Chapter 2. Prehension

142 THE PHASES OF PREHENSION
torques as well as passive ones. In order to use an opposition space
after contact with the object, the reaching and grasping schemas in the
CCP must be activated and receive the necessary information, in order
to set up an opposition space12. The planning process entailed 'seeing
the opposition vector' in the object, appropriate for the task at hand,
and selecting the opposition space parameters including: the type of
opposition; the virtual finger mapping; and the virtual finger state
variables when in contact with the object at stable grasp. After
planning, the fundamental problem for setting up an opposition space
(the motor control system from movement initiation to contact) is
through coordinate transformations, to configure the hand and arm for
placement on the object, consistent with the above opposition space
parameters. Reminiscent of Jeannerod's discussion of superimposing
'proprioceptive maps' and 'visual maps', the subgoal for setting up
an opposition space is to align the hand configuration with the
opposition vector seen in the object, satisfying the opposition space
parameters. Thus, for grasping, the opposition vector drives the
movement execution prior to contact.
What is known about reaching and grasping and the relationship
between the two schemas in Figure 5.1? With regard to the transport
and grasping components, Jeannerod (1981, 1984) placed markers on
the distal parts of the index finger and thumb and found systematic dif-
ferences in the effects of object properties on reaching and grasping.
He contrasted conditions in which subjects grasped small or large ob-
jects, with conditions in which subjects had to move to different
amplitudes in the sagittal plane. Analyses of the kinematics of the
hand transport were combined with aperture between the two markers
to infer central control. As seen in Figure 5.14, distance of the object
away from the subject affected the transport component (peak velocity
increased with the distance to be moved) but not the grasping compo-
nent. Conversely, object size affected the grasping component
(maximum aperture was bigger for a larger object), not the transport
component. Jeannerod made an important distinction between intrin-
sic obiect properties and extrinsic object properties13. He suggested
120pposition space terminology was introduced in Chapter 2, where we defined the
concepts of opposition space and virtual fingers. Virtual finger orientation is a state
variable, and the finger position constraint corresponds to the opposition vector's
magnitude. In Chapter 4, we made explicit the notion that subjects "see the
opposition vector" in the object, in the spirit of action oriented perception.
131ntroduced in Chapter 3, Jeannerod distinguished intrinsic obiect D rowrues
(identity constituents such as size, and shape) from extrinsic obiect D roDemeS . (or
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