Chapter 2. Prehension

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Chapter 5 - Movement Before Contact
660' I I
Moving Stationary
Target Speed
Grasping
Pointing
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Figure 5.15 Movement time to point to, or grasp objects that
were either stationary or moving (rolling down a ramp). Note that
grasping movements were better adapted to moving targets, and
that pointing movements were faster to stationary targets (from
Carnahan, 1992; reprinted by permission)
hand, and the appropriate opposition space for making contacts with
the environment are key variables in determining kinematic landmarks
in the arm transport. Reaching with the arm to grasp with the hand is
different from reaching with the arm to point. For grasping, an im-
portant question is whether and how the hand movements are coupled
to the arm movements. Although we will address this in more detail
later, Jeannerod suggested that they are under the control of parallel
controllers that get coupled in time. Wing and associates (Wing &
Fraser, 1983; Wing, Turton, & Fraser, 1986) argued instead for a
spatial relationship between the arm transport component and the
grasping component. In their experiments, subjects reached in the
sagittal plane to grasp a vertically standing dowel under conditions
where visual feedback and speed of movement varied. In all condi-
tions, the thumb tended to move less than the index finger, especially
after the fingers reached the peak aperture, as seen on the right side of
Figure 5.16. In addition, in trials without visual feedback and also in
the faster moves, subjects tended to open the hand wider, reaching a
larger, and more variable, peak aperture. Wing et al. (1986) sug-