Experiment Module: Identifying the Brain Structures
Involved in Conditioned Fear Conditioned fear is a phenomenon that
can be easily induced in the laboratory and that has been used to determine what
neural circuits are involved in fear behaviours. Conditioned fear is the
process by which a stimulus with no particular significance becomes, by association,
a sign of imminent danger. For example, if your neighbour’s dog has already bitten
you once, you are probably going to be afraid whenever you have to go into your
neighbour’s yard. For you, your neighbour’s trees and buildings have simply become
associated with the painful bite of his dog. In the laboratory, the most
common conditioned-fear experiments involve placing a rat in a cage where small
electrical shocks can be administered to the rat’s feet through the floor. The
researchers place the rat in the cage, subject it to a particular sound, then
immediately administer a slight shock to its feet. After only a few repetitions
of this procedure, the rat associates the sound with the shock and becomes afraid
when it simply hears the sound. For the rat, the neutral stimulus of the sound
has become a conditioned fear. Not only can conditioned fear be induced
very quickly, but it has the added advantage of lasting a very long time. It thus
represents a very strong association that is unlikely to be forgotten and that
scientists can test without worrying about how long ago it was learned. Another
characteristic of conditioned fear is that scientists have been able to induce
it in just about any species, ranging from flies and molluscs to fish and monkeys.
Conditioned fear thus seems to have been preserved by evolution and to play a
fundamental role in the survival of species. But the two most interesting
things about conditioned fear are: (1) that the sensory system through which the
conditioning is applied often is well understood and can be used as a starting
point to trace the pathways involved in the brain; and (2) that the behavioural
response of fear involves several well known, measurable, physiological changes,
such as accelerated heart rate and elevated levels of stress hormones in the blood. The
question that researchers want to answer is, how does a new acoustic stimulus
that has no particular intrinsic meaning manage to trigger a defensive behavioural
response as the result of a conditioned fear? To answer this question, researchers
start with two solid pieces of information. They know where this phenomenon begins
(in the auditory system and its connections to the brain), and they know where
it ends (with the defensive behavioural response and its physiological manifestations).
All the researchers have to do is find the connections between the inputs and
the outputs. One researcher who helped to revive the scientific study of
emotions through his experiments on conditioned fear was Joseph LeDoux of New
York University. In a first set of experiments with laboratory rats, LeDoux investigated
what parts of the auditory system were essential for establishing conditioned
fear. First he made lesions in the auditory cortex of a number of rats (the part
of the brain where sounds are identified). But he found that he could still induce
conditioned fear in these rats. Next, he removed the auditory thalamus
(the structure just before the cortex) from the auditory systems of another set
of rats. Unlike those rats whose auditory cortex had been removed, these rats
could no longer acquire a conditioned fear. Lesions made in the underlying structure,
the midbrain, had the same effect. It was thus apparent that in conditioned
fear, the auditory stimulus had to reach the thalamus. But where did the stimulus
go after that to trigger the emotional reaction of fear? The neuroanatomy manuals
of the day showed the auditory cortex as the only output pathway for the auditory
thalamus. To answer this question, LeDoux and his colleagues began injecting
their animals with tracer substances that were absorbed by the neurons, thereby
allowing the pathways followed by their axons to be observed and traced. Using
this method, the researchers identified four subcortical regions that receive
axons from the auditory thalamus. In a series of experiments, each of these
pathways was then destroyed in turn. The researchers thereby discovered that for
a conditioned fear to be established, only one of these pathways was needed: the
one leading to the amygdala. The discovery of this new sub-cortical pathway
explained why the lesions in the auditory cortex, though they did prevent the
animals from identifying a sound precisely, did not prevent them from acquiring
a conditioned fear. The neural pathway from the thalamus to the amygdala simply
bypassed the cortex. At about the same time, another researcher, Bruce
Kapp, had discovered the central nucleus, a subregion of the amygdala with connections
to regions of the brainstem that are involved in controlling heart rate and various
responses of the vegetative nervous system. Kapp therefore proposed that
it was these brainstem regions that received the outputs from the amygdala and
enabled the numerous bodily responses involved in conditioned fear. Soon afterward,
a study of lesions in the central nucleus showed that Kapp’s intuition had been
correct: destroying the central nucleus interfered dramatically with the behavioural
response of conditioned fear. Thus, through the experimental methods of
lesion and tracing, scientists were able to identify the basic “wiring” and the
main structures in the brain that were involved in conditioned fear reactions.
Subsequent studies showed that the amygdala plays a central role in the emotion
of fear, whatever the nature of the sensory stimulus that triggers it. |