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Robert L. Whitwell, A. David Milner, Cristiana Cavina-Pratesi, Masih Barat, Caitlin M. Byrne, Melvyn A. Goodale; Visual Feedforward Grasping and Motor Adaptation to Actual Target Width in Visual Form Agnosic Patient DF. Journal of Vision 2014;14(10):904. doi: https://doi.org/10.1167/14.10.904.
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© ARVO (1962-2015); The Authors (2016-present)
Patient DF, who developed visual form agnosia following ventral stream damage, configures her hand in-flight to match the geometric properties of novel objects when picking them up, despite her inability to use these same properties to explicitly differentiate amongst these objects. We have proposed that her spared grasping is mediated by a feedforward-visuomotor system within the posterior parietal lobe. Another interpretation, however, proposes that her dissociated performance is due to the bimodal nature of grasps compared to unimodal visual perceptual judgments. According to this new account, DF uses haptic feedback from touching the goal objects to calibrate visual egocentric cues about the finger-contact positions on the object surfaces. Thus, the 'perception-action' dissociation could be an artifact of the presence or absence of visual-haptic calibration. To test this 'calibration hypothesis' directly, we presented DF with a grasping task in which the visual size of the target varied from trial to trial while its actual size remained the same. In so doing, we uncoupled haptic and visual input, disrupting visual-haptic calibration. According to the calibration hypothesis, DF's grasps should no longer reflect the visual size of the goal objects. Contrary to this prediction, however, DF continued to scale her grip aperture to the visual sizes of the targets. Furthermore, providing haptic feedback about perceptual judgments of visual size did not improve her chance performance. Finally, we also show that DF does not require online visual feedback to scale her grip aperture to target size. Together, these findings strengthen the notion that DF's spared grasps are driven by visual-feedforward processing. They also suggest that tactile contact with an object keeps the visuomotor dorsal stream engaged, preventing the grasps from defaulting to pantomimes. The need for actions to have a tangible endpoint provides an important modification of the Two Visual Systems hypothesis.
Meeting abstract presented at VSS 2014
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