Chapter 2. Prehension

Chapter 7 - Opposition Space Phases 289
hand postures, using expert systems and neural networks. Expert
systems make explicit the rules of these mappings; neural networks
can learn the rules. Yet, none of these are performing the mapping the
other way; i.e., how do the constraints of hand anatomy (level of
fatigue, motivations, etc.) drive the choice of the opposition vector?
Thirdly, a location and orientation in space for the hand to go to
must be planned. Planning a hand location and orientation will depend
on the grasp strategy chosen. Such a decision can be based on retinal
information or on knowledge about the hand. It has been shown how
wrist orientation is affected by task constraints. But this is the same
issue as raised above. Constraints of the anatomy and other biological
and motivational factors affect the perceived opposition vector, and
thus the choice of the opposition space of the hand.
How this plan is constructed in the brain is open to discussion. In
Chapters 3 and 4, task plans from a variety of fields were examined,
showing possible mechanisms for programming phases, for
integrating feedforward and feedback controllers, and for including
contingencies for error. Some models suggested distributed control,
others hierarchical control. Other models suggested how plans could
be mapped out across regions of cortical and sub-cortical areas in the
CNS. Neural recordings have amply demonstrated the CNS is
computing something; what it is computing is unknown. Importantly,
planning takes time, and reaction time studies have demonstrated that
the more complex the movement, the longer the reaction time. Of
course, subsequent movements can be planned during initial
movements.
From the experiments and computational models put forth in
Chapters 3-4, the underlying hypotheses are made explicit for the
reader's further evaluation and research:
1. A task plan is built in terms of sensory consequences (Abbs and
Cole, 1987).
2. Motor equivalence suggests task planning does not occur at the
muscle level.
3. A minimal amount of time is needed for planning. More time is
needed for a more complicated serial task (Henry & Rogers,
1960).
4. Kuperstein (1988) built associations of maps between the eye
position and arm configuration. This suggests that a person
must look at where he/she wants the arm to go before
movement occurs.