Chapter 2. Prehension

322 CONSTRAINTS AND PHASES
component to a prehensile posture, if so needed in a task. The
tradeoff for this extra power is in not having the finger pulps in contact
with the object; a compromise would be to use a combination of side
and palm opposition, where the thumb pad contacts the object and
applies a force transverse to the palm. If the task goal is to 'move as
quickly and as accurately as possible', timing parameters are chosen
consistent with Fitts' Law while also consistent with kinematic and
dynamic constraints acting on the hand and arm. Not only must the
anticipated forces be matched, an opposition space must be chosen that
allows perceptual systems access to the information needed to ensure
the accuracy requirements. Accuracy suggests pad opposition, but
only if the forces are not too great. With greater forces, one
compromise is to use more fingers in the VF opposing the thumb.
The final mapping to be discussed from the Opposition Space level
(Figure 8.1) is the Sensorimotor level. Opposition space parameters
are re-represented at this level in terms of the activation level of the
muscles and receptors acting on the fingers, which, in effect, is the
implementation level. Anatomical constraints, for example, limit the
number of ways real fingers can be mapped into VFs. The task goal of
'not dropping the object', expanded by the ways that forces can be
generated to achieve the goal, is further translated into a posture that
judicially uses the skin mechanoreceptors, optimally placing the pulps,
with all their receptors, against the object. The chosen VFs must have
adequate sensory resolution and sensitivity at the contact points. With
large task forces, pad opposition would not be effective due to loss of
skin sensitivity and lack of muscle strength. Being able to anticipate
much of the object's behavior upon contact allows part of the choice to
be influenced by the VF parameters that will correctly position task-
specific sensors for triggering the next action. If the object starts
moving, the tonic vibration reflex will force the fingers to close
tighter. Perhaps in contour following, where the fingers are moving
instead of the object, similar principles apply. The advantage of
performing contour following is that one gains additional knowledge
about the object's various properties, and the fingers can perhaps be
better placed for effecting a stable grasp. If palm opposition is used in
order to handle the larger forces, the posture still gives information
about the object state, particularly because the palm can adapt to the
object's shape and therefore place more skin surface in contact with
the object. In terms of constraints on the mapping from virtual to real
fingers, there are significant distinctions in how the individual fingers
contribute to prehension (number of muscles, types of articular
surfaces, lengths and widths of phalanges, type of support at wrist,