Consciousness, the Brain, and Behavior
249
Limbic System Dysfunction
The case of H.M. illustrates that formation of declarative
memories requires limbic structures of the temporal lobe.
In
1953, a young man known as H. M. underwent bilateral
removal of the amygdala and large parts of the hippocampus
as a treatment for persistent, untreatable epilepsy. Although
his epileptic condition improved after this surgery, he was
now affl icted with anterograde amnesia. He still had a
normal IQ and a normal working memory. He could retain
information for minutes as long as he was not distracted;
however, he could not form long-term memories. If he was
introduced to someone on one day, on the next day he did
not recall having previously met that person. Nor could he
remember any events that occurred after his surgery, although
his memory for events prior to the surgery was intact.
Interestingly, H. M. had normal procedural memory, and
could learn new puzzles and motor tasks as readily as normal
individuals. This case was the fi rst to draw attention to the
ADDITIONAL CLINICAL EXAMPLES
critical importance of the temporal lobe in the formation of
long-term declarative memories and to suggest that structures
in this region are necessary for the conversion of short-term
into long-term memories. Additional cases now demonstrate
that the hippocampus is the primary structure involved in this
process. Because H. M. retained memories from before the
surgery, his case showed that the hippocampus is not involved
in the
storage
of declarative memories.
The case of S. M. illustrates that the amygdala processes
fearful emotions.
The patient in this study suffered from a rare
disease (
Urbach-Wiethe
disease) in which the anterior and
medial portions of the temporal lobe atrophied, essentially
destroying the amygdala bilaterally. Intelligence and memory
formation remained intact. However, this individual now
lacked the ability to express fear in appropriate situations
and could not recognize fearful expressions in other people.
Therefore, in humans the amygdala is important for at least
one emotion—fear.
identify an object such as a ball in their left or right hand
behind a barrier that prevented them from seeing the object.
Subjects who held the ball in their right hand, were able to say
that it was a ball, but persons who held the ball in their left
hand were unable to name it. Because the processing of sen-
sory information occurs on the side of the brain opposite to
the sensation, this result demonstrated conclusively that the
left hemisphere contains a language center that is not present
in the right hemisphere. These striking results provide the
strongest evidence that at least some of the functions of the
two hemispheres are distinct.
Although language skills emerge spontaneously in all
normal children in all societies, there is a critical period during
childhood when exposure to language is necessary for these
skills to develop, just as the ability to see depends upon effec-
tive visual input early in life. The critical period is thought to
end at puberty or earlier. The dramatic change at puberty in
the possibility of learning language, or the ease of learning
a second language, occurs as the brain attains its structural,
biochemical, and functional maturity at that time.
As part of these basic language skills, the left hemi-
sphere contains the “rules” for general grammatical prin-
ciples; thus, it is much more skilled at changing verb tenses
and constructing possessives than is the right hemisphere. It
is also dominant for language usages that occur in sequences
over time, such as speaking the fi rst part of a word before
the last. In addition, the left hemisphere seems to constantly
form theories about how the world works, to fi nd relation-
ships between events, and to assess where one stands in rela-
tion to the world. The left hemisphere has been called “the
interpreter.”
The right hemisphere, on the other hand, more typically
handles sensory information in basic ways, such as the percep-
tion of faces and other three-dimensional objects.
Memories are handled differently in the two hemispheres,
too. Verbal memories are more apt to be associated with the
left hemisphere, and nonverbal memories (e.g., visual patterns
or nonverbal memories that convey emotions) with the right.
Even the emotional responses of the two hemispheres seem to
be different; for example, the left hemisphere has more ability
to understand the emotional states of oneself or others. When
electroconvulsive therapy is administered in the treatment of
depression, however, better effects are often obtained when the
electrodes are placed over the right hemisphere. The two sides
of the brain also differ in their sensitivity to psychoactive drugs.
Conclusion
Mental tasks are the result of synchronized activity in vast
neuronal networks made up of many functional regions of the
cerebral cortex, subcortical nuclei, and brainstem. Also impor-
tant are the pathways that reciprocally connect these sites and
orchestrate their performance during specifi c tasks. In fact, the
nervous system is so abundantly interconnected that it is dif-
fi cult to know where any particular subsystem begins or ends.
Material in this chapter has often been highly qualifi ed
on a molecular level, largely because the information simply is
not known. For example, the scientifi
c literature is full of state-
ments such as, “The study of the neural basis of language is in
fl ux,” and “The understanding of learning is one of nature’s
most closely guarded secrets.” We also caution the reader that
listing brain sites that show increased activity during substance
dependence, sleep, language, or learning does little to explain
how or why these phenomena occur. For the topics discussed
in this chapter, the answer to the question “How?” which
makes up the stuff of physiology, is often simply, “We don’t
yet know.” However, this leaves us with the exciting prospect
of future discoveries on the new frontier of the brain.
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