Chapter 2. Prehension

144 THE PHASES OF PREHENSION
related with the beginning of finger closure. This correlation between
the time of peak deceleration of the wrist and the time of peak aperture
of the grip led Jeannerod to hypothesize a central program or pattern
for the coordination of a unitary act, and that reaching and grasping
movements can be separated into two phases. These are an initial,
faster arm movement during which the fingers preshape, and a slower
arm movement beginning after peak aperture, during which the fingers
capture the object. During the slow second phase, Jeannerod noted
many corrective type movements. He argued that the arm (the trans-
port component) is controlled separately from the hand (the grasping
component), but that these are temporally linked for the coordination
of prehensile movement.
In the Jeannerod experiments, some reaches had no visual feed-
back, while others were performed in such a way that the object could
be seen but not the arm and hand as the reaching occurred. It is inter-
esting to note that the low velocity phase was still observed in the ab-
sence of visual feedback. This would suggest that the low velocity
phase is not reflecting only the visual corrections as the hand gets
closer to the object. However, in studies where the subject's gaze was
monitored, visual information is sought after, if available. The kine-
matics during this second phase are affected by the type and amount of
visual information available, by the type of posture, and by the object
size. Jeannerod (1984) explained that movements having no visual
feedback often fell short of the target by 1-2 mm. Movements under
visual control were longer in duration.
In examining the behavior of reaching to grasp an object, we focus
in this section on different aspects of the kinematic profiles14 . Some
of these have been discussed previously. We can examine the nature
of the velocity or acceleration functions, looking at kinematic land-
marks like peaks and their times of occurrence. If the profiles are
normalized in time or amplitude, we can evaluate whether or not they
come from the same family, as we did with the arm pointing and aim-
l4 Jeannerod's seminal work (1981,1984) was performed using cinematography, at
50 frames/s. The introduction of computerized motion analysis technologies in the
1980s and 1990s has changed dramatically the possibilities, nature and scope of
kinematic analyses of natural human movements like grasping. Prior to this time,
a 2D or 3D view obtained from film required manual digitizing of body segments.
With the advent of computer-based optoelectric, video and sonic systems, the time
consuming and tedious process of manual digitizing was eliminated and 3D
kinematics were obtainable at higher sampling rates than required (e.g., 200 Hz).
However, new challenges concerned procedures for data management and analysis.