Chapter 2. Prehension

76 THE PHASES OF PREHENSION
and basal ganglia) from execution (motor cortex and cerebellum).
With this in mind, we turn to study the processing that is necessary
for the planning of prehensile behaviors.
4.3 Perceiving Object Properties
Intrinsic object properties are the physical identity constituents of
objects, such as size, weight and shape. Extrinsic object properties
are spatial properties of objects in an egocentric body space, such as
distance, orientation with respect to the body, and, if in motion,
direction and velocity of the object. In this section, visual information
during the planning of prehension is discussed.
4.3.1 Perceiving intrinsic object properties
Objects have properties that are intrinsic to their design. These can
include structural properties, such as shape, size, distribution of mass,
and weight, and also surface properties, such as texture, temperature,
and hardness. Intrinsic properties affect the selection of a grasp pos-
ture, 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 can be perceived
primarily through vision or haptics. During the planning phase, only
visually perceived object properties are available. After contact with
the object, object properties can be perceived haptically, and this is
addressed in Chapter 6.
Some intrinsic object properties in particular are accessible to the
vision system. Klatzky and Lederman (Klatzky & Lederman, 1987;
Klatzy, Lederman, & Reed 1987; Lederman & Klatzky, 1987) have
determined that spatial density (a surface property and aspect of
texture), volume (or size), and shape (both global and exact shape) are
accessible to the visual system. From this, assumptions about other
object characteristics are made as well. For example, weight tends to
covary with size (large objects tend to be heavier).
Research has shown that humans can judge object size; i.e., they
can relate visually perceived object size to how wide the fingers must
be open in order to grasp the object. Jeannerod and Decety (1990)
asked six subjects to separate their index finger and thumb to an
aperture that matched the diameter of a seen target object, ranging in
size from 1.4 to 8.4 cm. Subjects could not see their own hand. The
tips of the thumb and finger were videotaped and the grip size