Chapter 2. Prehension

292 THE PHASES OF PREHENSION
Fitts’ Law, one of the few lawful relationships in motor control,
demonstrates a speed-accurarcy tradeoff in many movements.
Asymmetric velocity profiles point to the possibility of a two-phase
controller for complex movements where interaction with the
environment is required. The environmentally-defined goal seems to
suggest that a precision effect might be in play in these situations.
This precision effect shows up in the second part of the movement
where sensory information is needed for changing from an
unrestrained movement to a guarded movement. As seen in Figure
7.4, preshaping the fingers and orienting the palm are concurrent
activities, to get the hand configuration ‘in the ballpark’ of the object
during the first phase.
During the preshape, from movement onset to peak aperture, the
hand moves in an unrestrained way. The preshaping of the hand into
a suitable posture for the task is an inherently unique event. It sets up
the opposition space, with the posture chosen reflecting perceived
object properties, such as size and shape, and also task properties. The
force production muscles are stretched, preparing to be used. In order
to accomplish this setting up, sensory information is needed --
proprioceptive usually, but if it is not available through some sort of
neural dysfunction, then vision can be used. In situations where
sensory information is reduced (e.g., blind conditions, fast
movements), the hand opens wider. If planning is occurring in terms
of sensory consequences, then during preshaping, there would be no
anticipated tactile feedback. Accidental contact with the object is
avoided. In terms of a controller, if the free motion is ballistic, then
contact is of course ignored.
As the fingers are enclosing, a transition is occurring between the
previously unrestrained movements of preshaping and the upcoming
compliant motions to be made during contact with the object. During
this transition phase, the hand is performing a guarded motion, trying
to establish tactile contact. Sensory information is sought by the Drive
Fingers Guarded Schema, comparing the anticipated tactile feedback to
the current haptic information. The chosen pads (for pad opposition,
or more generally, grasping surface patches for all oppositions) are
being brought into a specific contact with the object. Research has
shown that the CNS knows the direction that the pads are pointing.
The arm is subservient to the hand, helping in the guarded move to
establish contact. The force-generating muscles are active to ‘drive the
fingers’ around the object. Central vision can compare the location of
some part of the hand with the object, since both are within a small
visual field. The controller can use the cone of friction as a ballpark to