Chapter 2. Prehension

338 CONSTRAINTS AND PHASES
transport and shaping components. Once the hand is preshaped and
the palm transported to a place near the object, a guarded enclosing
motion occurs, trying to establish tactile contact with the object.
Sensory information is sought, comparing the anticipated tactile
feedback to the current tactile information. The arm is subservient to
the hand, helping in the guarded move to establish contact. The force-
generating muscles become active to ‘close the fingers’ around the
object. During this setting up process, the CNS may be trying to get
some parameters into the ‘right ballpark’ which can then be fine tuned
(Arbib, Iberall, & Lyons, 1985; Greene, 1972; Iberall, 1987a).
During the use of opposition space, several subphases have been
noted (Johansson & Westling, 1988b; MacKenzie, 1992; Westling &
Johansson, 1987; Weir, 1991; Weir 8z MacKenzie, 1993). Capturing
the object involves compliant motion against the object. Lifting the
object involves a differential response by the normal component of the
active force and gravitational component (Johansson & Westling,
1988b; Westling & Johansson, 1987; Weir, 1991; Weir &
MacKenzie, 1993; Winstein, Abbs & Petashnick, 1991). If the active
forces are of insufficient magnitude, microslips from the cutaneous
receptors will signal motor adjustments to be made. This pressure
must be maintained during the lifting, holding, manipulating, and
replacing of the object.
For example, 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 sensory information consistent with
the accuracy requirements of the task. 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. Of course, anatomical constraints limit the number of ways
real fingers can be mapped into VFs.
The words ‘functionally effective’ are used to highlight the fact
that the forces must be applied within the functional constraints of the
task; i.e., while forces can be used to effect a stable grasp and impart
motions as needed in a task, there are functionally specific demands
on how this is accomplished. In a functional sense, grip and load
forces are applied by virtual fingers so that the object is not dropped
and so that the object is not crushed. (See Johansson & Westling,
1990; Johansson & Westling, 1984; Westling, 1986; Westling &
Johansson, 1984; Winstein, Abbs & Petashnick, 1991). Prehensile