Chapter 2. Prehension

Chapter 5 - Movement Before Contact 167
et al. (1990). This is a large and important difference. Again for the
small to large perturbations, there was a two peaked pattern in the grip
aperture, but only a single peak in grip aperture for the large to small
perturbations. The first kinematic landmark of corrective digit
movement to switch from pad to palm opposition was at 342 ms,
derived from the grip velocity profiles. Thus these changes in hand
configuration took at least a visual reaction time, and were always
preceded and facilitated by changes in the deceleration of the limb.
They considered that the proximal control channel uses information
about object size to modify the duration of its low velocity phase.
Object size seems to determine the computational requirements of
networks for both distal and proximal control.
The perturbation study of Castiello, Bennett and Paulignan (1992),
using pad opposition for small and palm opposition for large cylindri-
cal objects points to the importance of considering different opposition
types, given object characteristics and task requirements. They
showed that when human subjects grasped small dowels (1.5 cm di-
ameter) using pad opposition, and larger dowels (6 cm diameter) using
palm opposition in blocked sets of trials, the following differences
were observed: movement times were longer for pad opposition (574
ms) than palm opposition (552 ms). This increased time was spent in
the deceleration phase of the movement (374 ms for pad opposition
vs. 353 ms for palm opposition). Castiello, Bennett 8z Paulignan’s
(1992) analysis of normalized data on unperturbed trials revealed that
the two velocity functions did not belong to the same family of curves,
i.e., a greater proportion of time was spent in the deceleration phase
when using pad opposition compared to palm opposition. The aper-
ture profile revealed, as expected, a larger maximum aperture between
the thumb and index finger for the larger object using palm opposition
(128 mm) than for the smaller object using pad opposition (92 mm).
Peak aperture occurred during the deceleration phase, earlier for pad
than palm opposition, and subject spent a greater proportion of time
enclosing with pad opposition (42%) than with palm opposition
(37%). Thus the increased precision requirements of grasping the
small object in pad opposition appear to have influenced both transport
and grasp components, leading to a longer time spent in the second
‘homing in’ phase of the movement. These results replicated closely
an earlier study by Gentilucci, Castiello, Corradini, Scarpa, Umilta
and Rizzolatti (1991) who found the effects of grasp type always ad-
ditive to their distance manipulations.
Related findings were reported by Sivak (1989, Experiment 5)
who had subjects reach for a dowel (2.5 cm diameter) placed 30 cm in