Chapter 2. Prehension

294 THE PHASES OF PREHENSION
accelerations or torques. Treating muscles as tunable springs is
another possible method of control, suggesting that the CNS need
only set a new equilibrium point for the muscles, thus avoiding the
need for a more active controller. Of course, the CNS never seems to
have a unique solution, evidenced by the issue of motor equivalence
and also of the exhibited variability. One possibility for how
computations are performed is that the CNS uses linear
approximations of the exact nonlinear relation between limb segment
angles and target locations.
The CNS may be trying to get some parameters into the ‘right
ballpark’ which can then be fine tuned. This could be the threshold of
motoneuron recruitment as a parameter that will cause movement once
it is set. The setting of such a parameter, in terms of prehension, must
be related to high level task goals. It was shown that kinematic
parameters can be used for satisfying high level goals. The palm is
placed into a ballpark of its final orientation and location, and an
opposition space controller puts virtual finger parameters into the right
ballpark of their goal configuration. A desired posture and position
are computed from estimated object location, orientation, and size, and
a feedforward controller gets the hand and arm into the right ballpark
during the first phase of the movement. Feedback mechanisms then
overcome errors in perception during the second phase of the
movement.
Importantly, during the setting up of an opposition space,
movements are concerned with directions and distances. Polar
coordinate frames, whether centered in the head, sternum, shoulder,
wrist, or palm, set up a convenient way to view this setting up
process. Direction and distance appear to be parallel dimensions and
the CNS seems capable of computing directions quite accurately. The
opposition vector determines the directional planning for the hand
configuration, both for the hand with respect to the body midline, and
the grasping surface patches of the fingers with respect to the palm.
From the experiments and computational models put forth in
Chapter 5, the underlying hypotheses are made explicit for the reader’s
further evaluation and research:
1. Set up occurs in parallel, and therefore no extra time is needed
to set up a more complicated grasp.
2. There is a functional relationship between preshaping and
transporting (Jeannerod, 1984).
3. Pointing is different fiom reaching and grasping (Marteniuk et
al., 1987).