Figure 1 shows the inner surface of the left cerebral hemisphere and the brain stem cut through its length.

The limbic lobe forms the inner surface of each cerebral hemisphere. It makes a ring of interconnected structures around the "stem" that connects the cerebral hemispheres to the top end of the brain stem. This ring and its connections is often called the limbic system, which is especially important in motivation and emotion (see asgn4n, o, and z). The drawing at the right shows the location of several parts of the limbic system on the inner surface of the left cerebral hemisphere. These include the septal area, at the back end of the inner part of the frontal lobe, and the amygdala and hippocampus, deep inside the temporal lobe.

The limbic system is does much of its control of behavior through the hypothalamus, at the top end of the brain stem. Damage to parts of the limbic system severely affects ability to store and retrieve information in declarative (~conscious) memory (Squire, 1987). This part is hippocampus and related parts of the medial (inner, toward the middle) wall of the temporal lobe. Its role in memory is described further in asgn3l.

Evidence for the functions of the limbic system comes from many sources. For example, anatomical data show that the limbic system is strongly connected to the hypothalamus, which serves as the main output of the limbic system. Electrical stimulation in the hypothalamus can elicit motivated behavior, such as eating and drinking. Stimulation in the hypothalamus can produce intense reward. Animals will repeatedly turn on electrical stimulation to these parts of the hypothalamus thousands of times per hour for many hours in a row, even at the expense of ordinary motives like eating. These areas in the hypothalamus are also activated by cocaine and amphetamine ("speed").

Figure 8-2e shows a cross section of the cerebral hemispheres at about the level of the ears (imagine a slice from the top of the head straight down).

Figure 8-2e. Cross section through the cerebral hemispheres and front end of brain stem as viewed from the front

It is positioned as if the head faces you. It shows some of the most important structures on the inside of the cerebral hemispheres and the top of the brain stem. It illustrates the location of most of the brain structures described below.

Stimulating different parts of the limbic system can, among other things, affect the functioning of the hypothalamus. Such stimulation can also trigger emotional behavior, such as aggression. Damage to various areas of the limbic system disturbs many behaviors related to motivation and emotion.

The amygdala, deep in the temporal lobe, is a very important link in the limbic system. Damage to it disrupts emotional reactions in monkeys and makes them social outcasts. The problem appears to be that monkeys without the amygdala cannot recognize the meaning of emotionally and socially important signals from other monkeys. Recent evidence from humans supports the idea that the amygdala is important (though not essential) for recognizing emotion. (Hamann et al.,1996; Morris et al., 1996)

Damage to the septal area makes animals very irritable and easily aroused to aggression, indicating that an inhibitory effect has been lost. Stimulation here serves as a reinforcer. An animal will turn on electrical stimulation through electrodes located here, though not nearly as fast as for stimulation to parts of the hypothalamus.

For example, Parkinson's disease is the result of losing of a set of neurons that form a major pathway in the basal ganglia. Some of you probably know an older person who suffers from Parkinson's disease. The first sign of Parkinson's disease is usually a slow shaking of the resting hand or foot. As the disease progresses, voluntary movement becomes harder to start, walking becomes a slow shuffle, and the face becomes mask-like and unexpressive. In its latest stages, patients are unable to move voluntarily. Surprisingly, patients with Parkinson's Disease can make quick, automatic reactions to specific triggering stimuli, especially under stress. For example, a former baseball player who was paralyzed this way could quickly raise his hands to catch a ball thrown at him unexpectedly. Mohammed Ali, the former heavyweight boxing champion, is one well known figure who suffers from it; his boxing career may have helped the disorder to develop. It usually appears later in life, but some young people developed devastating cases because of an impurity that can form in certain improperly prepared "recreational" street drugs.

For a summary of the causes and treatment of Parkinson's disease, click HERE.

For more information about Parkinson's Disease, click HERE.

Parts of the basal ganglia also appear to be important in storing and retrieving automatic, non-conscious memories, like memory for motor skills and habits (Petri & Mishkin, 1994).