Chapter 2. Prehension

58 THE PHASES OF PREHENSION
untary but depends on the person’s intent. If one chooses to ignore
the fragility of the object, there will be no response.
The power of conceptual models like Arbib’s, Paillard’s, and
Greene’s is that they provide a summary of what is known and a
starting framework for detailing the planning and control processes.
Their validity can be argued from the fact that they are based on exper-
imental evidence. They rely on perceptual and motor units all working
together, not under one central controller, but under distributed con-
trol. They also represent plausible views which in turn suggest
testable hypotheses about central nervous system (CNS) mechanisms,
planning and control algorithms for such a complex behavior as
reaching and grasping.
Conceptual models are useful as a starting ground for understand-
ing the complex interaction between a performer and the environment.
Sensory information is needed at crucial times for completion of the
motor task. The CNS, however, being highly parallel and redundant,
can solve problems in many ways. It exhibits motor equivalence, in
that motor commands are nonspecific and different spatial- temporal
patterns of action can produce the same outcome. Muscles can vary
their functional roles from movement to movement. It has been
shown that handwriting, for example, is the same regardless of the
writing implement, speed, size, or limb used. Another issue is the
availability of sensory information. If available from a given modal-
ity, it has the potential of being used in the control of the movement; if
not, other modalities of sensory information will be used. If visual in-
formation is available, it will be used, causing movements to be
slower. If not, subjects rely more on touch. Finally, an important is-
sue in studying human motor control is the fact that there is a great
deal of variability in movements. Seemingly identical movements are
not exactly alike. However, these variations occur within a restricted
bandwidth. As a result, one approach to studying motor control is to
examine the variability of movements (Marteniuk & MacKenzie, 1990;
Newel1 & Corcos, 1993; Worringham, 1987,1991).
When our ‘triangular’ modelling strategy was introduced in
Chapter 1 (Figure 1.3), it was noted that existing models suggest fur-
ther experiments, the results of which might suggest more compre-
hensive models. Since Arbib’s conceptual model first appeared in the
literature, experimental evidence has shown that size information not
only affects the grasping component but also the transport component.
Recent experiments indicate that visual and mechanical perturbations
affect the kinematics of the transport component as well as the manipu-
lative or grasping component. This suggests a functional coupling of