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Robert L. Whitwell, Cristiana Cavina-Pratesi, A. David Milner, Melvyn A. Goodale; Preserved grip scaling to visual size despite non-veridical haptic feedback in a patient with visual form agnosia. Journal of Vision 2013;13(9):339. https://doi.org/10.1167/13.9.339.
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Compelling support for the two-visual-systems model (Goodale & Milner, 1992) comes from studies of patient DF who has visual form agnosia. Despite her inability to discriminate the form of objects, DF’s in-flight grip aperture scales to the width of objects when she reaches out to grasp them. DF’s perceptual deficits are thought to be a result of bilateral lesions in her ventral stream, while her spared visuomotor abilities are attributed to her relatively intact dorsal-stream structures. Schenk (2012) has recently argued, however, that DF uses haptic feedback "to compensate for her deficit in size perception". Using a mirror apparatus, Schenk found that DF failed to scale her grasp to objects of different sizes viewed in the mirror unless a corresponding goal object, which she could actually grasp, was present behind the mirror. When the object behind the mirror was absent, she failed to scale her grasp to the visual size of the objects (in contrast to normal controls). But it could also be the case that DF failed to scale because the absence of a felt object induced a shift in the nature of the task from real grasping, which is mediated by the dorsal stream, towards something that resembles ‘pantomimed’ grasping, which is mediated by the ventral stream (damaged in DF). Here we show that placing an object of constant size behind the mirror preserved DF’s grip scaling for different sized objects viewed in the mirror – even though there was no veridical haptic feedback since the felt object was always the same size. These data suggest that the presence of a felt object is necessary and sufficient to engage DF’s intact dorsal stream, allowing her to scale her grasp to changes in the size of a visible object – and that Schenk’s earlier findings reflect DF’s well-known inability to pantomime.
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