From the discussion thus far, it is already clear that several limbic structures are particularly concerned with the affective nature of sensory sensations—that is, whether the sensations are pleasant or unpleasant. These affective qualities are also called reward or punishment, or satisfaction or aversion. Electrical stimulation of certain limbic areas pleases or satisfies the animal, whereas electrical stimulation of other regions causes terror, pain, fear, defense, escape reactions, and all the other elements of punishment. The degrees of stimulation of these two oppositely responding systems greatly affect the behavior of the animal.
Reward Centers
Experimental studies in monkeys have used electrical stimulators to map out the reward and punishment centers of the brain. The technique that has been used is to implant electrodes in different areas of the brain so that the animal can stimulate the area by pressing a lever that makes electrical contact with a stimulator. If stimulating the particular area gives the animal a sense of reward, then it will press the lever again and again, sometimes as much as hundreds or even thousands of times per hour. Furthermore, when offered the choice of eating some delectable food as opposed to the opportunity to stimulate the reward center, the animal often chooses the electrical stimulation.
Through use of this procedure, the major reward centers have been found to be located along the course of the medial forebrain bundle, especially in the lateral and ventromedial nuclei of the hypothalamus. It is strange that the lateral nucleus should be included among the reward areas—indeed, it is one of the most potent of all—because even stronger stimuli in this area can cause rage. However, this phenomenon occurs in many areas, with weaker stimuli giving a sense of reward and stronger ones a sense of punishment. Less potent reward centers, which are perhaps secondary to the major ones in the hypothalamus, are found in the septum, the amygdala, certain areas of the thalamus and basal ganglia, and extending downward into the basal tegmentum of the mesencephalon.
Punishment Centers
The stimulator apparatus discussed earlier can also be connected so that the stimulus to the brain continues all the time except when the lever is pressed. In this case, the animal will not press the lever to turn the stimulus off when the electrode is in one of the reward areas, but when it is in certain other areas, the animal immediately learns to turn it off. Stimulation in these areas causes the animal to show all the signs of displeasure, fear, terror, pain, punishment, and even sickness.
By means of this technique, the most potent areas for punishment and escape tendencies have been found in the central gray area surrounding the aqueduct of Sylvius in the mesencephalon and extending upward into the periventricular zones of the hypothalamus and thalamus. Less potent punishment areas are found in some locations in the amygdala and hippocampus. It is particularly interesting that stimulation in the punishment centers can frequently inhibit the reward and pleasure centers completely, demonstrating that punishment and fear can take precedence over pleasure and reward.
Association of Rage With Punishment Centers
An emotional pattern that involves the punishment centers of the hypothalamus and other limbic structures and that has also been well characterized is the rage pattern, described as follows.
Strong stimulation of the punishment centers of the brain, especially in the periventricular zone of the hypo thalamus and in the lateral hypothalamus, causes the animal to (1) develop a defense posture, (2) extend its claws, (3) lift its tail, (4) hiss, (5) spit, (6) growl, and (7) develop piloerection, wide-open eyes, and dilated pupils. Furthermore, even the slightest provocation causes an immediate savage attack. This behavior is approximately the behavior that one would expect from an animal being severely punished, and it is a pattern of behavior called rage.
Fortunately, in the normal animal, the rage phenome non is held in check mainly by inhibitory signals from the ventromedial nuclei of the hypothalamus. In addition, portions of the hippocampi and anterior limbic cortex, especially in the anterior cingulate gyri and subcallosal gyri, help suppress the rage phenomenon.
Placidity and Tameness. Exactly the opposite emotional behavior patterns occur when the reward centers are stimulated: placidity and tameness.
