lar sensation. These may be called association areas
and perhaps are what truly make us individuals. It is
probably these areas that give each of us a personality,
a sense of humor, and the ability to reason and use
logic. Learning and memory are also functions of
these areas.
Although much has been learned about the forma-
tion of memories, the processes are still incompletely
understood and mostly beyond the scope of this book.
Briefly, however, we can say that memories of things
such as people or books or what you did last summer
involve the hippocampus (from the Greek for
“seahorse,” because of its shape), part of the temporal
lobe on the floor of the lateral ventricle. The two hip-
pocampi seem to collect information from many areas
of the cerebral cortex. When you meet a friend, for
example, the memory emerges as a whole: “Here’s
Fred,” not in pieces. People whose hippocampi are
damaged cannot form new memories that last more
than a few seconds.
The right hippocampus is also believed to be
involved in spatial cognition (literally: “space think-
ing”). For example, if you are in school and a friend
asks you the shortest way to your home, you will prob-
ably quickly form a mental map. You can see how
much memory that involves (streets, landmarks, and
so on), but the hippocampus can take it a step further
and make your memories three-dimensional and men-
tally visible. You can see your way home. That is spa-
tial cognition.
It is believed that most, if not all, of what we have
experienced or learned is stored somewhere in the
brain. Sometimes a trigger may bring back memories;
a certain scent or a song could act as possible triggers.
Then we find ourselves recalling something from the
past and wondering where it came from.
The loss of personality due to destruction of
brain neurons is perhaps most dramatically seen in
Alzheimer’s disease (see Box 8–6: Alzheimer’s Disease).Basal Ganglia
The basal gangliaare paired masses of gray matter
within the white matter of the cerebral hemispheres
(see Fig. 8–6). Their functions are certain subcon-
scious aspects of voluntary movement, and they work
with the cerebellum. The basal ganglia help regulate
muscle tone, and they coordinate accessory move-
ments such as swinging the arms when walking or ges-
turing while speaking. The most common disorder of
the basal ganglia is Parkinson’s disease (see Box 8–7:
Parkinson’s Disease).Corpus Callosum
As mentioned previously, the corpus callosumis a
band of nerve fibers that connects the left and right
cerebral hemispheres. This enables each hemisphere
to know of the activity of the other. This is especially
important for people because for most of us, the left
hemisphere contains speech areas and the right hemi-
sphere does not. The corpus callosum, therefore, letsThe Nervous System 183BOX8–6 ALZHEIMER’S DISEASE
of another protein called beta-amyloid that are
damaging to neurons.
A defective gene has been found in some
patients who have late-onset Alzheimer’s disease,
the most common type. Yet another gene seems to
trigger increased synthesis of beta-amyloid. Some
research is focused on the interaction of these
genes and on inflammation as a contributing factor
to this type of brain damage.
It is likely that the treatment of Alzheimer’s dis-
ease will one day mean delaying its onset with a
variety of medications, each targeted at a different
aspect of this complex disease. Early diagnosis will
be very important, and this is yet another avenue of
research.In the United States, Alzheimer’s disease, a pro-
gressive, incurable form of mental deterioration,
affects approximately 5 million people and is the
cause of 100,000 deaths each year. The first symp-
toms, which usually begin after age 65, are mem-
ory lapses and slight personality changes. As the
disease progresses, there is total loss of memory,
reasoning ability, and personality, and those with
advanced disease are unable to perform even the
simplest tasks or self-care.
Structural changes in the brains of Alzheimer’s
patients may be seen at autopsy. Neurofibrillary
tangles are abnormal fibrous proteins found in cells
of the cerebral cortex in areas important for mem-
ory and reasoning. Also present are plaques made