Localization of Function • 29In addition to recording action potentials from single neurons, Adrian made other
discoveries as well. He also found that each action potential travels all the way down
the axon without changing its size. This property makes action potentials ideal for
sending signals over a distance, because it means that once an action potential is started
at one end of an axon, the signal is still the same size when it reaches the other end.
At about the same time Adrian was recording from single neurons, other research-
ers were showing that when the signals reach the end of the axon, a chemical called
a neurotransmitter is released that makes it possible for the signal to be transmitted
across the synaptic gap that separates the end of the axon from the dendrite or cell body
of another neuron (see Figure 2.4).
Although all of these discoveries about the nature of neurons and the signals that
travel in them were extremely important (and garnered a number of Nobel prizes for
their discoverers), our main interest is not in how axons transmit signals, but in how
these signals contribute to the operation of the mind. So far our description of how
signals are transmitted is analogous to describing how the Internet transmits electrical
signals without describing how the signals are transformed into words and pictures that
people can understand. Adrian was acutely aware that it was important to go beyond
simply describing nerve signals, so he did a series of experiments to relate nerve signals
to stimuli in the environment and therefore to people’s experience.
Adrian studied the relation between nerve fi ring and sensory experience by measur-
ing how the fi ring of a neuron from a receptor in the skin changed as he applied more
pressure to the skin. What he found was that the shape and height of the action poten-
tial remained the same as he increased the pressure, but the rate of nerve fi ring—that
is, the number of action potentials that travel down the axon per second—increased
(● Figure 2.6).
What this means in terms of cognition is that the intensity of a stimulus can be rep-
resented by the rate of nerve fi ring. So, for example, increasing the pressure to the skin
causes neurons in the touch system to fi re more rapidly, and this causes an experience
of increased pressure. Or increasing the intensity of light presented to visual receptors
in the retina causes more rapid fi ring of neurons in the visual system and an increased
perception of brightness. Thus, the rate of neural fi ring is related to the intensity of
stimulation which, in turn, is related to the magnitude of an experience such as feeling
pressure on the skin or experiencing the brightness of a light.
If the amplitude of experience—our perception of a 100-watt light as brighter than
a 40-watt bulb—is related to the rate of nerve fi ring, what about the quality of experi-
ence? For the senses, quality refers to the different experience associated with each of
the senses—perceiving light for vision, sound for hearing, smells for olfaction, and so
on. We can also ask about quality within a particular sense. How do we perceive differ-
ent shapes, different colors, and various directions of movement, for example?
One way to answer the question of how action potentials determine different
qualities is to propose that the action potentials for each quality might look different.
However, Adrian ruled out that possibility by determining that all action potentials are
basically the same.
If all nerve impulses are basically the same whether they are caused by seeing a
red fi re engine or remembering what you did last week, how can these impulses stand
for different qualities? The answer to this question is that neurons serving different
cognitive functions transmit signals to different areas of the brain, a principle called
localization of function.Localization of Function
One of the basic principles of brain organization is localization of function—specifi c
functions are served by specifi c areas of the brain. Most of the cognitive functions
are served by the cerebral cortex, which is a layer of tissue about 3 mm thick that● FIGURE 2.6 Action potentials
recorded from an axon in
response to three levels of
pressure stimulation on the skin:
(a) light; (b) medium; (c) strong.
Increasing stimulus intensity
causes an increase in the rate of
nerve fi ring.Time(a)(b)(c)Copyright 2011 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part.