Chapter 2. Prehension

82 THE PHASES OF PREHENSION
c
Opposable surfaces
length
radius of curvature
spatial density
Opposition Vector
magnitude
orientation
Anticipated Forces
weight
inertial
forces
torques
Functional Degrees of Freedom
direction of movement
range of movement
resolution of movement
Figure 4.7. An opposition vector is perceived in the object, given
task requirements and object properties. The opposition vector has
a magnitude, corresponding to the diameter of the cylinder, and an
orientation, in an egocentric reference frame. In action-oriented
perception, task requirements, intentions and knowledge about task
mechanics lead to the percption of task-relevant object properties.
The opposition vector perceived would be different if the task
required looking inside the glass, compared to precise placement of
the glass on a coaster. The opposition vector, seen in the object,
is used to select a grasp strategy, and drives the alignment of
visual and proprioceptive maps.
than precision grasps).
Just as important as object properties are the task considerations of
applying forces and imparting motions. Contributing to the anticipated
forces acting on the object are the perceived weight, torques, and
inertial forces that will be acting on the object during the interaction.
Assumptions are made that weight covaries with size. But it is the
center of mass acting at the opposition vector that the hand cares
about. Using an opposition vector at the handle of the mug causes a
moment with a length the distance from the opposition vector to the
center of mass and with a magnitude the weight of the object. For this
reason, we tag the task requirements to the opposition vector because