Chapter 2. Prehension

Chapter 5 - Movement Before Contact 191
(1991) and Paulignan, MacKenzie, Marteniuk, and Jeannerod (1991).
This includes the bell-shaped velocity profiles and the occurrence of a
peak aperture. Prolonged deceleration is seen when target location is
perturbed, and two peaks in grip aperture are observed. When object
size decreases, the aperture levels out before proceeding to the new
maximum.
Following Jeannerod’s hypothesis that these two components are
linked in time, the Hoff and Arbib model is a powerful model. It is
useful for making predictions. Hoff and Arbib predict that grasping
nearby objects, with a tight time constraint, will lead to low velocity
profiles, since there is a lower bound on the movement time. The
model seems to work for perturbation data, but have limited generality
since constant enclose time was not found in many studies, for
example, Marteniuk et al. (1990) where disk diameter varied. As disk
size decreased, the movement time increased and the time after peak
aperture increased.
5.5.2 Enclosing as guarded motion
An argument can be made that preshaping (up to the time of maximum
aperture) and enclosing (from maximum aperture until contact)
are uniquely different movements. While both are free motions (no
interaction with the environment), two fundamentally different things
are happening. During preshaping, the hand is opening up and there
is a desire to avoid contacting anything in the environment until this is
complete. In terms of a controller, if the free motion is ballistic, then
contact is of course ignored. But after the previously unrestrained
movements of preshaping, during enclosing, there is anticipation of
the upcoming compliant motions to be made during contact with the
object. During enclose, the hand is trying to establish tactile contact,
whereas in the preshaping phase, contact was being avoided. In
robotics terms, this enclosing motion would be called a guarded mo-
- tion. During a guarded motion, sensory information is sought,
whereas in unrestrained movements, contact with the environment is
avoided. In fact, if human prehensile motions were planned in terms
of sensory consequences, then the CNS might compare the anticipated
tactile feedback to the current tactile information. During preshaping,
there would be no anticipated tactile feedback, and during enclosing,
there would be a tactile or haptic pattern anticipated on the palmar pads
as determined by the chosen posture and object characteristics such as