Chapter 2. Prehension

204 THE PHASES OF PREHENSION
about the state of interaction with the object during the task in order to
ensure grasping and manipulative stability. As an effector, it applies
task appropriate forces for grasping and manipulative stability, using
the muscles (somatically innervated sensorimotor system) in parallel
with the eccrine sweat glands (autonomically innervated sudomotor
system’), given the inherent ‘passive’ structural characteristics of the
hand (e.g., the compliant pads of the fingers and palm). The object
has characteristics relevant for stable grasping which may be assessed
with varying degrees of accuracy through the visual, kinesthetic, or
tactile systems. As well, physical characteristics of the object’s
structure determine the nature of the interaction in stable grasping: for
example, the object and the hand surfaces together determine the
coefficient of friction.
Some object properties are found through direct interaction with
the object, or hapticallyz. Grasping activates complex patterns of
cutaneous, muscle and joint receptors. Gibson (1962) suggested that
the covariance of cutaneous and articular motion provides information
for haptic form perception. For example, Iwamura and Tanaka (1978)
identified types of units in Area 2 of the parietal lobe which did not
respond to ordinary passive cutaneous stimulation of the hand, nor
with manipulation of wrist or finger joints by the experimenter. These
neurons were selectively responsive to properties of the object being
grasped. Gibson (1962) suggested that some movements are
‘exploratory’ and can be distinguished from ‘performatory’ hand
movements.
Prehension is defined as the application of functionally effective
forces by the hand to an object for a task, given numerous constraints.
Key questions are how the hand can effect these forces and what is the
nature of the forces arising in the task. This chapter addresses these
questions by looking at how the hand meets the functional demands of
applying forces to match the anticipated forces in the task, imparting
motion to the object, and gathering sensory information about the state
of the interaction with the object. First, as a key to the interface
between object and hand, the skin and its properties are discussed.
Then touch and active touch are addressed through a discussion of
cutaneous sensors and sensorimotor integration. Force application is
Sudomotor refers to the motor innervation of sweat glands. It is derived from
Latin, sudor, sudoris, meaning sweat.
2Haptics is a perceptual system that uses both cutaneous (including thermal) and
kinesthetic inputs to derive information about objects, their properties, and their
spatial layout (Loomis and Lederman, 1986).