Human Physiology, 14th edition (2016)

Sensory Physiology 273

neurons whose receptive fields are stimulated most strongly

inhibit—via interneurons that pass “laterally” within the

CNS—sensory neurons that serve neighboring receptive fields.

Lateral inhibition is a common theme in sensory physi-

ology, though the mechanisms involved are different for each

sense. In hearing, lateral inhibition helps to more sharply

tune the ability of the brain to distinguish sounds of different

pitches. In vision, it helps the brain to more sharply distinguish

borders of light and darkness; and in olfaction, it helps the

brain to more clearly distinguish closely related odors.

different receptive field and a different sensory neuron—two
separate points of touch will thus be felt. If the distance is suf-
ficiently small, both points will touch the receptive field of
only one sensory neuron, and only one point of touch will be
felt ( fig. 10.5 ).
The two-point touch threshold, which is the minimum
distance at which two points of touch can be perceived as
separate, is a measure of the distance between receptive fields.
If the distance between the two points of the calipers is less
than this minimum distance, only one “blurred” point of touch
can be felt. The two-point touch threshold is thus an indica-
tion of tactile acuity ( acus 5  needle), or the sharpness of touch
perception.
The tactile acuity of the fingertips is exploited in the read-
ing of braille. Braille symbols are formed by raised dots on
the page that are separated from each other by 2.5 mm, which
is slightly greater than the two-point touch threshold in the
fingertips ( table  10.3 ). Experienced braille readers can scan
words at about the same speed that a sighted person can read
aloud—a rate of about 100 words per minute.

Lateral Inhibition

When a blunt object touches the skin, a number of receptive
fields are stimulated—some more than others. The receptive
fields in the center areas where the touch is strongest will be
stimulated more than those in the neighboring fields where the
touch is lighter. Stimulation will gradually diminish from the
point of greatest contact, without a clear, sharp boundary. What
we perceive, however, is not the fuzzy sensation that might be
predicted. Instead, only a single touch with well-defined bor-
ders is felt. This sharpening of sensation is due to a process
called lateral inhibition ( fig. 10.6 ).
Lateral inhibition and the resultant sharpening of sensa-
tion occur within the central nervous system. Those sensory

Figure 10.5 The two-point touch threshold test. If each point touches the receptive fields of different sensory neurons, two
separate points of touch will be felt. If both caliper points touch the receptive field of one sensory neuron, only one point of touch will
be felt.

Perception

of one point

of touch

Sensory

neuron

Sensory

neurons

Skin surface

Perception

of two points

of touch

Body Region

Two-Point Touch

Threshold (mm)

Big toe 10

Sole of foot 22

Calf 48

Thigh 46

Back 42

Abdomen 36

Upper arm 47

Forehead 18

Palm of hand 13

Thumb 3

First finger 2

Table 10.3 | The Two-Point Touch

Threshold for Different Regions of the Body

Source: From S. Weinstein and D.R. Kenshalo, editors, The Skin Senses,

© 1968. Courtesy of Charles C. Thomas, Publisher, Ltd., Springfield, Illinois.