154 Touch
both of which should alter vibration; they were also unaf-
fected by either low- or high-frequency vibrotactile adapta-
tion (Lederman, Loomis, & Williams, 1982). Vibratory
coding of roughness does, however, occur with very fine mi-
crotextures. LaMotte and Srinivasan (1991) found that
observers could discriminate a featureless surface from a tex-
ture with height .06–.16 microns and interelement spacing
~100microns.Subjectsreportedattendingtothevibration
from stroking the texture. Moreover, measures of mechanore-
ceptor activity in monkeys passively exposed to the same sur-
faces implicated the FAII (or PC) units, which respond to
relativelyhigh-frequencyvibrations(peakresponse~250Hz;
Johansson & Vallbo, 1983). Vibrotactile adaptation affected
perceived roughness of fine but not coarse surfaces (Hollins,
Bensmaia, & Risner, 1998).
Somewhat surprisingly, the textural scale where spatial
coding of macrotexture changes to vibratory coding of mi-
crotexture appears to be below the limit of tactile spatial res-
olution (.5–1.0 mm). Dorsch, Yoshioka, Hsiao, and Johnson
(2000) reported that SAI activity, which implicates spatial
coding, was correlated with roughness perception over a
range of gratings that began with a .1-mm groove width.
Using particulate textures, Hollins and Risner (2000) found
evidence for a transition between vibratory and spatial cod-
ing at a similar particle size.
Weight
The perception of weight has been of interest for a time
approaching two centuries, since the work of Weber
(1834/1978). Weber pointed out that the impression of an ob-
ject’s heaviness was greater when it was wielded than when it
rested passively on the skin, suggesting that the perception of
weight was not entirely determined by its objective value. In
the late 1800s (Charpentier, 1891; Dresslar, 1894), the dis-
covery of the size-weight illusion—that given equal objec-
tive weight, a smaller object seems heavier—pointed to the
fact that multiple physical factors determine heaviness per-
ception. Recently, Amazeen and Turvey (1996) have inte-
grated a body of work on the size-weight illusion and weight
perception by accounting for perceived weight in terms of re-
sistance to the rotational forces imposed by the limbs as an
object is held and wielded. Their task requires the subject to
wield an object at the end of a rod or handle, precluding vol-
umetric shape cues. Figure 6.3 shows the experimental setup
for a wielding task. Formally, resistance to wielding is de-
fined by an entity called the inertia tensor, a three-by-three
matrix whose elements represent the resistance to rotational
acceleration about the axes of a three-dimensional coordi-
nate system that is imposed on the object around the center
of rotation. Although the inertia tensor will vary with the
coordinate system that is imposed on the object, its eigenval-
ues are invariant. (The eigenvaluesof a matrix are scalars
that, together with a set of eigenvectors—essentially, coordi-
nate axes—can be used to reconstruct it.) They correspond to
the principal moments of inertia: that is, the resistances to ro-
tation about a nonarbitrary coordinate system that uses the
primary axes of the object (those around which the mass is
balanced). In a series of experiments in which the eigenval-
ues were manipulated and the seminal data on the size-weight
illusion were analyzed (Stevens & Rubin, 1970), Amazeen
and Turvey found that heaviness was directly related to the
product of power functions of the eigenvalues (specifically,
the first and third). This finding explains why weight is not
dictated simply by mass alone; the reliance of heaviness per-
ception on resistance to rotation means that it will also be
affected by geometric factors.
But the story is more complicated, it seems, as weight per-
ception is also affected by the material from which an object is
made and the way in which it is gripped. A material-weight re-
lation was documented by Wolfe (1898), who covered objects
of equal mass with different surface materials and found that
objects having surface materials that were more dense were
judged lighter than those with surfaces that were less dense
(e.g., comparing brass to wood). Flanagan and associates
(Flanagan, Wing, Allison, & Spencely, 1995; Flanagan &
Wing, 1997; see also Rinkenauer, Mattes, & Ulrich, 1999)Figure 6.3 Experimental setup for determining the property of
an object by wielding; the subject is adjusting a visible board so
that its distance is the same as the perceived length of the rod. For
weight judgments, the subject assigns a number corresponding to
the impression of weight from wielding. Source:From Turvey
(1996; Figure 2). Copyright © 1996 by the American Psycholog-
ical Association. Reprinted with permission.[Image not available in this electronic edition.]