Chapter 2. Prehension

296 THE PHASES OF PREHENSION
27. Central vision is used to preshape the hand (Sivak &
MacKenzie, 1990).
28. Proprioception is needed to orient the palm and preshape the
hand.
29. The parietal lobe is needed to orient the palm and preshape the
hand (Jeannerod, 1986).
30. For grasping, the arm is subservient to the hand (Marteniuk et
al., 1990).
3 1. Enclosing in pad opposition aligns the pads (if possible) (Cole
and Abbs, 1987).
32. The force-generating muscles become active at the start of the
Enclose Phase (Smith et al., 1983).
33. The coordinate frame for transporting the hand to the object is
polar-centered, with the origin in the sternum.
34. The coordinate frame for finger movements is polar-centered,
with the origin in the palm.
7.3 Using an Opposition Space
Once the opposition space is set up, it can then be used. At
contact, stable grasp or stable manipulation is effected, by correctly
positioning the opposition space around the object. This occurs
during the Captu re Phase, which involves compliant motion against
the object. The Capture Phase is triggered by contact with the object.
Contour following can possibly occur. During the Preload subphase,
the grip force, or normal component of the active force, increases.
Then, during the Load subphase, both the grip force and the load force
(tangential or gravitational component) increase in parallel, with the
load force counteracting the object’s weight. If the active forces are of
insufficient magnitude, microslips from the cutaneous receptors will
signal motor adjustments to be made. This pressure must be main-
tained during the lifting, holding, manipulating, and replacing of the
object.
During these compliant motion phases, the opposition vector
specifies directions and magnitudes of forces to be applied. As in the
Setting Up of Opposition Space, direction and magnitude of forces
may be parallel dimensions as well. A complex interplay between
kinesthetic, tactile and motor processes allows the hand to apply forces
and obtain required sensory information for the interaction.
The hand, as both an input and output device, has a somewhat
unique capability in that it can grasp to feel and feel to grasp (see
Figure 7.5, first introduced as Figure 6.5). Active touch allows object