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Geoffrey Bingham, Young Lee, Mark Mon-Williams; Large perspective changes (>45°) allow metric shape perception used to guide grasping. Journal of Vision 2009;9(8):1109. doi: 10.1167/9.8.1109.
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© ARVO (1962-2015); The Authors (2016-present)
Background. Lee, et al. (2008a) found that binocular viewing (wtih head movement) of cylinders (5 aspect ratios) did not yield accurate metric shape perception used to guide reaches-to-grasp. Metric shape perception was not calibrated by grasp feedback. However, Bingham & Lind (2008) found large perspective changes (≤45°) allowed accurate metric shape perception. Lee, et al. (2008b) investigated judgments of SFM and stereo displays and found metric shape only became accurate with changes ≤45°. We now investigated whether such information allows accurate reaches-to-grasp.
Methods. A different group of 10 Ss performed in each of 3 experiments. Elliptical cylinders (5 aspect ratios) were viewed in a mirror while rotated 50°. Feedforward reaches-to-grasp actual and virtual cylinders behind the mirror were measured using Mini-Bird markers on index and thumb nails and wrist. Major and minor axes of each target were grasped and measured apertures (at MGA and TGA) were used to compute aspect ratios. Exp 1 entailed no delay between viewed perspective change and reaches-to-grasp. In Exp 2, there was a 5s delay. In Exp 3, three different targets were viewed and rotated simultaneously and then grasped in sequence with a delay for the third object of ≈25 s.
Results. Aspect ratios, as reflected in grasping, were now accurate at both MGA and TGA in all three experiments. In previous studies, regression of produced versus actual aspect ratios had yielded low slopes of ≈0.6. Now slopes were 0.8–0.9.
Conclusions. Large perspective changes (as generated during approach to a workspace) yield accurate perception of the metric shape of multiple objects in a scene, that remains stable with continued viewing from a single perspective, and that allows accurate control of subsequent feedforward reaches-to-grasp. Future study of space perception used to guide actions requires consideration of nested actions at different spatial-temporal scales.
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