Chapter 2. Prehension

Chapter 9 - Reevaluation and Future Directions 339
behaviors demonstrate mechanisms that help prevent errors from
occurring, especially when there is uncertainty; this includes opening
the the thumb and finger to a wider aperture during the Setting up of
the Opposition Space under uncertainty (Wing et al., 1986) and safety
margins during Using the Opposition Space (Johansson & Westling,
1990). Mechanisms have been noted that help maintain the grasp:
magnet phenomenon (Johansson & Westling, 1990), eccrine secretion
enhancing adhesion (Cutkosky & Howe, 1990), vibrations against the
skin exciting cutaneous mechanoreceptors (Torebjork, Hagbarth &
Eklund, 1978).
Ultimately, the versatility of the human hand stems from what
Napier pointed out in 1956, that precision and power are not mutually
exclusive. The human hand (and brain!) can resolve these multiple
task components and in doing so, find a set of oppositional forces that
are functionally effective for satisfying the competing task
requirements for arbitrary objects. This is true, whether the task is to
hold one or even 10 oddly-shaped objects at a time, to do either one or
many things with them! The human hand has a variety of ways to
grasp objects stably; the decision that the CNS must make in choosing
which oppositions to use depends on balancing the functional
requirements of the task with the functional abilities of the hand and
body.
9.2 Phases of Prehension Revisited
One of the key points of this text has been that prehensile behavior
unfolds over time, as introduced in Chapter 3, detailed in Chapters 4
through 6 and summarized in Chapter 7. Recalling Figure 7.1, we
note the four main phases of prehension, planning, setting up, using,
and releasing an opposition space. In Chapter 8 it was suggested that
there are also levels of analysis for understanding prehension. High
level goals (from the opposition space level) travel downward to the
Biomechanical level and Sensorimotor level. In turn, from these
levels, constraints work upwards. Sources of these constraints come
from evolution, anatomy, biomechanics, functionality, and
motivations.
A model is presented in Figure 9.2 that combines these two
perspectives. Starting from a resting posture during the Planning of an
Opposition Space, the hand opens during the Setting up of the
Opposition Space. Once opened, it can enclose around the object. On
contact, the Using of the Opposition Space occurs. Finally, the