Fig. 17-1. Interconnections between structures of the limbic system.
Emotion is another of the behaviors for which there is really no satisfactory definition. It has both unobservable internal and observable external aspects. It is a cognitive process in that one must perceive the causative situation and evaluate it in light of past experience and cultural variables. For example, a particular situation can be potentially beneficial or deleterious to one's self, family, or country. The person may not even be aware that the evaluation is occurring. Emotion is either pleasant or unpleasant and need not be the same to every individual. It is expressed outwardly in the form of facial expressions, gestures, vocalizations, body postures, and other movements, or it may be expressed in the form of an absence of movement. There may be concomitant autonomic activity resulting in changes in heart rate, blood pressure, piloerection, sweating, and flushing. Movements of emotion can propel the individual toward another object, as in attacks, or away, as in flight. Finally, emotional behavior can result in excitement and alertness or depression, dullness, and sluggishness.
The initiation and regulation of emotional behavior by the brain is thought to involve the limbic system. Knowledge of the limbic system is quite old; it was named by the surgeon, Broca, in the last century. Actually, little research was done to link the limbic system with emotion until about 1937 when the neuroanatomist, Papez, published a proposed mechanism of emotion involving the limbic system. Figure 17-1 is a summary diagram of most of the parts of the limbic system and their interconnections. The word limbic means "in a form of a ring." The system is given this name because the various parts are organized into rings of interconnected structures. The major parts of the limbic system are as follows: (1) structures of the temporal lobe of the cortex: the amygdala, hippocampus, and parahippocampal cortex; (2) other cortical structures: the orbital and cingulate cortices; (3) anterior and dorsomedial thalamic nuclei; (4) the hypothalamic structures: the hypothalamus proper, septum, and preoptic area; (5) the mammillary bodies; (6) the habenula; (7) the interpeduncular nuclei; and perhaps others. The limbic system receives input from enteroceptors and exteroceptors. It is intimately related to the olfactory system, and it also receives inputs from every sensory modality. It sends its output to various autonomic nuclei in the brain stem and, through the hypothalamus, influences every part of the body through release of hormones from the pituitary gland.
It has already been noted that removal of structures of the hemisphere above the mammillary bodies results in sham rage, a rage that is typical of normal aggressive behavior except in its lack of direction toward the offending object. The rage is most intense if the lesion is made immediately above the mammillary bodies, less if the hypothalamus is spared, and least if only the neocortex is removed. If a lesion is made in the ventromedial hypothalamic nuclei, a similar rage results but, unlike sham rage, the attacks are well directed. Similarly, a lesion in the septal area makes a normally placid animal savage. Interestingly, a subsequent lesion of the amygdala makes the animal tame again. Bilateral removal of the amygdala without a concomitant septal lesion leads to decreased aggressive behavior in normally savage animals.
Stimulation in the ventromedial and other areas of the hypothalamus elicits aggressive or attack behavior or what appears to be stalking and prey-killing behavior in animals that begins when the stimulus is turned on and ends when it is turned off. Aggressive behavior is also elicited by stimulation in the amygdala, but, unlike that due to hypothalamic stimulation, it develops more slowly and outlasts the stimulus by some period of time. In addition to aggressive behavior, amygdalar stimulation also leads to cessation of movements, changes in spinal reflex activity, controversive head and eye movements, swallowing, licking, chewing, changes in heart and respiration rates, changes in gastrointestinal motility, micturition, defecation, pupillary dilation, and piloerection. These changes are all characteristic of emotional responses, though not necessarily of aggressive behavior. In humans, stimulation of the amygdala leads to sensations of fear, disturbances of mood, a sense of unreality, and distortions of bodily perceptions.
Bilateral removal of the temporal lobe leads to a constellation of effects that are called the Kluver-Bucy syndrome. The symptoms include the following:
Attempts have been made to localize the structures whose removal produced each of these symptoms. There has been little success in such efforts.
Stimulation in the region of the medial forebrain bundle or the preoptic area produces a sensation described as pleasurable by humans. Apparently, animals also find it pleasurable because they will press a lever in order to receive an electrical stimulus in this area, some people call the pleasure center. They will elect to press the lever and receive the shook rather than eat when they are hungry, drink when they are thirsty, or engage in sexual activity with a receptive female. In fact, rats will cross an electrified grid that they normally would not cross, in order to press the lever and receive the shock. Obviously, the sensation is a powerful one in influencing behavior.
It is probable that the hypothalamus cannot, by itself, initiate directed emotional behavior, but requires the modulating, directing, and regulating influence of the rest of the limbic system. As we have seen, stimulation of various parts of the limbic system can activate all of the autonomic and behavioral concomitants of emotional behavior, but as yet we do not know how or where the behavior is initiated or what parts of the system play what roles in what behaviors. It is clear that the limbic system plays an important role in emotional behavior.
