Chapter 2. Prehension

Chapter 6 - During Contact 231
6.2.3 Object properties: Exploratory procedures
Lederman and Klatzky (1987) developed a set of exploratory
procedures that summarize the exploratory uses of hands to extract
salient object features (Figure 6.9). An exploratory procedure (EP) is
a highly stereotypic motion pattern for obtaining information about
some characteristic of an object. Lateral motion between the skin and
object is used to extract texture. The typical features of this motion are
rapidity, repetition, and positioning on a homogeneous portion of the
object surface rather than along an edge. Pressure, involving a normal
force or torque on one part of the object, is used for determining
hardness. Static contact, where the object is supported externally and
the hand rests on the object without molding to it, is used for thermal
sensing. Unsupported holding, or hefting, where the object is lifted
and maintained in the hand and molded to the object, is used for
weight. Enclosing around the object (i.e., static molding of the hand)
provides knowledge about volume and global shape i.e., the coarse
envelope of the object. Contour following, where the hand makes
smooth nonrepetitive movements within a segment of the object's
contour, is used to learn exact shape, ie., the fine, spatial details.
Through a battery of experiments (Klatzky, Lederman, 8z Reed,
1987; Klatzky & Lederman, 1990), Klatzky and Lederman identified
properties of objects to which either the visual system or the haptic
system have access. These include surface properties (texture,
hardness, and temperature) and structural properties (global shape,
exact shape, volume, and weight). Various properties affect the
method in which the hand can apply the forces. For example, shape
and size constraints affect how many fingers can be used and where
they can be placed, shape interacts with weight to constrain where the
fingers must be placed; and hardness interacts with weight to constrain
how many fingers can be used. Hardness also affects how forces can
be applied to the object in order to impart motion, and texture affects
the application of forces, due to frictional considerations. Lederman
and Klatzky argued that humans use EPs to gather sensory
information. They have noted invariant and typical features with
which subjects get the sensory information needed for a specific task.
For example, to gain texture information, lateral motion between the
skin and object is used. The typical features of this motion are
rapidity, repetition, and positioning on a homogeneous portion of the
object surface rather than along an edge. Any kind of movement will
encode texture; therefore, the EPs of enclosing and contour following
will also provide textural information (see Table 6.2). To test weight,