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Jennifer Charles, Sam Kent, Erik Jansson, Mark Mon-Williams; Visible surface area and prehension movement patterns. Journal of Vision 2008;8(6):304. doi: 10.1167/8.6.304.
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Visual information about an object's distance and size determines the reach-to-grasp movement pattern. If vision specifies that object A has a smaller grasp surface area than object B, then slower movements will be programmed to A. Smaller grasp surfaces afford slower movements because more on-line visual feedback corrections are required. But what happens if the thumb and index finger have different sized grasp surface areas? In experiment 1, participants (n=8) reached-to-grasp one of nine objects directly in front of their hand in the sagittal plane at one of three distances (10, 30, 50cm). Each object consisted of a block with a dowel attached so that the ends extended to either side. The objects were the same width (5cm) but made with three different dowel diameters (3, 2, 1cm). The dowel was either the same width on both sides, or one of the six possible asymmetric configurations (10 trials per condition, total=270). Duration was affected by the surface size (duration decreased linearly as size increased) but only as a function of the thumb's surface: the finger's surface had no effect. The position of the object provided a clear line-of-sight of the thumb's surface (but not the finger's). Thus, these results can be explained by: (i) the thumb and index finger's ‘opposition vector’ being controlled as a single coordinative structure; (ii) the opposition vector being maneuvered with respect to the visible (fixated) object surface. Experiment 2 directly tested this by manipulating the vertical height of conical frusta so that: (a) the upper surface was visible but not the lower surface; (b) the lower surface was visible but not the upper; (c) the upper and lower surfaces were equally visible. Our findings show the importance of considering the visible surface area as a critical task constraint when attempting to understand prehension movement patterns.
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