Influences of Awkwardness and Eccentric Fixation on Visuomotor Susceptibility to Pictorial Illusions
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Investigation of the Goodale and Milner (1992) model of dorsal vision for action and ventral vision for perception streams of cortical visual processing has yielded controversial results. Some studies using three-dimensional versions of pictorial illusions in neurologically intact individuals have found that grip scaling (largely controlled by the dorsal stream) remains accurate despite robust perceptual illusion effects (e.g. Aglioti, DeSouza, and Goodale, 1995). However, others have found that the visuomotor system is as susceptible to visual illusions as the visuoperceptual system (Franz, 2001). Goodale (2008) has suggested that the less automatic an action, the more likely that the visuoperceptual system will be called upon, making such actions more susceptible to visual illusions. The current set of experiments explored the effects of grasp awkwardness and eccentric fixation on visuomotor and visuoperceptual susceptibility to pictorial illusions; an effort to explain a recent study s visuomotor illusion findings (Radoeva, Cohen, Corballis, Lukovits, and Koleva, 2005). The first experiment investigated the effect of practice over the course of three days with a potentially awkward measuring device on grip scaling when strongly right-handed participants grasped bars embedded in Muller-Lyer arrowhead illusions with their right and left hands. Results demonstrated that grasps and estimations are less susceptible to the illusion after practice, but further experiments are needed to determine whether the visuomotor and visuoperceptual practice effects were the result of separate mechanisms. The second experiment demonstrated that after correction for scaling, manual estimation and adjustment estimation provide similar assessments of perceptual sensitivity to the Muller-Lyer illusion, and it provided preliminary evidence for an effect of eccentric fixation on the relative magnitudes of visuomotor and visuoperceptual susceptibility to the illusion.