Subcortical Structures

cortex in pairs, one in each hemisphere. Many of these structures are

highly interconnected with one another and are therefore seen to be part of

a system. Furthermore, most of the subcortical structures can be categorized

as belonging to one of two major systems. Surrounding the thalamus is

one system called the basal ganglia, which is most prominently involved in

one system called the basal ganglia, which is most prominently involved in

movements and muscle tone. The basal ganglia deteriorate in Parkinson’s

and Huntington’s diseases, both disorders of motor activity. The three major

structures of the basal ganglia are the caudate nucleus and putamen,

which form the striatum, and the globus pallidus. The activities of the basal

ganglia extend beyond motor control. The striatum, for instance, plays a

significant role in the learning of habits as well as in obsessive-compulsive

disorder, a disorder of excessive habits. In addition, disorders of memory, attention,

and emotional expression (especially depression) frequently involve

abnormal functioning of the basal ganglia.

The nucleus basalis, while not considered part of the basal ganglia, nevertheless

is highly interconnected with those structures (and the hypothalamus)

and receives direct input from them. Nucleus basalis activity is essential

for attention and arousal.

The other major subcortical system is the limbic system. The limbic system

was originally thought to be involved in motivated or emotional behaviors

and little else. Later research, however, demonstrated that many of

these structures are crucial for memory formation. The fact that people

have heightened recall for emotionally significant events is likely a consequence

of the limbic system’s strong involvement in both memory and motivation

or emotion.

Two limbic structures are essential for memory formation. The hippocampus

plays the key role in making personal events and facts into longterm

memories. For a person to remember information of this nature for

more than thirty minutes, the hippocampus must be active. In people with

Alzheimer’s disease, deterioration of the hippocampus is accompanied by

memory loss. Brain damage involving the hippocampus is manifested by

amnesias, indecisiveness, and confusion. The hippocampus takes several

years to develop fully. This is thought to be a major reason that adults tend to

remember very little from their first five years of life, a phenomenon called

infantile amnesia.

The second limbic structure that is essential for learning and memory is

the amygdala. The amygdala provides the hippocampus with information

about the emotional context of events. It is also crucial for emotional perception,

particularly in determining how threatening events are. When a

person feels threatened, that person’s amygdala will become very active.

Early experiences in life can fine-tune how sensitive a person’s amygdala will

be to potentially threatening events. A child raised in an abusive environment

will likely develop an amygdala that is oversensitive, predisposing that

person to interpret too many circumstances as threatening. Two additional

limbic structures work with the amygdala in the perception and expression

of threatening events, the septal nuclei and the cingulate gyrus. High activity

in the former structure inclines one to an interpretation that an event is

not threatening. Activity in the latter structure is linked to positive or negative

emotional expressions such as worried, happy, or angry looks.