Chapter 2. Prehension

Chapter 9 - Reevaluation and Future Directions 337
do, as shown in Chapter 6. The natural frequency for manipulative
movements, in humans, has been observed to be in the 4-7 Hz range
(Kunesch, Binkofski & Freund, 1989).
9.1.4 Mapping between inputs and outputs
The act of grasping an object involves the planning, setting up,
using, and releasing of an opposition space for the task, as outlined in
Chapters 3 through 7.
During the planning phase, task-related object properties are
perceived, as noted above. This entails seeing an opposition vector in
the object that will satisfy the required degrees of freedom of the task.
It depends on previous knowledge about the behavior of objects, and
our ability to predict and anticipate task-specific prehensile
occurrences. A grasp strategy is chosen, selecting appropriate
opposition types, mapping virtual fingers into real anatomical fingers,
and determining opposition space parameters. The choice of the
opposition vector must satisfy the constraints imposed by the object,
task, and hand. Computational models have demonstrated possible
ways to map object and task characteristics to hand postures, using
expert systems (Cutkosky & Howe, 1990; Iberall et al., 1988;
Stansfield, 1991) and neural networks (Iberall, 1988; Uno et al.,
1993). Finally, 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 (Klatzky et al., 1987, Sivak & Mackenzie, 1992)
or on knowledge about the hand. As was shown in Chapters 4 and 5,
cortical parietal areas, premotor and motor areas, and spinal areas are
all processing information relevant to arm reaching and grasping
(Alstermark, Gorska, Lundberg, & Pettersson, 1990; Georgopoulos
1990). For example, Georgopoulos (1990) shows evidence of motor
cortex and area 5 computing a Populational Vector that specifies the
direction of arm movement prior to movement.
Setting up the opposition space involves preshaping the hand and
transporting it towards the object. At first, the hand moves in an
unrestrained way opening to a posture suitable for the task (Jeannerod,
1984) and to stretch the finger flexors (Smith et al., 1983). The
relationship between the transport and grasping components appears to
be functionally specific to the high level goal of the task (Jeannerod,
1984; Marteniuk et al., 1987; Wing, Turton, & Fraser, 1986).
Noting the interwoven action of wrist, forearm, elbow, and shoulder
muscles, we suggest that the palm is the likely interface between the