Chapter 2. Prehension

Chapter 5 - Movement Before Contact 159
In contrast to the above studies, Haggard and Wing (1991) intro-
duced mechanical perturbations of the arm, and looked for compen-
satory responses in hand aperture. They used a torque motor to de-
liver pulling or pushing forces on the arm (extending the shoulder, and
moving it backwards from the target, or flexing the shoulder and
moving it towards the target). Perturbations occurred on 25% of the
trials (randomly between 1 and 560 ms after trial onset; net force 5,10,
15, or 20 N, for 250 ms duration). Push perturbations did not disrupt
hand transport and were unnoticed by the subjects. In contrast,
pulling the arm backward from the target showed disruptions through
‘transport reversals’, i.e. changes in direction of hand transport
(typically, about 120 ms after perturbation). Figure 5.18 shows the
pattern obtained on 67% of perturbed trials with transport reversals
where ‘hand-aperture reversals’ were observed also (about 70 ms later
than the hand transport reversals, or 100 ms after the backwards accel-
eration). On these trials, hand aperture would continue to increase for
a short time after hand transport had been slowed. As the hand began
to be pulled backwards, hand aperture would decrease for a short
time. Then, after the perturbation ended, the hand’s transport towards
the target would resume, followed by resumption of grasp opening.
Haggard and Wing suggested the hand aperture reversals were not a
biomechanical result of the perturbation because of the long latencies;
rather they suggest that information about the two effector systems is
monitored for sensor-based coordination and control. The latency of
over 100 ms from arm transport reversal to aperture reversal appeared
to them more consistent with a voluntary response than a spinal reflex.
They suggested that the compensatory response returns the subject to
the spatial track they had before the perturbation, and that the spatial
basis for coordination of transport and grasp is an important one.
Visual perturbation experiments have shown also a functional
coupling between transport and grasp components. Like Haggard and
Wing, the perturbations have affected both transport and grasp com-
ponents but with different times required to reorganize each compo-
nent. A repeated finding is the linking of deceleration of hand trans-
port with aperture closing, and reacceleration of the limb with hand
opening, or aperture increase. These perturbation studies have,
through optical and illumination techniques altered the distance, direc-
tion, shape and size of the object to be grasped. First we consider the
extrinsic object properties of direction and distance, then we consider
object shape and size perturbations.