Chapter 2. Prehension

Chapter 9 - Reevaluation and Future Directions 33 1
spatial properties of objects in an egocentric body space. Extrinsic
object properties, perceived visually, include orientation with respect
to the body (Chan et al., 1990; Jeannerod & Decety, 1990), distance
(Sivak & MacKenzie, 1992), and direction (Paulignan, MacKenzie,
Marteniuk & Jeannerod, 1991). Neurally, Georgopoulos et al. (1988)
showed that direction is encoded by a population vector in the motor
cortex.
Intrinsic object properties are the physical identity constituents of
objects, and include structural properties, such as shape, size,
distribution of mass, and weight, and also surface properties, such as
texture, temperature, and hardness. Intinsic properties affect the
selection of a grasp posture, as was observed in the discussion on
grasp taxonomies in Chapter 2. For example, the shape and size
constrains the type of opposition used, how many fingers can be used
and where they can be placed on an object.
Intrinsic object properties are perceived primarily through vision
or haptics, though some may be inferred from audition. During the
planning phase, only visually perceived object properties are available,
as noted in Chapter 4. These include the visual perception of object
shape (Jeannerod, 1984; Klatzky & Lederman, 1987), volume or size
(Klatzky & Lederman, 1987), and surface spatial density (Klatzky &
Lederman, 1987). Humans seem to infer the location of the center of
mass (Mason, 1986). After contact with the object, object properties
can be perceived haptically, as discussed in Chapter 6. Using
exploratory procedures, the human hand can extract surface
roughness, temperature, and weight (Klatzky & Lederman, 1987).
Using wielding, length, weight, moment of inertia and center of mass
can be perceived (Hoisington, 1920; Solomon, Turvey, & Burton,
1989). Using holding and jiggling, weight can be perceived (Brodie &
Ross, 1985; Victor Raj, Ingty & Devanandan, 1985). Neurally,
Iwamura and Tanaka (1978) showed somatsensory cortical responses
to differently shaped objects.
9.1.2 Tasks
Grasping occurs within the constraints of some task, e.g.,
grasping a screwdriver to turn a screw, grasping a screwdriver to lever
open a can of paint. The functional requirements can be summarized as
follows:
a) apply forces to match the anticipated forces in the task (stable
grasp)