Chapter 2. Prehension

316 CONSTRAINTS AND PHASES
level fine adjustments. These data suggest physiological limitations on
prehensile movements, specifically on the underlying sensory and
motor pathways.
Other types of damage to different neural substrates constrains
available grasping behaviors. It has been shown that cutting the
pyramidal tract, which is a direct pathway between various cortical
areas and the motoneurons in the spinal cord, results in the loss of
fractionated finger movements (Lawrence & Kuypers, 1968a,b).
Primates can still perform collective hand grasping movements
because less direct pathways are still intact, but are unable to oppose
thumb and fingers. One other aspect of the nervous system lies in the
issue of sensorimotor representations (c.f. Iwamura et al., 1981).
While the motor and somatosensory cortical representation for the
hand is quite large, the type of processing is still not known.
In summary, human prehensile behavior is further constrained by
anatomical and physiological aspects of the body and nervous system,
just as robots and their computer controllers are constrained by their
construction and interfaces. However, without a ‘reason’ for the
movement, which in itself can constrain the formation and modulation
of the movement, there would be no need for this complex machinery.
Therefore, higher level goals are discussed in the next subsection.
8.3 High level constraints
At the highest level, four classes of constraints in living systems
can be grouped together. These constraints are imposed for a variety
of reasons. The importance is that there are probably large individual
differences. They are labeled here as follows: semiotic/informational,
motivational, social/cultural, and functionalhntentional.
Semiotic, symbolic, or emotional information may be conveyed in
the way an object is grasped (Nespoulous & Lecours, 1986). For
example, one might hold a cigarette in a suggestive manner. Anxiety
or anger may be communicated by gripping an object tightly.
Motivations, such as needs being satiated, may add an urgency to the
movement. As well, sociocultural constraints create boundaries on
acceptable behavior. Refined behavioral interactions with objects in
one culture might be taboo in another. For example, sticking out the
little finger as one grasps a teacup may be a sign of good upbringing in
a culture (see Section 8.2 for a possible anatomical reason). On the
other hand, sticking out one’s elbow while reaching might be an
invasion of someone else’s extrapersonal space.