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R. H. Cuijpers, E. Brenner, J. B. J. Smeets; On the role of shape perception when grasping objects. Journal of Vision 2002;2(7):720. doi: https://doi.org/10.1167/2.7.720.
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© ARVO (1962-2015); The Authors (2016-present)
Grasping has successfully been used as a tool to study the visual processing of object properties such as size and orientation. Even the grasping of irregularly shaped objects has been studied. In this study we want to examine the processing of a simple object shape property: the aspect ratio of ellipses.
For that purpose we tracked the finger positions of eight subjects while they picked up cylinders with an elliptical base. We used seven 10 cm tall cylinders. One axis of the elliptical base was always 5cm whereas the length of the other was determined by the aspect ratio, which varied from 0.4 to 1.6 in steps of 0.2. On each trial a cylinder was placed either 30cm or 60cm in front of the subject in one of six orientations, which varied in steps of 30 degrees. The starting position of the hand was always 30cm in front and 30cm to the right of the subject.
Our subjects usually picked up the non-circular cylinders along the cardinal axis that was most perpendicular to the direction in which their hand was moving. When neither axis was close to perpendicular, an “arbitrary” axis was chosen. Surprisingly, this behaviour was independent of the aspect ratio, and was the same even for nearly circular cylinders. The maximum grip aperture was related to the grip aperture at pickup but was independent of the object orientation. The cylinder with a circular base was picked up at convenient locations depending on the direction of approach of the hand.
We conclude that the global shape determines where the object will be grasped, but that the way the fingers move to the chosen positions only depends on the positions themselves. This grasping behaviour corresponds to the predictions of a simple minimum-jerk model of the movements of the digits towards the end positions. Nonetheless, the grasping of an object is very sensitive to the distinction between circular and elliptical objects: as soon as an object is non-circular it will be grasped at one of its cardinal axes.
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