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Robert Whitwell, Melvyn Goodale, James Enns; Touchpoints reveal sensitivity to object shape in an individual with visual agnosia and in another who is cortically blind. Journal of Vision 2018;18(10):435. doi: https://doi.org/10.1167/18.10.435.
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© ARVO (1962-2015); The Authors (2016-present)
Object shape is exploited for object recognition and object-directed actions like reaching and grasping. Classic work has shown that information about object shape is expressed in the grasp kinematics and surface-contact points of individuals with compromised shape perception that followed damage to ventral-stream structures (e.g., Goodale et al. 1991). These studies constitute a cornerstone of the two visual systems hypothesis (Goodale & Milner, 1992) and its multi-visual-system descendants. Recent work with normally-sighted populations has shown that the freely chosen end points from point-to-touch movements directed at targets reveal the target-shape's medial axis – a type of shape 'skeleton' (Blum, 1973; Firestone & Scholl, 2014). Moreover, this occurs without participants' having any explicit knowledge of what a medial axis is or what it looks like (Firestone & Scholl, 2014). Here we ask whether or not this medial-axis phenomenon extends to exclusively dorsal-stream representations of shape, by testing DF, who has visual form agnosia resulting from lesions that encompass the shape-sensitive ventrolateral cortical area (LOC) bilaterally, and MC, who is cortically-blind following lesions that also encompass area LOC bilaterally. Each patient touched pebble-like shapes shown on a touchscreen in random positions and orientations. DF and MC could not reliably discriminate amongst the shapes in same/different, oddball, and 1-back tasks, confirming their deficits in visual shape perception. Nevertheless, both DF and MC manually localized the shapes with a high degree of accuracy, showing intact shape localization by the preserved dorsal stream. Moreover, DF's and MC's touchpoints each fell significantly closer to the centre-of-mass and the medial axis of each of the shapes, when compared to sampling distributions for squared- and absolute-mean-deviation metrics derived from random points. These findings indicate that, in addition to the centre-of-mass, other shape metrics such as the medial axis inform dorsal-stream mediated action in the absence of ventral-stream input.
Meeting abstract presented at VSS 2018
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