In
order to carry out the correct behavior—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' behavior. Finally, he made minute lesions by means of these
electrodes and again observed the effects on behavior. With this technique
he showed that certain kinds of behavior 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 behavior 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.
A great deal of behavior 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 behavior. 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 behavior; that is done by the
cerebral hemispheres—insofar as one may abstract any single part from the
whole.
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 behavior 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 behavior
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 behavior 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 behavior 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 behavior, but it is thought that correct behavior 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 behavior
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.
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
behavior 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 behavior 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 behavior 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 behavior 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 behavior 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–behavior 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 behavior.
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