Chapter 2. Prehension

118 THE PHASES OF PREHENSION
5.3 Control of Arm Movements
As noted earlier in this chapter, simple movements not involving
contact with the environment may be distinct from more complex
movements such as grasping, which involve configuration of the hand
and complex interactions with the environment. In this section,
control of the arm is looked at, comparing pointing movements with
aiming movements making contact. Various computational models for
transforming goal locations into motor commands are examined.
5.3.1 Spatial transformations
Behavioral studies provide some evidence for the nature of
transformations in biological systems. Development of visuomotor
control requires experience with both movement and vision together.
Monkeys raised without seeing their own motions are unable to
perform accurate reaching movements (Hein, 1974; Held & Bauer,
1967, 1974). They are unable to map between visual space and motor
coordinates for limb control. Recalling the Kuperstein (1988) model
discussed in Chapter 4, integrating visuomotor control of head and eye
movement with proprioceptive control of limb movement requires the
specific experience of viewing the moving hand6. Jeannerod (1988,
1989) studied the mapping between the extrinsic coordinates computed
for target position (the ‘visual map’) and the position of the moving
limb with respect to the target (the ‘proprioceptive map’). He
suggested that there must be a comparison of target position relative to
the body and the moving limb position relative to the target. An
important mechanism for superimposing the visual and proprioceptive
maps is the process of foveation, whereby the gaze axis is aligned
with the position of the target in space. Provided the hand is also
visible at the same time, foveation will align the two maps. A
complementary possibility is that ‘tactile foveation’ will also align the
visual and proprioceptive maps. A key brain area involved in this
superposition of visual and proprioceptive maps for accurate reaching
is the posterior parietal cortex. Visually directed hand movements are
impaired in human patients with lesions to this area; as well, they may
have a variety of other spatial processing deficits and problems with
oculomotor control to visual targets (Perenin & Vighetto, 1988;
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6Held and Bauer (1974) noted that in addition to the reaching deficits, these animal
reared without vision of the moving hand also showed deficits in the tactile
guidance of grasp, with fumbling movements once contact is made.