The Nervous System - Advanced Version / Emotion & Behavior

In order to carry out the correct behaviour—that is to say, correct in relation to the survival of the individual and the group—humans and other higher animals developed innate drives, desires, and emotions, and the ability to remember and learn. These fundamental features of living depend on the entire brain, yet there is one part of the brain that organizes metabolism, growth, sexual differentiation, and the desires and drives necessary to achieve these aspects of life. This is the hypothalamus and a region in front of it comprising the septal and preoptic areas. That such basic aspects of life might depend on a small region of the brain was conceived in the 1920s by the Swiss physiologist Walter Rudolf Hess and later amplified by Erich von Holst. Hess implanted electrodes in the hypothalamus and in septal and preoptic nuclei of cats, stimulated them, and observed the animals' behaviour. Finally, he made minute lesions by means of these electrodes and again observed the effects on behaviour. With this technique he showed that certain kinds of behaviour were organized essentially by just a few neurons in these regions of the brain. Later, von Holst stimulated electrodes by remote control after placing the animals in various biologically meaningful conditions.

When such acts result from the artificial stimulation of the neurons, the accompanying emotion also occurs, as do the movements expressing that emotion.

The hypothalamus, in company with the pituitary gland, controls the emission of hormones. It is in control of body temperature, maintains the blood pressure and the rate and force of the heartbeat, and controls the body's need for water and electrolytes. The maintenance of these and other changing events within normal limits is called homeostasis; this includes behaviour aimed at keeping the body in a correct and thus a comfortable environment.

The hypothalamus is also the centre for organizing the activity of the two parts of the autonomic system, the parasympathetic and the sympathetic (see the autonomic system). Above the hypothalamus, regions of the cerebral hemispheres most closely connected to the parasympathetic regions are the orbital surface of the frontal lobes, the insula, and the anterior part of the temporal lobe. The regions most closely connected to the sympathetic regions are the anterior nucleus of the thalamus, the hippocampus, and the nuclei connected to these structures.

In general, the regions of the cerebral hemispheres that are closely related to the hypothalamus are those parts that together constitute the limbic lobe, first considered as a unit and given its name in 1878 by the French anatomist Paul Broca. Together with related nuclei it is usually called the limbic system, consisting of the cingulate and parahippocampal gyri, the hippocampus, the amygdala, the septal and pre-optic nuclei, and their various connections.

The autonomic system also involves the hypothalamus in movement. The expression of emotion and signaling to others depend greatly on the sympathetic nervous system. Emotional expression is also carried out by regions of the cerebral hemispheres above the hypothalamus and by the midbrain below it.
 

Emotion

A great deal of behaviour in social animals, such as humans, involves social interaction. Although the whole brain contributes to social activities, certain parts of the cerebral hemispheres are particularly concerned. The operation of leucotomy, cutting through the white matter that connects parts of the frontal lobes with the thalamus, upsets this aspect of behaviour. This operation used to be performed for severe depression or obsessional neuroses. After the operation, patients lacked the usual inhibitions that were socially demanded, appearing to obey the first impulse that occurred to them. They told people what they thought of them without regard for the necessary conventions of civilization.

Which parts of the cerebral hemispheres produce emotion has been learned from patients with epilepsy and from operations under local anesthesia in which the electrical stimulation of the brain is carried out. The limbic lobe, including the hippocampus, is particularly important. Stimulating certain regions of the temporal lobes produces an intense feeling of fear or dread; stimulating nearby regions produces a feeling of isolation and loneliness, other regions a feeling of disgust, and yet others intense sorrow, depression, anxiety, and, occasionally, guilt. An ecstatic feeling can also occur in which it appears to patients that all problems have been or are just about to be solved.

In addition to these regions of the cerebral cortex and the hypothalamus, regions of the thalamus also contribute to the genesis of emotion. The hypothalamus itself does not initiate behaviour; that is done by the cerebral hemispheres—insofar as one may abstract any single part from the whole.

The defense reaction: fighting and fleeing

When certain neurons of the hypothalamus are excited, an animal either becomes aggressive and eventually attacks, or it flees. These two opposite ways of behaving are together called the defense reaction; both are in the repertoire of all vertebrates. The defense reaction is accompanied by strong sympathetic activity. Aggression is also influenced by the production of androgen hormones.

The total act of copulation is organized in the anterior part of the hypothalamus and the neighbouring septal region. In the male, erection of the penis and ejaculation are organized in this area, which is adjacent to the area for urination. Under normal circumstances the neurons that organize mating behaviour do so only when they receive relevant hormones in their blood supply. But when the septal region is electrically stimulated in conscious patients, sexual emotions and thoughts are produced.

There are visible differences between the male and female sexes in nuclei of the central nervous system related to reproduction. These differences are a form of sexual dimorphism.

