Sensory Aspects of Touch 151BACKTHIGH BELLYSHOULDERUPPER
LIP12 34BREASTCHEEKSOLE CALF05101520253035404550MEAN THRESHOLD (mm)RIGHT SIDE
LEFT SIDEHALLUX
FOREHEADNOSEUPPER
ARMFOREARMTHUMBPALMFINGERSFigure 6.2 The minimal separation between two points needed to perceive them as separate (2-point threshold),
when the points are applied at different sites of the body. Source:From Weinstein (1968), in D. R. Kenshalo,
The Skin Senses, 1968. Courtesy of Charles C. Thomas, Publisher, Ltd., Springfield, Illinois. Reprinted with
permission.Gescheider, Bolanowski, Verrillo, Hall, & Hoffman, 1994;
Verillo, 1993). The rise in the threshold with age has been at-
tributed to the loss of receptors. By this account, the Pacinian
threshold is affected more than are other channels because it
is the only one whose response depends on summation of re-
ceptor outputs over space and time (Gescheider, Edwards,
Lackner, Bolanowski, & Verrillo, 1996). Although the ability
to detect a vibration in the Pacinian range is substantially
affected by age, the difference limen—the change in ampli-
tude needed to produce a discriminable departure from a
baseline value—varies little after the baseline values are ad-
justed for the age-related differences in detection threshold
(i.e., the baselines are equated for magnitude of sensation rel-
ative to threshold; Gescheider et al., 1996).
Cutaneous spatial acuity has also been demonstrated to de-
cline with age. Stevens and Patterson (1995) reported an ap-
proximate 1% increase in threshold per year over the ages of
20 to 80 years for each of four acuity measures. The measures
were thresholds, as follows: minimum separation of a 2-point
stimulus that allows discrimination of its orientation on the
finger (transverse vs. longitudinal), minimum separation be-
tween points that allows detection of gaps in lines or disks,
minimum change in locus that allows discrimination between
successive touches on the same or different skin site, and dif-
ference limen for length of a line stimulus applied to the skin.
The losses in cutaneous sensitivity that have been de-
scribed can have profound consequences for everyday life in
older persons because the mechanoreceptors function criti-
cally in basic processes of grasping and manipulation.Sensory-Guided Grasping and ManipulationPersons who have sustained peripheral nerve injury to their
hands are often clumsy when grasping and manipulating ob-
jects. Such persons will frequently drop the objects; more-
over, when handling dangerous tools (e.g., a knife), they can
cut themselves quite badly. Older adults, whose cutaneous
thresholds are elevated, tend to grip objects more tightly than
is needed in order to manipulate them (Cole, 1991). Experi-
ments have now confirmed what these observations suggest:
Namely, cutaneous information plays a critical role in guid-
ing motor interactions with objects following initial contact.
Motor control is discussed extensively in the chapter written
by Heuer in this volume.