Chapter 2. Prehension

64 THE PHASES OF PREHENSION
ence, we have acquired knowledge about how objects can be expected
to behave when we grasp them, a field of research called ‘task me-
chanics’. The term ‘grasp strategy’ refers to selecting appropriate op-
position types, mapping virtual fingers into real, anatomical fingers,
and determining opposition space parameters to achieve a hand con-
figuration and aperture appropriate for the task and object at hand.
Selecting a strategy is quite dependent on a person’s anatomy, emo-
tional state, intentions, fatigue level, motivations, etc. Planning a
hand location and orientation will depend on the grasp strategy
chosen. Before any movement occurs, a sensorimotor set is es-
tablished, ‘tuning’ the spinal circuitry and motoneuron pool, allowing
for gating of sensory information and motor outputs.
4.1 Types of Planning
The nature of plans and programs has been of central debate in
psychology, physiology, movement science and computer science.
Miller, Galanter and Pribram (1960) suggested that plans have time
scales, i.e., life goals, yearly plans, daily plans through to the plans
for imminent movements. As the time frame shrinks, there are differ-
ences in the nature of goals and the levels of analysis for procedures to
achieve those goals. Life plans are abstract, while upcoming pro-
grams of action must be more specific and concrete. In the analysis of
human motor behavior, views on the issue of motor planning are di-
verse and heated. At one end of a continuum, there are those individ-
uals who would deny any central representation for movement (e.g.,
Kugler, Kelso, and Turvey, 1982). Called direct or action theory, this
approach appeals to physical, deterministic, environmental and
dynamical levels of explanation, and denies the relevance of concepts
such as intention, learning and memory. In contrast, there are those
who would posit a central representation or motor program to specify
the details of an upcoming movement (see Keele, Cohen & Ivry,
1990). For reviews of the debates concerning physical and
representational analyses of human movement planning, see Whiting,
Meijer, and van Wieringen (1990); for related discussions on the
interface between the neurosciences and cognitive sciences, refer to
Churchland (1989).
For our purposes in this book, we are concerned with representa-
tion and computation by the CNS or another computational system in
the planning of prehensile movements. Planning is discussed at two
different levels. One is concerned with the movement process (how to
execute the movement, from the perspectives of kinematic analyses of