Chapter 2. Prehension

Chapter 5 - Movement Before Contact 173
tween pad and palm opposition as separate prehensile categories, for
planning andor execution. Once pad opposition is chosen, either one
or more fingers can be mapped onto VF2, to oppose the thumb as
VF1. In Sivak (1989) the transport velocity and deceleration phase of
the movement was sensitive only to the distinction between pad and
palm opposition. For cylindrical objects 2.5 cm in diameter or less,
the number of real anatomical fingers mapped onto VF2 in opposition
to thumb may discriminate the aperture evolution between collective
fingers in palm opposition, collective fingers in pad opposition and in-
dependent fiiger movement in pad opposition.
Recent evidence by Castiello, Bennett and Stelmach (1993),
showed a clear distinction between whether VF2 is mapped into one or
more fingers in opposition to VF1 (as the thumb). Further, they
demonstrated the importance of a natural mapping between object size
and grasp. They compared blocked size and perturbed size conditions
in which subjects grasped and lifted with natural grasps (i.e, VF2
mapped onto the index finger for a 0.7 cm diameter object, compared
to VF2 mapped onto index, middle, ring and little fingers for an 8.0
cm diameter object) or with instructions for one of these grasps (called
precision or whole hand grasps) for both small and large cylinders.
Results indicated that with a natural grasp, there was little or no
increase in movement time for selecting a new grasp type matched to
the object size. In contrast, with instructed grasps, movement
durations increased for adjustments of aperture to a size perturbation.
Like Paulignan, Jeannerod, MacKenzie, and Marteniuk (1991),
adjustments for perturbations from small to large were more difficult
than from large to small. They also found that perturbations requiring
a change from collective fingers to the index finger in opposition to the
thumb (i.e., large to small perturbations) yielded a differentiable
pattern in hand shaping (independent spatial path of the index finger
from the others) about 173 ms after onset of perturbation of object
size. Note that this time is substantially shorter than the 300 - 350 ms
reported by Paulignan et al. for a readjustment to increase aperture
between thumb and index (using pad opposition) with perturbation to
object size.
Further research on the kinematics prior to contact is needed to un-
derstand how opposition space is set up with different types of oppo-
sitions. It is clear that the different opposition types show distinctive
hand configurations (obviously!), as well as corresponding changes in
the kinematics of transport. Following from the above research, more
experiments are needed with a greater range of object properties and
task requirements, as we discussed earlier in our analysis of different