Chapter 2. Prehension

288 THE PHASES OF PREHENSION
approach the object. We know that gravity will make objects drop.
These all contribute to the selection of an opposition vector. Torques
caused by locating the opposition vector to the side of the center of
mass are anticipated in the frame of reference created by the opposition
vector.
Opposable surfaces
1-e
radius of curvature
spatial density
mposition Vector
magnitude
orientation
Anticipated Forces
weight
inertial
forces
tagues
Functional Degrees of Freedom
direclionof movement
range of movement
resolution of movement
Figure 7.3. Opposition vector seen in the object, perceived with a
magnitude between two opposable surfaces and an orientation
relative to an approach vector from the palm of the hand.
Secondly, the choice of an opposition space depends on choosing
a grasp strategy. This is quite dependent on a person’s anatomy,
emotional state, intentions, fatigue level, motivations, sociocultural
milieu, and so on. The term ‘grasp strategy’ refers to selecting
appropriate opposition types, mapping virtual fingers onto real
anatomical fingers, and determining opposition space parameters that
will ultimately define the shape of the posture. A grasp strategy is
chosen in accordance with the task, so that functionally effective
forces of a given direction and magnitude may be applied. The key,
though, is that the choice of the opposition vector must satisfy the
constraints imposed by the object, task, and hand. As Napier (1956)
pointed out, the precision and power capabilities of the human hand
can match the requirements of the task. Computational models have
demonstrated possible ways to map object and task characteristics to