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Rebecca Kwok, Oliver Braddick; The effect of the Titchener circles illusion on grasping and manual estimation of two and three dimensional targets. Journal of Vision 2002;2(7):54. doi: 10.1167/2.7.54.
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Goodale and Milner (1992) proposed a dual visual system, with a ventral cortical stream involved in visual perception and identification, and a dorsal stream mediating visually guided action. This has been supported by findings that perceptual judgement is affected by pictorial illusions, while action is relatively unaffected (Aglioti et al 1995; Haffenden and Goodale 1998), although some contrary results have been reported (reviewed by Bruno, 2001). Previous studies have found that pantomimed grasping is fundamentally different from natural grasping (Goodale et al 1994) and action towards an illusion shows increased illusory bias as the delay between viewing and action is increased (Gentilucci et al 1996), indicationg that ventral stream representations are involved in driving these actions. It is therefore suggested that an action must be both goal directed and immediate in order for dorsal stream information to be utilised.
Two- and three-dimensional stimuli were compared to discover whether a stimulus must be ‘graspable’ in order for the dorsal stream to dominate in guiding a goal-directed action towards it. Subjects made grasping actions and manual estimations of a flat circle (2D) or to a disk of 3mm thickness (3D) in the Titchener Circles illusion. Movements were recorded using the ELITE motion tracking system (BTS, Milan) and analysis identified maximum in-flight aperture during grasping, and mean aperture in estimation. Results confirmed previous findings: manual estimation of the 3D target showed the illusory bias, whereas grasping did not, and was scaled to the true size of the disks. Maximum aperture in grasping to 2D targets was smaller overall, but showed the same pattern of effects; an illusion effect was found in estimation but not in grasping. These results suggest that although action towards a 2D stimulus is different to action towards a 3D stimulus in some respects, it still appears to be driven by dorsal stream visual information.
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