The parts of the brain that appear to be of paramount importance in the neural basis of emotional states are the hypothalamus (in the diencephalon) and the limbic system. The limbic system consists of a group of forebrain nuclei and fiber tracts that form a ring around the brain stem (limbus = ring). Among the components of the limbic system are the cingulate gyrus (part of the cerebral cortex), the amygdaloid nucleus (or amygdala), the hippocampus, and the septal nuclei (fig. 8.14).
The limbic system was once called the rhinencephalon, or "smell brain," because it is involved in the central processing of olfactory information. This may be its primary function in lower vertebrates, whose limbic system may constitute the entire fore-brain. It is now known however, that the limbic system in humans is a center for basic emotional drives. The limbic system was derived early in the course of vertebrate evolution, and its tissue is phylogenetically older than the cerebral cortex. There are thus few synaptic connections between the cerebral cortex and the structures of the limbic system, which perhaps helps to explain why we have so little conscious control over our emotions.
There is a closed circuit of information flow between the limbic system and the thalamus and hypothalamus (fig. 8.14) called the Papez circuit. (The thalamus and hypothalamus are part of the diencephalon, described in a later section.) In the Papez circuit, a fiber tract, the fornix, connects the hippocampus to the mammillary bodies of the hypothalamus, which in turn project to the anterior nuclei of the thalamus. The thalamic nuclei, in turn, send fibers to the cingulate gyrus, which then completes the circuit by sending fibers to the hippocampus. Through these interconnections, the limbic system and the hypothalamus appear to cooperate in the neural basis of emotional states.
Studies of the functions of these regions include electrical stimulation of specific locations, destruction of tissue (producing lesions) in particular sites, and surgical removal, or ablation, of specific structures. These studies suggest that the hypothalamus and limbic system are involved in the following feelings and behaviors:
1. Aggression. Stimulation of certain areas of the amygdala produces rage and aggression, and lesions of the amygdala can produce docility in experimental animals. Stimulation of particular areas of the hypothalamus can produce similar effects.
2. Fear. Fear can be produced by electrical stimulation of the amygdala and hypothalamus, and surgical removal of the limbic system can result in an absence of fear. Monkeys are normally terrified of snakes, for example, but they will handle snakes without fear if their limbic system is removed. Humans with damage to their amygdala have demonstrated an impaired ability to recognize facial expressions of fear and anger.
3. Feeding. The hypothalamus contains both a feeding center and a satiety center. Electrical stimulation of the former
Cortex of right hemisphere
Cortex of right hemisphere
■ Figure 8.14 The limbic system. The pathways that connect the structures of the limbic system are also illustrated. Note that the left temporal lobe of the cerebral cortex has been removed to make these structures visible.
The Central Nervous System causes overeating, and stimulation of the latter will stop feeding behavior in experimental animals.
4. Sex. The hypothalamus and limbic system are involved in the regulation of the sexual drive and sexual behavior, as shown by stimulation and ablation studies in experimental animals. The cerebral cortex, however, is also critically important for the sex drive in lower animals, and the role of the cerebrum is even more important for the sex drive in humans.
5. Goal-directed behavior (reward and punishment system). Electrodes placed in particular sites between the frontal cortex and the hypothalamus can deliver shocks that function as a reward. In rats, this reward is more powerful than food or sex in motivating behavior. Similar studies have been done in humans, who report feelings of relaxation and relief from tension, but not of ecstasy. Electrodes placed in slightly different positions apparently stimulate a punishment system in experimental animals, who stop their behavior when stimulated in these regions.
One of the most dramatic examples of the role of higher brain areas in personality and emotion is the famous crowbar accident of 1848. A 25-year-old railroad foreman, Phineas P. Gage, was tamping blasting powder into a hole in a rock with a metal rod when the blasting powder suddenly exploded. The rod—three feet, seven inches long and one and one-fourth inches thick—was driven above his left eye and through his brain, finally emerging through the top of his skull.
After a few minutes of convulsions, Gage got up, rode a horse three-quarters of a mile into town, and walked up a long flight of stairs to see a doctor. He recovered well, with no noticeable sensory or motor deficits. His associates, however, noted striking personality changes. Before the accident Gage was a responsible, capable, and financially prudent man. Afterward, he appeared to have lost his social inhibitions, engaging, for example, in gross profanity (which he had never done before the accident). He also seemed to be tossed about by chance whims. He was eventually fired from his job, and his old friends remarked that he was "no longer Gage."
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