Chapter 2. Prehension

Chapter 5 - Movement Before Contact 145
ing velocity profiles. One approach is to look for kinematic invari-
ances. Referring to Figure 5.14, Jeannerod (1984) showed that pg&
grip aperture remained invariant with changes in movement distance.
In contrast we note svstematic variation in that the value of peak hand
velocity increases with movement distance (confirmed through infer-
ential statistics as more than one might expect to find due to random
variation alone).
For the transport component, the velocity profile is roughly
shaped, and in this case svmmetrical, i.e., about the same proportion
of time is spent in the acceleration phase prior to peak tangential veloc-
ity, as in the deceleration phase after peak velocity. We would say that
the velocity profile for transport was scaled to movement distance, if
for the three distances, the shape of the velocity profile was similar
when normalized in time, and only the amplitude varied. In this case,
normalizing in the amplitude domain would lead to superimposed ve-
locity profiles. Differentiating the resultant velocity profile to examine
the rate of change of speed along the path of the movement, we could
similarly address the acceleration-time function, noting the amplitude
and timing of peak acceleration, zero crossing (corresponding to peak
velocity), and peak deceleration. Lack of smoothness in the decelera-
tion phase of the movement, local peaks and valleys, or multiple zero
crossings in the acceleration function might indicate corrections to
movement or changes in the plane of movement. Insight might be
gained from examination of the phase planes, e.g., some variable plot-
ted against its derivative. The spatial path reveals another view of the
motion pattern of the hand. The variability of all the above variables
over trials and experimental manipulations can be analyzed to reveal
underlying planning and control processes (for a review, see
Marteniuk & MacKenzie, 1990).
With respect to the grasp component, one metric that researchers
have used in studying pad opposition or precision grasping has been
the separation between the thumb and index finger pads. Jeannerod
(1981, 1984) found that the griD aperture between the thumb pad and
index finger pad increases until it reaches some peak separation, and
then the fingers close in around the object until contact is achieved (see
Figure 5.14). This is a highly stereotypic pattern, the preshaping prior
to and the enclosing after maximum aperture. The peak aperture, the
time spent in preshaping and enclosing, as well the rate of hand open-
b(finger extension) and rate of enclosing are all potentially valuable
dependent measures to understand control of the movement prior to
contact. Note the relativity and power of grip aperture as an explana-
tory dependent measure. In pad opposition, when the fingers contact