Chapter 2. Prehension

298 THE PHASES OF PREHENSION
characteristics, such as surface texture, hardness, temperature, object
size and shape, to be explored. Since the visual system supplied only
an approximation of many of these (e.g., spatial density, weight, size,
shape), active touch makes available more accurate information for the
ensuing phases. When the desired placement of the hand surfaces is
achieved, the force-generating muscles begin their second function,
that of ‘generating the forces’ against the object. If holding the object
is part of the task, then a stable grasp must be effected. The contacting
surfaces of the hand must support the weight of the object. Therefore,
the muscles must be set to lengths that generate the necessary forces
without undue strain on themselves as well as on the bones, joint
capsules, and tendons. Using the frictional components of skin, such
as the epidermal ridges, fatty pads and sweat glands, the amount of
active force needed can be reduced, as long as the pads are placed
correctly.
From the experiments and computational models put forth in
Chapter 6, the underlying hypotheses are made explicit for the reader’s
further evaluation and research:
1. Perturbations can occur during contact. To compute a new
opposition space while using an opposition space would take
time. To deal with perturbations that fit within constraints of
chosen opposition space wouldn’t take much extra time
(Paulignan, MacKenzie, Marteniuk, & Jeannerod, 199 1;
Paulignan, Jeannerod, MacKenzie, 8z Marteniuk, 199 1).
2. During contact, haptic perturbations will be more powerful than
visual perturbations.
3. The highest goal in a hierarchy of goals will tune others.
4. If the goal is to place an object on a support surface, and the
support surface is moving, this will affect grasp.
6. Knowledge of task mechanics is used to predict the behavior of
the object as the hand makes contact and mes to establish a
stable grasp (Mason, 1986; Rosenbaum et al., 1990).
7. A stable grasp is not always formed before manipulation
begins.
8. There are subphases to using an opposition space (Johansson &
Westling, 1984, 1987, 1990).
9. Analogous control processes and organizational principles apply
to the fovea of the eye and the pads of the fingers.
10. The cone of friction creates a ballpark for placement of the
grasping surface patch of a virtual finger.