suddenly moving dot in a leftward or rightward direction toward the center of the display. Contingent upon a centered gaze location, pre-transected lines were then presented for 150 ms. Subjects made forced-choice judgments of transector location relative to line midpoint. No-scanning and manual line bisection (transector moved rightward or leftward via mouse from one line endpoint until judged to be centered) conditions served as controls. There was a main effect of scanning direction where, contrary to previous reports (Chokron & Imbert, 1993), leftward scanning resulted in leftward error, and vice versa. This was true for the manual scanning condition as well. Smooth pursuit was more potent than saccadic scanning, and overt scanning was more potent than covert scanning. The strongest effects occurred with leftward overt smooth pursuit scanning. If bisection error is due to differential attentional magnification of line halves, then our results imply that visuospatial attention is deployed asymmetrically ahead of a scanned target.
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