404
UNIT THREE
Another area of the temporal lobe, the amygdala, assigns
value to a memory, such as whether it was pleasant. Clinical
Application 11.4 discusses some common causes of damage
to the cerebrum.
Unusual behaviors and skills of people who have damaged the hip-
pocampus have taught researchers much about this intriguing part
of the brain. In 1953, a surgeon removed parts of the hippocampus
and the amygdala of a young man called H. M., to relieve his severe
epilepsy. His seizures became less frequent, but H. M. suF
ered a pro-
found loss in the ability to consolidate short-term memories into
long-term ones. As a result, events in H. M.’s life faded from memory
as quickly as they occurred. He was unable to recall any events that
took place since surgery, living always as if it was the 1950s. He would
read the same magazine article repeatedly with renewed interest
each time.
Basal Nuclei
The
basal nuclei
(basal ganglia) are masses of gray matter
deep within the cerebral hemispheres. They are called the
cau-
date nucleus,
the
putamen,
and the
globus pallidus,
and they
develop from the anterior portion of the forebrain
(f g. 11.19)
.
The basal nuclei produce the inhibitory neurotransmitter
dopamine.
The neurons of the basal nuclei interact with
other brain areas, including the motor cortex, thalamus, and
cerebellum. These interactions, through a combination of
stimulation and inhibition, facilitate voluntary movement.
Clinical Application 11.5 discusses Parkinson disease, in
which neurons in the basal nuclei degenerate.
The functioning of the basal nuclei may be very speciF
c,
such as controlling the movements necessary to speak. A
family in London with many members who have unintel-
ligible speech led to the discovery of a single gene,
FoxP2,
that controls the ability to combine words into meaningful
speech. The gene, which controls several other genes, also
enables a person to understand and use grammar. Songbirds
have the gene, too. Impairing
FoxP2
function in birds pre-
vents them from learning their songs. In both humans and
songbirds,
FoxP2
acts on a speciF
c part of the basal nuclei
called “area X.”
PRACTICE
21
What is hemisphere dominance?
22
What are the functions of the nondominant hemisphere?
23
Distinguish between short-term and long-term memory.
24
What is the function of the basal nuclei?
Diencephalon
The
diencephalon
(di
en-sef
ah-lon) develops from the poste-
rior forebrain and is located between the cerebral hemispheres
and superior to the brainstem (see F gs. 11.15 and 11.19). It
In most persons, the left hemisphere is dominant for the
language-related activities of speech, writing, and reading.
It is also dominant for complex intellectual functions requir-
ing verbal, analytical, and computational skills. In other per-
sons, the right hemisphere is dominant, and in some, the
hemispheres are equally dominant.
In addition to carrying on basic functions, the nondominant
hemisphere specializes in nonverbal functions, such as motor
tasks that require orientation of the body in space, understand-
ing and interpreting musical patterns, and visual experiences.
It also provides emotional and intuitive thought processes. ±or
example, the region in the nondominant hemisphere that cor-
responds to the motor speech area does not control speech, but
it infl uences the emotional aspects of spoken language.
Nerve F
bers of the
corpus callosum,
which connect the
cerebral hemispheres, enable the dominant hemisphere to
control the motor cortex of the nondominant hemisphere.
These F
bers also transfer sensory information reaching the
nondominant hemisphere to the general interpretative area
of the dominant one, where the information can be used in
decision making.
Memory
Memory, one of the most astonishing capabilities of the brain,
is the consequence of learning. Whereas learning is the acqui-
sition of new knowledge, memory is the persistence of that
learning, with the ability to access it at a later time. Two types
of memory, short term and long term, have been recognized
for many years, and researchers are now beginning to realize
that they differ in characteristics other than duration.
Short-term, or “working” memories are thought to be
electrical. Neurons may be connected in a circuit so that the
last in the series stimulates the F rst. As long as the pattern
of stimulation continues, the thought is remembered. When
the electrical events cease, so does the memory—unless it
enters long-term memory.
Long-term memory probably changes the structure or
function of neurons in ways that enhance synaptic trans-
mission, perhaps by establishing certain patterns of synap-
tic connections. Synaptic patterns fulF ll two requirements
of long-term memory. ±irst, there are enough synapses to
encode an almost limitless number of memories—each of the
10 billion neurons in the cortex can make tens of thousands
of synaptic connections to other neurons, forming 60 trillion
synapses. Second, a certain pattern of synapses can remain
unchanged for years.
Understanding how neurons in different parts of the
brain encode memories and how short-term memories are
converted to long-term memories, a process called
memory
consolidation,
is at the forefront of research into the func-
tioning of the human brain. According to one theory,
long-
term synaptic potentiation,
near simultaneous repeated
stimulation of the same neurons strengthens their synaptic
connections. In response, in an area of the temporal lobe
called the
hippocampus,
more frequent action potentials are
triggered in postsynaptic cells.
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