Chapter 2. Prehension

Chapter 1 - Introduction 11
how can hand movements be quantified so that we can explore how
the CNS might be controlling it? Should the movement of each indi-
vidual joint be studied? But then, what happens in an injured hand
when fingers are lost? Does the CNS change? Is the brain of the per-
son born with fewer than five fingers fundamentally different from the
person with a more usual hand?
This book provides a framework and comprehensive study of the
functionality of human prehension. From our earlier discussion about
hand and tool use, several key points are identified that will be the fo-
cus for this text. Analyzing how the hand interacts with various ob-
jects, we see that the hand adopts a wide variety of task-specific pos-
tures for applying forces. In the next chapter, we summarize ways
that researchers in a variety of fields have looked at prehensile behav-
iors. These include classifications from anthropology, hand surgery,
hand rehabilitation, robotics, and developmental psychology. This is
a useful starting ground for analyzing prehension. More important,
however, is to find a way to go beyond classification toward quantifi-
cation in order to seek out the variables that the CNS is controlling in
prehension.
Part I1 of the book has to do with serial order. When reaching to
grasp an object, the hand and arm move in an unrestrained fashion to
get the hand to the object. Then contact is made with the object. This
involves a sequence in which first an unrestrained movement occurs,
then a guarded one, and finally a compliant movement occurs as the
hand grasps the object. These steps are identified as distinct phases
during prehension. In Chapters 3-7, prehension is analyzed as it un-
folds in time. Each phase is examined individually to identify what
makes it distinct from previous phases. Here existing evidence for
movement characteristics and the sensorimotor integration that
uniquely defines each phase is considered in detail. This analysis
comes from the behavioral, biomechanical, neural, and robotics litera-
ture. Experimental results are used to examine object properties and
task requirements, addressing issues such as the effect of context and
intention, and the transitions between the phases. Chapter 7 summa-
rizes the evidence from experiments and ideas from computational
models into a new conceptual model of the phases of prehension. We
identify hypotheses for empirical and computational studies to help
validate and extend the model. From a research perspective, computa-
tional modelling and empirical studies are closely intertwined and can
be used to validate and further develop the conceptual models.
Part I11 of the book looks at the constraints over the serial order.
A major challenge in developing a comprehensive model is that