Chapter 2. Prehension

Chapter 9 - Reevaluation and Future Directions 333
Finally, in terms of gathering sensory information, there are
analytic measures for the design of sensors. The sensivity required in
a task deals with sensor spatial resolution, sensor spacing, and the
sensitivity of the sensors. The required speed of response is a
bandwidth issue. An important question relative to tasks is what
information must be transduced (e.g., finger normal forces, tangential
forces, contact, joint torques, vibrations changes in forces). (See
Cutkosky & Howe, 1990; Fearing & Hollerbach 1985).
From the experimental literature, tasks are described at the level of
‘move as quickly and as accurately as possible’ or ‘pick up the
dowel. ’ At this level, an environmentally-defined goal is made
explicit, Goals can be subdivided into sub-tasks occurring in serial
order, and a task plan constructed that consists of desired subgoals
without regard to the details of the actions necessary to accomplish
them. Task plans have been constructed for robots (Lozano-Perkz &
Winston, 1977) and as models for the CNS in terms of coordinated
control programs (Arbib, 1985). Goals can be represented in terms of
sensory consequences (Schmidt, 1975; Cole & Abbs 1987). Mis-
matches between the actual outcome and the anticipated sensory
consequences cause reprogramming, but the error processing can also
be incorporated into the plan, as modelled with TOTES (Miller,
Galanter & Pribram, 1960).
In terms of human understanding of tasks, it is hard to relate these
high level descriptions to the lower level analytic measures (Cutkosky,
1989). Yet, at the level of human performance, it is observable that
postures are chosen with implicit knowledge of issues such as
stability, resistance to slipping, force closure, connectivity, etc.
9.1.3 Hands
The human hand has both precision and power capabilities (Napier
1956). It also has a variety of ways to grasp objects. Prehensile
postures present different degrees of available force, of available
motion, and of available sensory information. From the variety of
possible postures, the CNS matches the precision and power
requirements of the task with the precision and power capabilities of
the human (Napier, 1956). But there are constraints on the ways that
the hand can be postured, as well as on the potential success of a
chosen posture. Postures are created by the muscles of the hand
directing the bones into some configuration, based on the motion
capabilities of the various joints.