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Winona Snapp-Childs, Rachel Coats, Jing Samantha Pan, Mark Mon-Williams, Geoffrey P. Bingham; Intrinsic scaling of reaches-to-grasp predicted by affordance-based model: Testing men and women with large and small grip spans. Journal of Vision 2011;11(11):973. doi: 10.1167/11.11.973.
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© ARVO (1962-2015); The Authors (2016-present)
Reaching-to-grasp has been extensively studied to discover the functional timing of the movements, but, not the spatial scaling relations. However, Mon-Williams and Bingham (2005; submitted) discovered the affordance properties of objects that determine the spatial structure of reach-to-grasp movements. Using these findings, they formulated a model that contained a single free parameter and other variables that were determined by object, maximum object extent (MOE), and actor, maximum grip span (MGS), properties. In this experiment, we investigated the generality of this model by assessing the spatial structure of the reaches-to-grasps of participants with greatly differing maximum grip spans: males with large and small grip spans (18.4, 14 cm) and females with large and small grip spans (16, 12.4 cm). Participants reached, at a medium or fast pace, for objects that systematically varied in object width (3, 5, 7 cm) and contact surface size (1, 2, 3 cm), and thus in MOE, while taking care not to move the objects. The model predicts that the margin of safety at the time of maximum grasp aperture (MGA) for medium and fast paced reaches should be 24% and 34% of the available span (which equals MGS-MOE), respectively. The average safety margin at MGA for all males and females with large grasp spans replicated the model predictions (large-spanned males: 22%, 32%; small-spanned males: 25%, 33%; large-spanned females: 24%, 32%). These results confirm the affordance-based model in predicting the spatial structure of reaches-to-grasp. The safety margins for females with small grasp spans were somewhat different (37%, 45%). This may reflect proximity to a critical action mode boundary for them. Results for terminal grasp aperture and object width were similarly consistent with the model and previous results.
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