Chapter 2. Prehension

324 CONSTRAINTS AND PHASES
and the potential use of many prehensile strategies at one's disposal,
the choice for using one strategy over another can be based on
anything from physical laws to whimsy. Roboticists and
experimentalists seem to be currently at odds in terms of identifying
the important variables to be examined in the quantification of motor
control. By identifying and separating hard physical constraints from
softer functional constraints, the hope is to find ways to integrate the
various views of motor behaviors.
8.5 Time Varying Constraints
One might ask whether or not the constraints listed in Table 8.1 are
time varying constraints. Given the importance of time scale
considerations in the adaptation process (A. S. Iberall, 1972; Piaget,
1953), each constraint must be analyzed in terms of whether it varies
over the time course of the opposition space phases for a single grasp
instance. Some of these are constant within the time scale of the reach
and grasp. Other constraints may change within a person's lifetime of
grasping objects. Finally, others may be changing on a much longer
time span, over the course of evolution. For the case of a single
grasp, the time span is in the second range, although longer grasps are
possible (e.g., climbing, trapeze swinging). An important question to
address for computational modelling is how these constraints might fit
into an artificial network model. Possibilities include: acting as an
input to a system, acting as a rule mapping the inputs to the outputs, or
part of the structure of the network itself. Reviewing the constraints in
terms of time scales:
1. Social/Cultural--not time varying within the time scale of the
current reach and grasp. Can change within a person's
lifetime, thus acting as an input, influencing the selection of the
grasp and reach strategy.
2. Motivational--not time varying within the time scale of the
current reach and grasp. Changes between grasps, thus acting
as an input, influencing the selection of the grasp and reach
strategy.
3. Informational--not time varying within the time scale of the
current reach and grasp. Changes between grasps, thus acting
as inputs, influencing the selection of the grasp and reach
strategy.
4. Functional--not time varying within the time scale of the
current reach and grasp. Changes between grasps, thus acting