Chapter 2. Prehension

Chapter 3 - Serial Order in Prehension 51
The peak aperture of the grip was reached at about the same time as
peak deceleration, at about 70% of the total movement time. During
the slow second phase, Jeannerod noted many corrective type
movements in the transport, as the hand enclosed around the object.
Jeannerod (1984) made an important observation, often forgotten
or ignored, that these corrective type movements occurred during the
deceleration phase even when only the target and not the hand was
visible. He thus concluded that the slow phase was not due to visual
feedback processing but was a centrally generated part of the prehen-
sion pattern, a positioning or target acquisition phase. To summarize,
Jeannerod hypothesized that reaching and grasping movements can be
separated into two phases: an initial, faster arm movement during
which the fingers preshape, and a slower arm movement beginning
after maximum aperture, during which the fingers enclose to make
contact with the object.
Interestingly, Jeannerod (1984) reported a temporal coupling as
revealed by correlations between the time of peak deceleration of the
wrist and the time of peak aperture of the grip. Figure 3.1 shows that
the time of maximum aperture corresponds with the onset of the low
velocity phase. Jeannerod argued that the arm, which is the trans~ort
component carrying the hand to a location, is controlled separately
from the hand which is the manipulation or grasp component shaping
the hand in anticipation of the grasp. Further, these are temporally
linked for the coordination of prehensile movement. He hypothesized
a central program or pattern for the coordination of the transport com-
ponent with the manipulation (or grasp) component of the unitary act.
Jeannerod suggested that “the synchronization of the low velocity
phase with finger closure indicates that both are controlled by a com-
mon program which achieves the timing of coordination” (1984, p.
253).
With regard to the transport and grasping components, Jeannerod
presented systematic differences in the effects of object properties on
reaching and grasping. He contrasted conditions in which subjects
grasped small or large objects, such as rods and cylinders, with con-
ditions in which subjects had to move to different amplitudes in the
sagittal plane. As seen in Figure 3.2, distance of the object away from
the subject affected the transport component (peak velocity increased
with the distance to be moved) but not the grasping component.
Conversely, Figure 3.2 shows the object size affected the grasping
component (maximum aperture was bigger for a larger object), not the
transport component. Jeannerod made an important distinction be-
tween intrinsic obiect uroperties (identity constituents such as size,