Grasping movement plans
DAVID A. ROSENBAUM
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ERIN S. HALLORAN
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RAJAL G. COHEN
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This work was supported by Grant SBR-94-96290 from the Na- tional Science Foundation and Grants KO2-MH0097701A1 and R15- NS41887-01 from the National Institutes of Health, and by the Research and Graduate Studies Office of the College of Liberal Arts, Pennsyl- vania State University. The second author completed this research as part of her undergraduate psychology studies at Penn State. All three authors thank the team of anonymous reviewers for their very useful sent to D. A. Rosenbaum
, 642 Moore Building,
Department of Psychol- ogy, Pennsylvania State University, University Park
,
PA 16802 (
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Pennsylvania State University, University Park
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Pennsylvania
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ROSENBAUM, HALLORAN, AND COHEN
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 muchor in some cases moreabout what they have planned than can recording their subsequent movements.
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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--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 Fittss 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 Fittss 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. Rather than focusing on
hand kinematics during movement, we focused on where
participants grasp an object to be moved to another
location. Previous research has shown that people grasp
objects differently, depending o (...truncated)