Effects of Changing Object Size During Prehension

ABSTRACT

The authors tested how fast the grasp component of prehension was able to adjust to a sudden change in object size. Participants grasped an object, the size of which could suddenly increase. Whereas previous researchers usually applied perturbations through a change in illumination at movement onset, the present perturbations involved a change in the object's physical size at 1 of 4 moments during the movement (125, 200, 275, and 350 ms after movement onset). The results showed that grasp adjustments came in many forms and could be as fast as 120 ms. The implications for the understanding of the coordination of reaching and grasping in prehension are discussed.

Keywords:

• coordination
• grasping
• perturbation
• reaching

ACKNOWLEDGMENTS

This research was supported by a VIDI grant (452–03–356) from the Netherlands Organization for Scientific Research (NWO) awarded to Frank Zaal. The authors thank Marcel Lamberts for his assistance in collecting the data; Patrick Haggard for kindly sending a copy of part of his dissertation; Henry van de Crommert, Wim Kaan, Hans Thole, and Wolter de Goede for developing the set-up that we used in the study; and Gert-Jan Pepping for useful comments on an earlier draft.

Notes

1. We used two thresholds in the present study: (a) the hand-aperture-speed threshold, used in the offline analyses and applied to filtered data, to define the start and end of the grasping movement and align the hand-aperture time series and (b) a digit-speed threshold, applied to the unfiltered data and used in an online fashion to trigger the actual size perturbation. The hand-aperture-speed threshold was reached, on average, a little later than the digit-speed threshold. The times of the perturbations reported in Table 1 were with reference to the hand-aperture-speed threshold.

2. The difference in height of the two targets, in combination with the fact that the bigger target fit around the smaller object, forced the participants in Paulignan, Jeannerod et al.'s (1991) experiment to grasp the target objects at different heights. Any difference in kinematics might also have been affected by this difference in final hand position.