Chapter 2. Prehension

300 THE PHASES OF PREHENSION
are seen in the object and drive the Planning process, given the task
requirements and object properties. The Setting Up Phase seems to
demonstrate two different subphases. The first gets the hand posture
‘in the ballpark’. During preshaping, the hand is driving the arm
using anticipatory mechanisms and the hand is shaping according to
the opposition space chosen. At the same time, the palm is being
oriented. Then, after maximum aperture the hand encloses, a guarded
movement involving feedback control anticipates tactile contact. The
goal of setting up an opposition space is the alignment of the grasping
surface patches of the hand with the seen opposition vector, given the
opposition space parameterization appropriate for the task. The Using
Phase is complex, involving rich interactions between vision, haptics
and proprioception for controlling the hand, arm, and object.
Movement seems to be driven strongly by sensory information and
sensory consequences. Contact triggers a controller which uses active
touch and controlled slipping to capture the object into the hand.
Subgoals caused by the environmental goal can dominate in this
phase, determining the influence of sensory information on the motor
task; e.g., to hold a glass, a subgoal would be to squeeze fingers if the
object is slipping, whereas to twirl a baton, a subgoal would be to
reposition fingers if the object is slipping. The Releasing Phase is
another example where feedforward anticipatory mechanisms seem to
dominate, although it is unclear if the arm is driving the hand, the hand
is driving the arm, or they are totally uncoupled.
There is a distinction between free motion and compliant motion.
This distinction permeates the planning and selection of opposition
space parameters through to the motoneuron recruitment patterns and
muscle contractions for the application of functionally effective forces
to grasp and manipulate objects. Once an object is grasped stably, the
transport of the object can be viewed as a free motion with a ‘loaded’
limb. When the grasped object is being placed, the compliance of the
supporting surface comes into play. Replacing the object on a support
surface constitutes another guarded motion.
Opposition space analysis has vector description of motions and
forces. During free motion (trajectory related planning), the vectors
are concerned with directions and distances within a polar coordinate
system. Direction and distance appear to be parallel dimensions. In
contrast, during compliant motion (force related planning), the vectors
specify directions and magnitudes of forces to be applied, in a palm
centered coordinate frame. Here direction and magnitude of forces
may be different dimensions as well. In setting up an opposition
space, the opposition vector determines the directional planning for the