there are fewer afferent neurons in the internal organs and
each has a larger receptive ﬁ eld.
It is fairly easy to see why a stimulus to a neuron that has
a small receptive ﬁ eld can be located more precisely than a stim-
ulus to a neuron with a large receptive ﬁ eld (see Figure 7–6).
However, more subtle mechanisms also exist that allow us to
localize distinct stimuli within the receptive ﬁ eld of a single
neuron. In some cases, receptive ﬁ eld overlap aids stimulus
localization even though, intuitively, overlap would seem to
“muddy” the image. In the next few paragraphs we will exam-
ine how this works.
An afferent neuron responds most vigorously to stimuli
applied at the center of its receptive ﬁ eld because the receptor
density—that is, the number of receptors in a given area—is
greatest there. The response decreases as the stimulus is
moved toward the receptive ﬁ eld periphery. Thus, a stimulus
activates more receptors and generates more action poten-
tials if it occurs at the center of the receptive ﬁ eld (point A in
). The ﬁ ring frequency of the afferent neuron is
also related to stimulus strength, however. Thus, a high fre-
quency of impulses in the single afferent nerve ﬁ ber of Figure
7–7 could mean either that a moderately intense stimulus was
Central nervous system
Lips: Two distinct
points are felt
Back: Only one point
The inﬂ uence of sensory unit size and density on acuity. (a) The information from neuron A indicates the stimulus location more precisely
than does that from neuron B because A’s receptive ﬁ eld is smaller. (b) Two-point discrimination is ﬁ ner on the lips than on the back, due to
the lips’ numerous sensory units with small receptive ﬁ elds.
Make a prediction about the relative size of the brain region devoted to processing lip sensations versus that for the brain region that
processes sensations from your back.
Answer can be found at end of chapter.
Two stimulus points, A and B, in the receptive ﬁ eld of a single
afferent neuron. The density of nerve endings around area A is
greater than around B, so the frequency of action potentials in
response to a stimulus in area A will be greater than the response to
a similar stimulus in B.
ntral nervous system
Stimulus A Stimulus B