Chapter 2. Prehension

Chapter 5 - Movement Before Contact 111
muscle activation, and neural innervation.
One question that might be asked is whether all arm movements
exhibit two phases as Arbib modelled for reaching movements.
Jeannerod and Biguer (1982) made a distinction between simple and
complex movements. In the context to which they were referring,
pointing to a location in space is simple because it does not involve
interaction with the environment. Reaching and grasping in this con-
text would be complex, because the end goal involves interactions
with environment (the ‘actual grasp’ schema in Figure 5.1). While
the arm is transporting the hand to a location near the object (the
‘ballistic movement’ schema in the figure), the hand is preshaping in
anticipation of the actual grasp. The arm continues to move during the
actual grasp (the ‘adjustment’ schema).
The ‘ballistic movement’, ‘finger adjustment’, and ‘hand rotation’
schemas are examples of unrestrained or free space motions. Once
the hand touches the object, motion changes from being unrestrained
to a compliant motion, necessarily having to comply with the object
and its support surface. For example, in order to lift the object, the
posture effected by the hand must be able to apply forces against the
object surfaces that counterbalance the object’s weight, all the while
not crushing the object. This issue of force application to an object
and restrained or compliant motion will be dealt with in Chapter 6.
In looking at the CCP, an important issue is the nature of the con-
trol law used within each schema. For example, the term ‘ballistic
movement’ was used. A ballistic controller is a ‘bang-bang’ con-
troller, with the agonist muscles turning on to initiate the movement,
and near the end, the antagonist turning on to decelerate the arm. This
is similar to a feedforward controller, where a trajectory is preplanned.
An alternative is a feedback controller, which continually senses the
difference between a desired and current state of a system and makes
adjustments to reduce the difference.
In dealing with feedback control, the nature of sensory signals is
important, and in the human nervous system, sensory signals come
from interoceptors, exteroceptors and proprioceptors. Interoceptors
are sensitive to internal events and serve to maintain physiological
homeostasisl. Exteroceptors are sensitive to external stimuli and in-
clude vision receptors and the skin mechanoreceptors, which are re-
sponsive to crude touch, discriminative touch, skin deformation, pain,
lWe deal with interoceptors minimally in this book (e.g., in Chapter 6 we note
innervation to cutaneous mechanoreceptors and vasodilators in the blood vessels of
the hand).