Electrical stimulation in cats of regions in, and related to, the anterior part of the hypothalamus can induce the behaviour of expelling or retaining urine and feces. When electrodes planted in these regions are stimulated by radio waves, the cat stops whatever it is doing and behaves as though it were going to urinate or defecate. It goes through its usual behaviour of digging a hole, squatting, and assuming the correct posture, and then it passes urine or feces. At the end, it even goes through its customary ritual of hiding its excreta.

The eating and drinking centres are in the lateral and ventromedial regions of the hypothalamus, although such basic aspects of living concern most of the brain. If the lateral region is experimentally destroyed, the animal takes less food or stops eating altogether; if the ventromedial region is destroyed, it eats enormously. When neurons of the lateral region are electrically stimulated, the animal eats, and when those of the ventromedial area are stimulated, it stops eating. There is an increase in the activity of these neurons when a monkey looks at food, but only when it is hungry. In the lateral region are receptors that monitor blood glucose. They are stimulated into activity only when the blood glucose is low; satiety stops their response.

Hunger does not depend only on these glucose receptors. Severe hunger is associated with contractions of the stomach, which are felt almost as a sensation of pain. Yet neither is this an essential mechanism for feeling hungry, as patients who have had total removal of the stomach still feel hunger. In experiments in rats, it is found that stress may make the animal either increase or reduce the amount it eats. This is probably the same in humans.

When certain neurons in the same regions of the hypothalamus are experimentally destroyed, animals lose the urge to drink, although they continue to eat normally. Stimulation of these neurons make them drink excessively. The control of drinking depends on osmoreceptors throughout the hypothalamus. When the receptors detect a minimal increase in the concentration of dissolved substances in the extracellular fluid, which indicates cellular dehydration, the animal feels thirsty. A less important contributor is a reduction in blood volume. Dryness of the mouth can also be a component of thirst, noted by receptors in the mucous membrane. The feeling of having drunk enough depends not only on the hypothalamic neurons but also on receptors in the wall of the stomach, which report when the stomach is full.

Both glucose receptors and osmoreceptors are sensitive to the temperature of the passing blood. When the temperature starts to rise, one feels thirsty but not hungry; cooling the blood makes one feel hungry.

To maintain homeostasis, heat production and heat loss have to be balanced. This is achieved by both the somatomotor and sympathetic systems. The obvious behavioral way of keeping warm or cool is by moving into a correct environment. The posture of the body is also used, as is clearly seen in the behaviour of the cat. When lying in front of a fire, it is fully stretched out—in physiological terms, extended—thus presenting a large surface to the ambient air and losing heat. When it is cold, it curls itself up into a small volume—in physiological terms, fully flexed—thus presenting the smallest area to the ambient temperature. Humans also use these somatomotor methods.

The most important part of the nervous system for controlling body temperature is the sympathetic system. On a long-term basis, when the climate is cold, the sympathetic system produces heat by its control of certain fat cells called brown adipose tissue. From these cells, fatty acids are released, heat being produced by their chemical breakdown.

Body temperature fluctuates regularly within 24 hours; this is a type of circadian rhythm (see below). It also fluctuates in rhythm according to the menstrual cycle. During fever, the body temperature is set at a higher point than normally.

In a fundamental discovery made in 1954, James Olds and Peter Milner found that stimulation of certain regions of the brain of the rat acted as a reward in teaching the animals to run mazes and solve problems. The conclusion from such experiments is that stimulation gives the animals pleasure. The discovery has also been confirmed in humans. These regions are called pleasure or reward centres. One important centre is in the septal region, and there are reward centres in the hypothalamus and in the temporal lobes of the cerebral hemispheres as well. When the septal region is stimulated in conscious patients undergoing neurosurgery, there are feelings of pleasure, optimism, euphoria, and happiness.

Regions of the brain also clearly cause rats distress when electrically stimulated; these are called aversive centres. However, the existence of an aversive centre is less certain than that of a reward centre. Electrodes stimulating neurons or neural pathways may cause an animal to have pain, anxiety, fear, or any unpleasant feeling or emotion. These pathways are not necessarily centres having the function of providing punishment in the sense that a reward centre provides pleasure. Therefore, it is not definitely known that connections to aversive centres punish the animal for biologically wrong behaviour, but it is thought that correct behaviour is rewarded by pleasure provided by neurons of the brain.

Living organisms have inevitably become adapted to the orderly rhythms of the universe. These biologic cycles are called circadian rhythms, from the Latin circa (“about”) and dies (“day”). They are essentially endogenous, built into the central nervous system. The rhythmical activities usually covered by the word circadian are sleeping and waking, rest and activity, taking in of fluid, formation of urine, body temperature, cardiac output, oxygen consumption, cell division, and the secreting activity of endocrine glands. The rhythms are upset by shift work and by rapid travel into different time zones. After long journeys it takes several days for the endogenous rhythm generator to become synchronized to the local time.

