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Emel Gencer, Winona Snapp-Childs, Mark Mon-Williams, Geoffrey Bingham; Reaching With Altered-Grip-Spans: How Altering Effectivities (But Not Affordances) Influences Behavior. Journal of Vision 2013;13(9):748. doi: https://doi.org/10.1167/13.9.748.
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© ARVO (1962-2015); The Authors (2016-present)
Introduction: Mon-Williams and Bingham (2005; 2011) discovered the affordance properties of objects that determine the spatial structure of reach-to-grasp movements. They formulated a model containing a single free parameter and variables that were determined by object (maximum-object-extent or MOE) and actor (maximum-grip-span or MGS) properties. Snapp-Childs et.al (2011) showed the generality of this model testing people with different MGS. In the current experiments, we investigated the effect of altering effectivities (relevant actor properties) by restricting (Experiment 1) or extending (Experiment 2) MGS.
Method: Participants reached quickly for objects that varied in object-width (3, 5, 7cm) and contact-surface-size (1, 2, 3cm), thus in MOE. The levels of perturbation were: 1) no restriction/extension (no device), 2) no restriction/extension (with device on-hand), 3) 20% MGS restriction/extension, and 4) 35% MGS restriction/extension.
Results: The model predicted that the safety-margin at the time of maximum-grasp-aperture (MGA) should be 34% of the available-span (MGS-MOE). With no adaptation to MGS perturbations, the expected mean percentages were: 3) 49%, 4) 73% for 20% and 35% restrictions and 3) 24%, 4) 21% for 20% and 35% extensions, respectively. The actual mean percentages were, for Experiment 1: 1) 33%, 2) 30%, 3) 35% (significantly different from no adaptation (t(178) = 6.8, p <0.001)), 4) 40% (significantly different from no adaptation (t(178) = 10.1, p <0.001)) and for Experiment 2: 1) 33%, 2) 29%, 3) 24% (not significant (t(160) = 0.1, p > 0.9), 4) 21% (not significant (t(160) = -0.4, p > 0.6). Continuous adaptive trends were evident over trials with restricted grip-spans, but not with extended grip-spans.
Conclusions: When the effective size of their hand was reduced, participants exhibited adaptation to renormalize their grasping, essentially preserving the form and scaling of the behavior, but when the effective size of their hand was increased, they exhibited no adaptation.
Meeting abstract presented at VSS 2013
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