Chapter 2. Prehension

6 WHAT IS PREHENSION?
continue in this way, we modify our hand shape and insert the index
fingernail into the screw slot, and rotate the hand, turning the screw in
a way to get the job done adequately. Now, suppose the job were to
paint the wall. We look around for an object to lever open the lid of
the paint can, and perhaps all we see is a screwdriver. With its long
shape to provide an effective lever and a thin end to fit under the lip of
the lid, it will do the job. Then, with the paint can open, the paint
must be stirred. Having misplaced our paint stirrer, we once again
reach for our trusty screwdriver, which in a pinch also suffices for this
activity!
In these activities, the hand shapes differently. While the screw-
driver properties are not changing, the task is different in each case.
The grasp chosen for levering and stirring no longer resembles the
more standard way of grasping the screwdriver. And while the
screwdriver's physical properties exist to support its primary function,
those very properties can be used for other functions. The shape taken
on by the hand for grasping the screwdriver in these other tasks re-
flects the needs of these secondary functions. Importantly, we see that
the hand, as a general purpose tool, can shape itself into a posture
suitable to perform a given function. This can be to grasp an object to
do the function, or it can be to shape itself into a tool to perform that
function directly. In fact, most tools are designed to simplify accom-
plishing the tasks previously done by the naked hands.
How do our brains control our hands? The brain, as a complex
system, can be modelled using theories containing hypothetical expla-
nations, suppositions, and conjectures. If we think in terms of a black
box (see Figure 1.2), we can ask how such a system maps inputs into
outputs. In this case, the question is how the hand is being controlled
in prehensile behavior. The controller is the central nervous system
(CNS) or perhaps even a computational control system. The hand
could be a natural human hand or a dextrous prosthetic or robotic
hand. Our working definition of prehension is the application offinc-
tionally efSective forces by the hand to an object for a task, given nu-
merous constraints'. The inputs to the black box are an object and a
task. The output is prehensile behavior, measured as postures and
forces unfolding over time.
lhehensile behavior can also be accomplished by tails, trunks, tongues, teeth, or
other animal body parts. In this book, we have restricted our focus to the primate
hand.