Chapter 2. Prehension

Chapter 5. Movement Before Contact
‘lfyou don’t know where you are going, how can you expect to
get there?”
- Basil S. Walsh
109
Our subject is about to pick up a hammer placed directly in front of
her midline. Movement begins and, as the arm reaches out, anticipa-
tory shaping of the fingers occurs, appropriate for the task at hand.
Then, the fingers begin to enclose around the hammer. All of this ob-
served movement is effected for the goal of the task, i.e., grasping the
hammer.
From movement initiation until first contact with the hammer, the
goal of capturing the hammer into a stable grasp requires that the hand
be configured appropriately. In Chapter 4, a distinction was made
between task planning and trajectory planning. Task planning, prior
to movement initiation, was examined and it was observed that key
planning variables were opposition space parameters given the task
requirements and object characteristics. In this chapter, the focus is on
trajectory planning and the setting up of an opposition space. An
overview of the issues involved in trajectory planning is presented,
and a conceptual model for transforming a goal into motor comamnds
is discussed. We then consider control of unrestrained motion of the
arm in pointing and aiming tasks. Computational models and
experimental results related to coordinate transformations are
discussed, including the key roles for proprioceptive and visual infor-
mation in guiding the movement. When the hand is added to the arm
as an effector for grasping and manipulation, evidence is provided that
the arm acts differently than when the arm is moving alone in aiming
and pointing tasks. The kinematics of reaching and grasping are
examined as a function of task requirements and object properties.
Focussing on the hand, we consider hand opening to maximum
aperture @reshaping) and closing to first contact (enclosing) as unique
phases, noting that the precise functions of enclosing after maximum
hand opening remain elusive. Trajectories are considered for different
grasp types, pad and palm opposition, and the role of visual and
proprioceptive information in monitoring the free motion phase before
contacting the object. The two phases prior to contacting the objects
are viewed in terms of Greene’s ballpark model, introduced in Chapter
3. During the move fast phase the opposition space set up is placed
‘in the ballpark’; during the drive fingers guarded phase, contact is