Chapter 2. Prehension

Chapter 6 - During Contact 281
friction leads to reports that an axe or hammer is likely to fly out of the
hand.
Object characteristics relevant for stable grasping may be assessed
with varying degrees of accuracy through either vision or haptics.
Different sensory modalities, tactile or visual, seem to have access to
different object properties. Many object properties interact in our
perception, thus setting up anticipated values (e.g., large objects are
perceived to be heavy). With respect to task requirements, a
distinction has been made between exploratory movements, with a
goal to extract object property information, and performatory
movements, where sensory information is important to assess the state
of interaction with the object in transportation or manipulation. These
two classes of movements have different temporal frequency domains
and different modes of sensory control.
Physical characteristics of the object’s structure determine the
nature of the interaction in stable grasping: the object and the hand
surfaces together determine the coefficient of friction. A formal
analytic description of the mechanics of stable grasp was provided.
The hand generates appropriate forces along the opposition vector(s)
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.
A crucial variable for control during contact is that the system
being controlled is first the hand, then the hand plus object. The
process of acquiring objects into stable grasp can be broken down
further into subphases. ‘Triggering’ of these subphases seems
critically dependent on making and breaking contacts with the
environment (contact between the hand and object, contact between
the hand plus object with supporting surfaces).
We examined briefly, from human studies and robotics, stable
grasping for transporting objects and the more complex problem of
object manipulation, where an object is in ‘controlled slip’. In
considering object manipulation, we noted that while Elliott and
Connolly (1984) provided symbolic descriptions of some
manipulations, the coordinate frame for defining opposition space in
Chapter 2 was extended to describe such manipulations.