Chapter 2. Prehension

Chapter 5 - Movement Before Contact 161
To Derturb obiect direction, Paulignan, MacKenzie, Marteniuk,
and Jeannerod (1990, 1991) had 3 translucent dowels, placed 30 cm
away on a table, at 10,20 and 30 degrees from the body midline. In
blocked and control (nonperturbed) trials, one dowel was illuminated
and served as the target object. On 20% of trials, the first, center
dowel was illuminated, and at movement onset, the illumination
unexpectedly shifted left or right to another dowel. Subjects were in-
structed to grasp the illuminated dowel with pad opposition. Relative
to control trials, movement time on perturbed trials increased by 80
and 112 ms for perturbed-left and perturbed-right trials. Shown in
Figure 5.19, on perturbed trials, the transport of the hand showed an
abrupt change in direction at about 255 and 295 ms, corresponding to
the new target object (left and right respectively). Evidence for the
first change in the transport kinematics was that peak acceleration oc-
curred earlier in the perturbed conditions (about 100 ms) compared to
the control conditions (130 ms). Interestingly, on perturbed trials the
time between the two peaks in velocity was about the same as the time
from movement onset to the initial peak velocity, about 180 ms.
Although the perturbation affected no intrinsic object characteristics,
there were corresponding changes in the grip aperture, such that there
were two peaks in the aperture profile on most trials. At around the
time that the transport component was reoriented, the preshaping
phase was interrupted, and the hand began to close. The first peak in
grip aperture was smaller and earlier than in control trials, and the sec-
ond peak corresponded to the magnitude of peak aperture for control
trials. Paulignan et al. showed that the variability of spatial paths was
at a maximum around the time of peak velocity and decreased steadily
as the finger pads converged onto the object locations. They argue
that this might reflect two separate phases, where the first phase is in-
volved in directional coding of the movement; in contrast, the second
phase would involve comparisons between motor commands and sen-
sory feedback. The acceleration phase was interpreted as reflecting
mechanisms for directional control of movement. In contrast, the
deceleration phase reflects sensorimotor-based control based on
interaction between motor output and reafferent signals, visual and
proprioceptive. The reader is referred to related studies on pointing by
Pelisson and colleagues (Pelisson, Prablanc, Goodale & Jeannerod,
1986; Prablanc, Pelisson & Goodale, 1986).
To perturb obiect distance, Gentilucci, Chieffi, Scarpa and
Castiello (1992) had subjects grasp 3 spheres (diameter 4 cm) placed
at 15, 27.5 and 40 cm from the starting position along the subject’s
sagittal plane. Subjects grasped the illuminated sphere using pad op-