Chapter 2. Prehension

Chapter 9 - Reevaluation and Future Directions 34 1
posture is released during the Releasing Opposition Space, and the
hand comes back to a position of rest.
In order to effect these phases and this prehensile unfolding, motor
commands from the nervous system are generated at all three levels:
the Opposition Space level, the Biomechanical level, and the
Sensorimotor level. The environmental goal, along with the subgoals
for each phase filter downward. Movement over the phases occurs
satifying the constraints acting at the various levels. One of the key
planning parameters, we are suggesting, is the palm, as an interface
between the grasping component and the transport component, as
viewed over the last decade.
During the Planning of an Opposition Space, mental activities
perceive task-specific object properties, select a grasp strategy, and
plan a hand location and orientation. An opposition vector is seen in
the object. At the Biomechanical level, the perceived frictional state
and required forces are constraining the selection of an opposition
space. The posture must fit within the biomechanical constraints acting
on the hand and muscles chosen to effect that posture. At the
Sensorimotor level, there are further constraints dealing with the
requirements for sensory information and fine motor control. For
example, efficient use of the sensorimotor features of the hand is a
potential constraint. Motivational and functional goals are part of the
context within which these choices are made.
While Setting Up an Opposition Space, the palm is positioned and
oriented with respect to the opposition vector perceived in the object.
Biomechanical constraints on the extrinsic hand muscles as they cross
the wrist affect the chosen arm configuration. Certain wrist angles,
depending on the oppositions chosen, afford a mechanical advantage,
and therefore, the more proximal joints must be configured. In
parallel, the fingers open into the chosen posture. After peak aperture
and when the palm has been positioned and oriented, the fingers start a
guarded movement in order to capture the object. Task-specific circuits
await sensory information as the palm is transported closer to the
object and fingers enclose. The muscles that will supply the forces
needed in the task become active.
During Using of an Opposition Space, grip and load forces first
increase differentially and then stabilize to provide the active forces
necessary in the task. Sensorimotor features of the hand, such as the
sensitivity of mechanoreceptors and the strength of muscles, are used
to perform the task.