150 Touch
in comparison to cutaneous mechanoreceptors. For further
details on kinesthesis, see reviews by Clark and Horch (1986)
and by Jones (1999).
Pathways to Cortex and Major Cortical Areas
Peripheral units in the skin and muscles congregate into sin-
gle nerve trunks at each vertebral level as they are about to
enter the spinal cord. At each level, their cell bodies cluster
together in the dorsal root ganglion. These ganglia form
chains along either side of the spinal cord. The proximal ends
of the peripheral units enter the dorsal horn of the spinal cord,
where they form two major ascending pathways: the dorsal
column-medial lemniscal system and the anterolateral sys-
tem. The dorsal column-medial lemniscal system carries in-
formation about tactile sensation and limb kinesthesis. Of the
two systems, it conducts more rapidly because it ascends di-
rectly to the cortex with few synapses. The anterolateral sys-
tem carries information about temperature and pain—and to
a considerably lesser extent, touch. This route is slower than
the dorsal column-medial lemniscal system because it in-
volves many synapses between the periphery and the cortex.
The two pathways remain segregated until they converge at
the thalamus, although even there the separation is preserved.
The primary cortical receiving area for the somatic senses,
S-I, lies in the postcentral gyrus and in the depths of the central
sulcus. It consists of four functional areas, which when ordered
from the central sulcus back to the posterior parietal lobe, are
known as Brodmann’s areas 3a, 3b, 1, and 2. Lateral and some-
what posterior to S-I is S-II, the secondary somatic sensory
cortex, which lies in the upper bank of the lateral sulcus. S-II
receives its main inputs from S-I. The posterior parietal lobe
(Brodmann’s areas 5 and 7) also receives somatic inputs. It
serves higher-level associative functions, such as relating
sensory and motor processing, and integrating the various so-
matic inputs (for further details, see Kandel, Schwartz, &
Jessell, 1991).
SENSORY ASPECTS OF TOUCH
Cutaneous Sensitivity and Resolution
Tests of absolute and relative sensitivity to applied force
describe people’s threshold responses to intensive aspects of
mechanical deformation (e.g., the depth of penetration of a
probe into the skin). In addition, sensation magnitude has
been scaled as a function of stimulus amplitude, in order
to reveal the relation between perceptual response and stimu-
lus variables at suprathreshold levels. Corresponding psy-
chophysical experiments have been performed to determine
sensitivity to warmth and cold, and to pain. A review chapter
by Sherrick and Cholewiak (1986) has described basic find-
ings in this area in detail (see also Rollman, 1991; Stevens,
1991).
The spatial resolving capacity of the skin has been mea-
sured in a variety of ways, including the classical two-point
discrimination method, in which the threshold for perceiv-
ing two punctate stimuli as a single point is determined.
However, Johnson and Phillips (1981; see also Craig &
Johnson, 2000; Loomis, 1979) have argued persuasively that
grating orientation discrimination provides a more stable and
valid assessment of the human capacity for cutaneous spatial
resolution. Using spatial gratings, the spatial acuity of the
skin has been found to be about 1 mm.
The temporal resolving capacity of the skin has been eval-
uated with a number of different methods (see Sherrick &
Cholewiak, 1986). For example, it has been assessed in terms
of sensitivity to vibratory frequency. Experiments have
shown that human adults are able to detect vibrations up to
about 700 Hz, which suggests that they can resolve temporal
intervals as small as about 1.4 ms (e.g., Verrillo, 1963). A
more conservative estimate (5.5 ms) was obtained when de-
termining the minimum separation time between two 1-ms
pulse stimuli that is required for an observer to perceive them
as successive.
Overall, the experimental data suggest that the hand is
poorer than the eye and better than the ear in resolving fine
spatial details. On the other hand, it has proven to be better
than the eye and poorer than the ear in resolving fine tempo-
ral details.Effects of Body Site and Age on Cutaneous ThresholdsIt has long been known that the sensitivity, acuity, and mag-
nitude of tactile and thermal sensations can vary quite sub-
stantially as a function of the body locus of stimulation (for
details, see van Boven & Johnson, 1994; Stevens, 1991;
Weinstein, 1968; Wilska, 1954). For example, the face (i.e.,
upper lip, cheek, and nose) is best able to detect a low-level
force, whereas the fingers are most efficient at processing
spatial information. The two-point threshold is shown for
various body sites in Figure 6.2.
More recently, researchers have addressed the effect of
chronological age on cutaneous thresholds (for details, see
Verrillo, 1993). One approach to studying aging effects is to
examine the vibratory threshold (the skin displacement at
which a vibration becomes detectable) as a function of age. A
number of studies converge to indicate that aging particularly
affects thresholds for vibrations in the range detected by the
Pacinian corpuscles (i.e, at frequencies above 40 Hz; see