applied to the center at A or that a strong stimulus was applied
to the periphery at B. Thus, neither the intensity nor the loca-
tion of the stimulus can be detected precisely with a single
Since the receptor endings of different afferent neurons
overlap, however, a stimulus will trigger activity in more than
one sensory unit. In
, neurons A and C, stimu-
lated near the edges of their receptive ﬁ elds where the receptor
density is low, ﬁ re action potentials less frequently than does
neuron B, stimulated at the center of its receptive ﬁ eld. A high
action potential frequency in neuron B occurring simultane-
ously with lower frequencies in A and C provides the brain
Central nervous system
Action potential frequency
A stimulus point falls within the overlapping receptive
ﬁ elds of three afferent neurons. Note the difference in
receptor response (i.e., the action potential frequency
in the three neurons) due to the difference in receptor
distribution under the stimulus (fewer receptor endings
for A and C than for B).
in postsynaptic cell
in afferent neuron
Afferent pathways showing lateral inhibition. Three sensory units have
overlapping receptive ﬁ elds. Because the central ﬁ ber B at the beginning of
the pathway (bottom of ﬁ
gure) is ﬁ ring at the highest frequency, it inhibits
the lateral neurons (via inhibitory interneurons) to a greater extent than the
lateral neurons inhibit the central pathway.
with a more accurate localization of the stimulus near the cen-
ter of neuron B’s receptive ﬁ
eld. Once this location is known,
the brain can use the ﬁ ring frequency of neuron B to deter-
mine stimulus intensity.
The phenomenon of
is the most important
mechanism enabling the localization of a stimulus site. In lateral
inhibition, information from afferent neurons whose receptors
are at the edge of a stimulus is strongly inhibited compared to
information from the stimulus’s center.
neuronal arrangement that accomplishes lateral inhibition.
The afferent neuron in the center (B) has a higher initial ﬁ r-
ing frequency than the neurons on either side (A and C).
number of action potentials transmitted in the lateral pathways
is further decreased by inhibitory inputs from inhibitory inter-
neurons stimulated by the central neuron. While the lateral
afferent neurons (A and C) also exert inhibition on the central
pathway, their lower initial ﬁ ring frequency has less of an effect.
Thus, lateral inhibition enhances the
center and periphery of a stimulated region, thereby increasing
the brain’s ability to localize a sensory input. Lateral inhibi-
tion can occur at different levels in the sensory pathways but
typically happens at an early stage.
Lateral inhibition can be demonstrated by pressing the
tip of a pencil against your ﬁ nger. With your eyes closed, you
can localize the pencil point precisely, even though the region