Chapter 2. Prehension

Chapter 7 - Opposition Space Phases 285
In Chapters 3-6, we considered phases that seem to be distinct in
prehension. Planning, reaching, grasping, manipulating, and
releasing the object all entail different activities. A conceptual model is
presented in Figure 7.1 as a plan for effecting a prehensile movement.
Such a task plan ties together the serialization of multiple sub-tasks,
such as transporting the hand to the correct location and orientation
and shaping the hand into a posture suitable for the object and task.
Across the toD shows a rather high level descrimion of the task ~lan
for prehension, unfolded over -time. The taik plan include; the
following phases:
1. Planning an Opposition Space,
2. Setting Up an Opposition Space,
3. Using an Opposition Space, and
4. Releasing an Opposition Space.
Within each phase, computational processes and subphases occur, as
detailed below.
In evaluating prehensile behavior, experimenters have adopted one
of two main paradigms: focusing on the movement prior to contact
(paradigms discussed in Chapter 5), or focusing on the forces applied
during contact (paradigms discussed in Chapter 6). In Chapter 5,
detailed examination of the limb kinematics, spatial paths, and hand
configuration revealed organization and control while setting up an
opposition. In Chapter 6, detailed analyses of forces, EMG,
sensorimotor features and analytic measures revealed control while
using an opposition space. For lifting an object, Figure 7.2 shows an
integrated, synchronized view of the kinematics and grasping forces
from movement onset through acquisition into stable grasp, lifting,
replacing and releasing an object. Shown is a single trial in which a
human subject reached for a 155 g dowel (3.8 cm diameter, 11.5 cm
long) placed 20 cm in front of the midline sagittal starting point,
grasped it with pad opposition between thumb and index finger pads,
lifted it to match the height of a 4 cm block, replaced it on the contact
plate and returned to the starting posture. The entire movement
sequence had a duration of 2.4 s. Free motion occurs from movement
onset to peak aperture between the thumb and index finger. Guarded
motion occurs as the fingers enclose, anticipating contact with the
object. Compliant motion occurs from object contact until the object is
let-go. There is another period of compliant motion as the object is
replaced on the table until release. After letting go of the object, the
hand is in free motion. We now consider what is happening during
these phases in an Opposition Space analysis of prehension.