Handbook of Psychology, Volume 4: Experimental Psychology

156 Touch

contact position contact position

position origin

local

attitude

base-to-peak

height

difference

radius of curvature 

1/curvature

Figure 6.4 Definition of three different measures of curvature detectable

from touch (Pont et al., 1999; Figure 5, top)—the height difference, the at-

titude of the fingers, and the radius of curvature. The circles represent three

fingers touching a curved surface. Reprinted with permission.

other fingers by some height. The second is the difference in
the angles at which the two outer fingers lie: These fingers’
contact points have tangent lines tilted toward one another,
with the difference in their slopes constituting an attitude
difference, so to speak. In addition, the semicircle has some
objective curvature. All three parameters will change as
the semicircle’s radius changes size. For example, as the ra-
dius increases and the surface gets flatter, the curvature will
decrease, the difference in height between the middle and
outer fingers will decrease, and the attitudes of the outer
fingers approach the horizontal from opposing directions,
maximizing the attitude difference. The question is, which of
these parameters—height difference, attitude difference, or
curvature—determines the discriminability between edges
of different curvature? Pont et al. concluded that subjects
compared the difference in attitudes between surfaces and
used that difference to discriminate them. That is, for each
surface, subjects considered the difference in the slope at the
outer points of contact. For example, this model predicts that
as the outer fingers are placed further apart along a semicir-
cular edge of some radius, the value of the radius at which
there is a threshold level of curvature (i.e., where a curved
surface can just be discriminated from a flat one) will in-
crease. As the fingers move farther apart, only by increasing
the radius of the semicircle can the attitude difference be-
tween them be maintained.
As we report in the following section, when a stimulus has
an extended contour, moving the fingers along its edge is the
only way to extract its shape; static contact does not suffice.
For simple curves, at least, it appears that this is not the case,
and static and dynamic curvature detection is similar. Pont
(1997) reported that when subjects felt a curved edge by
moving their index finger along it, from one end to the other
of a window of exposure, the results were similar to those

with static touch. She again concluded that it was the differ-

ence in local attitudes, the changing local gradients touched

by the finger as it moved along the exposed edge, that were

used for discrimination. A similar conclusion was reached by

Pont, Kappers, and Koenderink (1999) in a more extended

comparison of static and dynamic touch. It should be noted

that the nature of dynamic exploration of the stimulus was

highly constrained in these tasks, and that the manner in

which a curved surface is touched may affect the resulting

percept (Davidson, 1972; Davidson & Whitson, 1974). We

now turn to the general topic of how manual exploration af-

fects the extraction of the properties of objects through haptic

perception.

Role of Manual Exploration in Perceiving

Object Properties

The sensory receptors under the skin, and in muscles, ten-

dons, and joints, become activated not only through contact

with an object but through movement. Lederman and Klatzky

(1987) noted the stereotypy with which objects are explored

when people seek information about particular object proper-

ties. For example, when people seek to know which of two

objects is rougher, they typically rub their fingers along the

objects’ surfaces. Lederman and Klatzky called such an action

an “exploratory procedure,” by which they meant a stereo-

typed pattern of action associated with an object property.

The principal set of exploratory procedures they described

is as follows (see Figure 6.5):

Lateral motion—associated with texture encoding; char-

acterized by production of shearing forces between skin

and object.

LATERAL MOTION/

TEXTURE

UNSUPPORTED

HOLDING/

WEIGHT

ENCLOSURE /

GLOBAL SHAPE,

VOLUME

STATIC CONTACT/

TEMPERATURE

PRESSURE/

HARDNESS

CONTOUR FOLLOWING/

GLOBAL SHAPE,

EXACT SHAPE

Figure 6.5 Exploratory procedures described by Lederman and

Klatzky (1987; Figure 1; adapted) and the object properties with which

each is associated. Reprinted with permission.