Chapter 2. Prehension

Chapter 5 - Movsrnent Before Contact 163
transport component. In contrast, there were obvious differences in
anticipatory hand posturing between the egg-like object and the spheri-
cal one. On perturbed trials, when the sphere suddenly expanded to
appear egg-like, the fmt evidence for alterations in hand configuration
occured at least 500 ms after perturbation onset. He noted this time
required elaboration of new commands, and the configuration of a
new, appropriate grip pattern before the beginning of fiiger closure.
Thus, in contrast to the short times for compensation to visual
perturbation of extrinsic characteristics, it appears that perturbing in-
trinsic object characteristics requires a longer time to reconfigure the
opposition space parameters. In a larger experiment (more subjects,
more trials with a computerized motion analysis system), with pertur-
bations to obiect size both grasp and transport components were af-
fected (Paulignan, Jeannerod, MacKenzie, 8z Marteniuk, 199 1).
Using pad opposition, subjects grasped one of two, nested dowels,
placed 35 cm away. The inner, ‘small’ dowel was 10 cm high and 1.5
cm in diameter; the outer, ‘large’ dowel was 6 cm high and 6 cm in
diameter. On 20% perturbed trials one of the two translucent dowels
would be illuminated, then at movement onset, the illumination would
suddenly switch to the larger (S->L) or smaller (L->S) dowel, giving
the impression of an expanding or shrinking dowel respectively.
Results showed that movement time for S->L trials, requiring a
change in direction of planned aperture size (i.e., a reopening,
compared to initial selection), increased by 175 ms, whereas L->S
trials, requiring a change in magnitude of planned aperture size (ie.,
greater closing), increased by only 98 ms. Compared to control, un-
perturbed trials, none of the kinematic landmarks up to peak decelera-
tion were affected by the perturbation; all changes to the transport
kinematics occurred after peak deceleration (about 300 ms), during the
low velocity phase. Shown in Figure 5.20, grip formation for the
S->L perturbation showed a first peak (corresponding in time and
amplitude to the peak aperture for the small dowel in control
conditions), some discontinuity, then reincreased to accommodate the
size of the new dowel. This reincreasing occurred 330 ms after
movement onset (as determined from the grip velocity profile), after
peak deceleration. The time between the two peaks in grip aperture for
S->L was about 180 ms. The aperture evolution for the L->S
perturbation was much attenuated, showing no discontinuities and
only one peak, and identical to control large conditions, except for a
longer enclose phase, until the fingers reached their contacting
locations on the smaller dowel. Remarkable in these data was the low
variability in the spatial paths of the hand, as enclosing occurred. This