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Benjamin Meek, Loni Desanghere, Jonathan Marotta; Posterior Cortical Atrophy: An investigation of grasping. Journal of Vision 2010;10(7):1087. doi: 10.1167/10.7.1087.
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© ARVO (1962-2015); The Authors (2016-present)
At last year's VSS meeting (journalofvision.org/9/8/1095/) we presented a patient with posterior cortical atrophy (PCA) who demonstrated a severe deficit in face recognition, problems interpreting and reproducing line drawings of common objects, simultanagnosia, and colour hallucinations. Despite these perceptual difficulties, she was able to accurately guide her hand to stable grasp sites on irregularly-shaped objects, and she showed appropriate grip scaling during reach-to-grasp movements. It has been suggested that such symptomatology represents a ‘ventral’ form of PCA, in which damage is predominantly restricted to occipitotemporal areas (Ross et al., 1996). The current study explored the visuomotor abilities of three patients with PCA in relation to their perceptual deficits. In one experiment, subjects were presented with two asymmetrical, irregularly-shaped objects and asked whether they were the same or different. They were then prompted to reach out and pick up one of these objects. We found that PCA patients unanimously performed worse on the object discrimination task than age- and gender-matched controls, yet they executed accurate grasps to these same objects. In another experiment, subjects had to reach out and pick up simple, rectangular blocks under free-viewing (closed-loop), no-vision (immediate open-loop), and delay conditions. Control subjects appropriately scaled their grasps in accordance with block size under all task conditions. In contrast, PCA patients scaled effectively during closed-loop and immediate open-loop conditions, but lost this ability following a three second delay. Previous work has demonstrated that introducing a delay in a grasping task forces visuomotor systems to rely on a stored ‘percept’ of the target object retrieved from the ventral stream (Goodale et al., 1994), which our patients clearly lack. This study supports the idea that PCA may include two unique variants - ‘dorsal’ and ‘ventral’ PCA, and reinforces the findings that there are separate neural pathways that mediate vision-for-perception and vision-for-action.
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