Chapter 2. Prehension

Chapter 3. Serial Order in Prehension
“Movements have the teleological property of anticipating
targets in space and time and can be jluently coordinated to
realize a given schedule.”
--Shaffer (1982, p. 112)
49
A hammer sits on the table in front of you. You reach out, pick it
up, and place it on a shelf. What was involved in this task of picking
up the hammer and placing it?
Woodworth (1 899) first described goal-directed aiming move-
ments as being two-phased: an initial, ungoverned motion, followed
by a final, currently controlled adjustment. The initial adjustment
phase transports the limb quickly towards the target location, and the
current control phase subsequently corrects any errors made along the
way, using sensory feedback to reach the target accurately. These
findings have been reported consistently, and extended in the litera-
ture since the time of Woodworth (for a comprehensive review, and
discussion of the speed accuracy tradeoff in aiming movements, see
Meyer, Smith, Kornblum, Abrams and Wright, 1990).
The notion of two phased movements was introduced into the
grasping literature by Marc Jeannerod (1981, 1984). In 1981,
Jeannerod published seminal data suggesting that two phases were oc-
curring during prehensile movement. He performed experiments in
which subjects grasped small objects such as rods, cylinders, and
spheres. These movements were made in the sagittal plane. Based on
cinematographical analyses of the positions of the index finger and
thumb during the grasping of objects, he suggested that there was a
fast (high velocity) phase and a slow (low velocity) phase. Figure 3.1
shows some of Jeannerod’s results. Jeannerod based his analyses on
the transport of the hand which can be seen in the top of the figure
showing results for two different sized objects, and the shaping of the
fingers into an aperture between the thumb and index markers, which
can be seen in the bottom of Figure 3.1. The first phase of the move-
ment, from initial movement to peak deceleration of the wrist, lasts for
approximately 70% of total movement time. It is important to note
that the tangential velocity profile is bell shaped but asymmetrical.
During this fast high velocity phase, the hand was opening as the
fingers were extending, thereby posturing appropriately for the grasp.