Chapter 2. Prehension

Chapter 4 - Planning of Prehension 75
riphery is fed back to cortical areas for further planning and com-
mands.
As a simplified view of the CNS, it is a useful model for localizing
some of the functions that task plans must perform. Motivations,
emerging from limbic structures, are converted to goal-directed, sen-
ally-ordered movements, using cortical and subcortical mechanisms.
Cortical areas include the parietal association cortex, involved in the
precise visual guidance of goal-directed arm and hand movements,
and the frontal association cortex, involved in the processing of per-
ceptual cues and memory stores. Subcortically, the basal ganglia,
converging on secondary motor areas (SMA and dorsomedial area 6),
are involved in strategy selection in accordance with goals and the
context for the actions. The role of motor cortex is more part of
movement execution, that of advanced tactile and proprioceptive
adjustment of the fingers and hand.
One way to study planning at the CNS level is to delay the signal
to initiate the movement. In monkey studies where the direction of
intended movement is given, but the GO signal is delayed (Wise,
1985), neurons in the dorsal premotor area show directionally tuned
activity during the delay period. Hocherman and Wise (1991) find
that these responses vary with both target location and degree and in-
tended curvature of the hand path.
Research has shown that CNS processing is different when
processing object property information for object recognition and
naming, compared to manual interactions with the object. For
example, after processing by the visual cortex, the parietal lobes are
involved more extensively in visuomotor processing for reaching and
grasping objects, whereas the temporal lobes are more involved in
processing information for object recognition. (Wise & Desimone,
1988; Goodale, Milner, Jakobson & Carey, 1991).
While neural models are a gross simplification of the brain, they
are useful for making important observations. The brain consists of
billions of neurons, each making thousands of connections on other
neurons. Information is processed to convert motivations into motor
activity through the help of sensory information. Information, goals,
control, and commands are distributed somehow across these arrays
of processors. Yet, studies on the primate brain suggest conflicting
evidence as to the nature of the information: there can be body-space
or world-space reference frames, the language can be kinematic or
dynamic, control can be feedforward or feedback, and commands can
be at the movement or muscle level. There is likely a separation of
neural computation for planning (frontal and parietal association areas,