Grasping Movement Plans

Article excerpt

Despite the great amount of research that has been done regarding the time it takes to move the hand to targets of varying distances and widths, it is unclear whether target distance and width are both represented in movement plans prior to movement initiation. We addressed this question by studying performance in an object manipulation task. Our participants reached out and took hold of a familiar object (a bathroom plunger) to move it to wide or narrow targets of varying heights. Grasp heights on the plunger were additively affected by target height and target width, suggesting that both factors were taken into account by participants prior to moving the plunger from its initial position. Another factor we manipulated was the width of the base from which the plunger was lifted on its way to its next position. This factor also affected grasp heights, but no more so than target widths. The latter result contradicts the view that movement starts are planned in more detail than movement ends, as might be expected from the fact that movement starts come sooner. Together, our results suggest that forthcoming movements are planned in considerable detail. A surprising methodological implication of this study is that recording how people prepare to move can reveal as much-or in some cases more-about what they have planned than can recording their subsequent movements.

This study was designed to contribute to the understanding of the planning and control of physical actions and, in particular, actions involving object manipulation, an important functional activity and a rich venue for exploring the information actors have about their own bodies vis-à-vis the external environment (MacKenzie & Iberall, 1994). Our starting point was a classic observation by Fitts (1954) on the time it takes to move the hand from one point to another. Fitts found that this time increases as the distance between points increases, and that this time increases as the width of the target gets smaller. This dual influence of distance and target width on movement time has been demonstrated so many times and in such a wide range of conditions that the relation, or its more specific quantitative formulation (which need not be repeated here), has come to be called Fitts 's law (for a review, see Elliott, Helsen, & Chua, 2001).

How distance and target width are internally represented prior to movement initiation remains unclear from the many studies that have been done on Fitts's law. Are both factors represented, or is only one factor represented in advance, so that the unrepresented or minimally represented factor is only dealt with while movement is underway? If both factors are considered before movement initiation, are they considered independently or in some dependent fashion?

The available methods for addressing these questions have relied mainly on the kinematics of ongoing hand movements (i.e., the positions of the hand over time), but these methods have been less than wholly satisfactory in illuminating premovement planning. Such studies have generally shown that target width has an observable effect on observed movement speed later than does required distance. Thus, the starting phase of the movement is strongly affected by the distance to the target but is largely unaffected by the size of the target, whereas the ending phase of the movement is strongly affected by the size of the target but is less affected by the distance of the target from the launch point (for a review, see Elliott, Helsen, & Chua, 2001). Such observations suggest that homing in to smaller targets occurs late in movement, but they do not prove that target width is not considered prior to movement initiation. Planning with respect to target width could be carried out before movements start but not be manifested kinematically until movements are under way.

Given this uncertainty about the nature of movement planning for manual positioning movements, we sought another way to address the issue. …


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