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Matthew Heath, Anika Maraj, Gordon Binsted; The anti-pointing task: Vector inversion is mediated by a perceptual representation of reaching space. Journal of Vision 2009;9(8):1101. doi: https://doi.org/10.1167/9.8.1101.
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The anti-pointing task entails reaching mirror-symmetrical to a presented target. A previous study by our group (Heath et al. 2008: Exp Brain Res) showed that anti-pointing movements in left and right space exhibit a respective under- and overshooting bias: a finding consistent with perceptual asymmetries of target extent (e.g., Elias et al. 2002: Brain Cogn). Unlike earlier work in which pro- and anti-pointing movements were completed in separate blocks of trials, the present investigation entailed a situation wherein the aforementioned response contexts were randomly interleaved on a trial-by-trial basis. In particular, a discriminant stimulus change (i.e., change in colour of fixation cross) occurring in time with response cuing signaled the appropriate movement context — a situation requiring visuomotor inhibition for both pro- and anti-pointing movements. We reasoned that premovement inhibition of direct stimulus-response relations would provide a framework for determining whether top-down visuomotor inhibition and/or the visual remapping of target location (i.e., vector inversion) mediates the visual field-specific bias of anti-pointing. As expected, pro-pointing reaction times were faster than anti-pointing counterparts (F=42.64, p [[lt]]0.001) indicating increased computational load for movement planning in the latter condition. Most notably, pro-pointing in left and right space exhibited a comparable and a robust level of accuracy whereas left and right space anti-pointing respectively under- and overshot veridical target extent (F=135.68, p [[lt]]0.001). Thus, the present results indicate that top-down visuomotor inhibition does not contribute to the endpoint bias of anti-pointing. Rather, results indicate that the remapping of target coordinates to a mirror-symmetrical location in space is mediated by a perception-based representation of target extent.
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