The alternation of night and day has been important in inducing rhythms affecting many physiological functions. Even in isolation, from clues giving information about light and dark, rhythms related to the time of day are maintained. Curiously, the endogenous sleep–wake rhythm deviates slightly from the Earth's 24-hour cycle; a bird's endogenous cycle is 23 hours, and the human cycle is 25 hours. In both cases the cycle is corrected by features of the environment. Such synchronizing agents are called zeitgebers (“time givers”). One zeitgeber is the Earth's magnetic field, which changes on a 24-hour cycle as the Earth turns on its axis. More obvious and important a zeitgeber is the alternation of dark and light.

One hypothalamic nucleus that is essential for the rhythms of sleeping, waking, rest, and activity is the suprachiasmatic nucleus. It is not surprising that this nucleus is adjacent to the incoming fibers from the eye, for the light–dark cycle appears to be the most important zeitgeber for circadian rhythms. The suprachiasmatic nucleus is most active in light. In experiments on the hamster, when the nucleus is destroyed, the rhythms of general activity, drinking, sleeping, waking, body temperature, and some endocrine secretion are disrupted.

The neurons of the cerebral cortex constitute the highest level of control in the hierarchy of the nervous system. Consequently, the terms higher cerebral functions and higher cortical functions are used by neurologists and neuroscientists to refer to all conscious mental activity of the kind normally described as thinking, remembering, and reasoning and to complex volitional behaviour such as speaking and carrying out purposive movement. They also refer to the processing of information in the cerebral cortex, most of which takes place unconsciously.

Analytical approaches

Neuroscientists investigate the structure and functions of the cerebral cortex, but the processes involved in thinking are also studied by cognitive psychologists, who group the mental activities known to the neuroscientist as higher cortical functions under the headings cognitive function or human information processing. From this perspective, complex information processing is the hallmark of cognitive function. Cognitive science attempts to identify and define the processes involved in thinking without regard to their physiological basis. The resulting models of cognitive function resemble flowcharts for a computer program more than neural networks—and, indeed, they frequently make use of computer terminology and analogies.

The discipline of neuropsychology, by studying the relationship between behaviour and brain function, bridges the gap between neural and cognitive science. Examples of this bridging role include studies in which cognitive models are used as conceptual frameworks to help explain the behaviour of patients who have suffered damage to different parts of the brain. Thus, damage to the frontal lobes can be conceptualized as a failure of the “central executive” component of working memory, and a failure of the “generate” function in another model of mental imagery would fit with some of the consequences of left parietal lobe damage.

The analysis of changes in behaviour and ability following damage to the brain is by far the oldest and probably the most informative method adopted for studying higher cortical functions. Usually these changes take the form of what is known as a deficit—that is, an impairment of the ability to act or think in some way. With certain provisos, one can assume that the damaged part of the brain is involved in the function that has been lost. However, people vary considerably in their abilities, and most brain lesions occur in subjects whose behaviour has not been formally studied before they become ill. Lesions are rarely precisely congruent with the brain area responsible for a given function, and their exact location and extent can be difficult to determine even with modern imaging techniques. Abnormal behaviour after brain injury, therefore, is often difficult to attribute to precisely defined damage or dysfunction.

It would also be naive to suppose that a function is represented in a particular brain area just because it is disrupted after damage to that area. For example, a tennis champion does not play well with a broken ankle, but this would not lead one to conclude that the ankle is the centre in which athletic skill resides. Reasonably certain conclusions about brain–behaviour relationships, therefore, can be drawn only if similar well-defined changes occur reliably in a substantial number of patients suffering from similar lesions or disease states.

The most prominent series of observations clearly belonging to modern neuropsychology were made by Paul Broca in the years following 1861. He reported the cases of several patients whose speech had been affected following damage to the left frontal lobe and provided autopsy evidence of the location of the lesion. In making his famous statement, “We speak with the left hemisphere,” he explicitly recognized the left hemisphere's control of language, one of the fundamental phenomena of higher cortical function.

In 1874, the German neurologist Carl Wernicke described a case in which a lesion in a different part of the left hemisphere, the posterior temporal region, affected language in a different way. In contrast to Broca's cases, language comprehension was more affected than language output. This meant that two different aspects of higher cortical function had been found to be localized in different parts of the brain. In the next few decades there was a rapid expansion in the number of cognitive processes studied and tentatively localized.

Wernicke was one of the first to recognize the importance of the interaction between connected brain areas and to think of higher cortical function as the buildup of complex mental processes through the coordinated activities of local regions dealing with relatively simple, predominantly sensory-motor functions. In doing so, he opposed the view of the brain as an equipotential organ acting en masse. Since his time, scientific fashion has swung between the localization and mass-action theories. Major changes in the 20th century have been both quantitative, with vast increases in knowledge, and methodological, with the discovery of new ways of studying the brain's anatomy and physiology and the introduction of better quantitative methods in the study of behaviour.

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