Learning and memory
Learning is usually defined as a relatively permanent change in behavior as a result of practice or experience. There is a vast psychological literature dealing with the kinds of learning and their characteristics, which we will not attempt to go into here. Animals can learn certain responses to stimuli just by associating the stimuli with other stimuli that normally elicit the desired response (classical conditioning). For example, if an animal is presented with a puff of air directed at the eye, it blinks. Repeated pairings of a brief light flash or tone and a puff of air effect some change in the nervous system such that the light flash or tone, by itself, evokes the blink. Another kind of learning is called instrumental or operant conditioning. In instrumental conditioning, an animal must perform some task in order to get a reward or avoid punishment. Thus, an animal will learn new behaviors in order to get a reward or relief from pain. This phenomenon is familiar to every animal trainer. We now know that animals will learn complex behavior patterns in order to receive electrical stimulation in parts of the hypothalamus, particularly the preoptic area, the pleasure center.
Some theorists claim that human learning can be explained in terms of classical and instrumental conditioning, whereas others maintain that there is something more, something unique about human learning ability. This something, if it exists, has yet to be identified. Much of what we know about learning has been obtained from animals, raising the question of the applicability of the results to humans. It has already been noted that humans may differ even from monkeys in the indispensability of the cerebral cortex in sensory functions, so it should be cautioned that there may be profound differences between man and animals, even in elementary learning processes. Because animal evidence is all that is available in many cases, we will treat it as if it also applied to humans.
In general, the following principles seem to emerge from studies on learning:
With instrumental conditioning, animals can change the behavior of neurons in the same way they change their overt behavior. These changes in the discharges of neurons, as a result of conditioning, could be brought about by making or changing some overt movement or behavior. We know that neurons in the CNS are responsive to sensory input from the periphery, sometimes evocable by an appropriate movement. A neuron, responsive to stimulation of a particular area of skin, may discharge if the animal brings the area into contact with some object, even its own body. In fact, in some of these conditioning experiments, movements or changes in movements have been observed concomitant with changes in neural activity. Clearly, the animal could be stimulating itself. It is also conceivable that a neuron's discharge could be altered by itself, that is, in isolation from any movement or sensation. Evidence suggests that this actually does not happen (Wyler, Burchiel, Robbins, 1979), but, because there is nearly always some muscle contracting somewhere in the body, this is a difficult demonstration. Work is in progress to find some way to use this control over nerve cells to run prosthetic devices for lost limbs. As yet, the work has just begun.
Attempts have been made to record both evoked potentials and discharges of single cells or small groups of cells during training of animals in order to find neural concomitants of learning. There are changes in evoked potentials that occur while an animal is learning a particular task. Unfortunately, these changes are difficult to interpret for several reasons. It is not known what events (postsynaptic potentials) are responsible for producing the various parts (positive and negative deflections) of the evoked potential, if, indeed, a single event is responsible for each part. Because the evoked potential reflects activity in all neurons neighboring the recording electrodes, we cannot say with certainty which neurons we are studying. Learning has been defined as a change in behavior; usually this means a change in movement. We cannot, as yet, separate the neural events, parts of an evoked potential, that are associated simply with the production of a movement from those associated with the learning process itself.
Interpretations of single-cell recordings during learning are even more difficult. The problem of separating changes due to motor activity from changes due to learning itself is also inherent in single-cell recordings. In addition, we cannot follow the behavior of a single cell long enough to observe it both before and after learning has occurred, and, over such a long period, we could not be sure it was, in fact, the same neuron. The neurons that are near the recording electrode after learning has occurred may behave differently either because they have "learned" or because they are different neurons or because they are damaged by the recording electrode.
Memory is the ability to recall events or tasks learned or experienced minutes, days, or years before. It is customary to divide memory into two types, immediate recall or short-term memory and long-term memory. Short-term memory usually has a duration of minutes to perhaps hours, whereas long-term memory has a duration from minutes (or hours) to years. It is not possible to say precisely when a given memory crosses the border from short-term into long-term memory or even if short- and long-term memory involve the same continuous process or two different processes.
Animals given electroconvulsive shocks or other treatments to interrupt brain activity within about 5 minutes after a training session do not remember the task they were trained to perform when tested some time later. When no treatment is given, the same animals remember the task 5 minutes and even weeks after the training sessions. If the same treatment is given to the animal 4 hours after training sessions, it has no effect at all on their ability to remember the task even weeks later. It thus appears that there is a period shortly after an event when the memory is vulnerable to disruption; then something happens to make the memory trace resistant to erasure. It is this event that constitutes the conversion from short-term to long-term memory.
Clinically, it is found that immediately following electroshock therapy or some brain concussions there is no memory of the therapy or the concussion or of events that occurred some time preceding it. In man, this lack of memory or amnesia may encompass longer periods than in animals, sometimes day, weeks, or even years, but more remote memories are usually intact. The patient, under these circumstances, is still capable of forming new long-term memories, but there are conditions where this ability is also impaired.
The amnesic syndrome, or Korsakoff's psychosis, is an impairment of the ability to convert short-term memory into long-term memory, without impairment of immediate recall and without elimination of already formed long-term memories, and it results from bilateral damage to the hippocampal formation of the temporal lobes. Some pathology is usually also found in the mammillary bodies, a major projection site of the hippocampus. The uncus, parahippocampal cortex, mammillary bodies, and amygdala do not seem to play a role in this syndrome, nor do bilateral lesions of the fornix, the output pathway from the hippocampus, produce impairment of memory. The patient can recall bits of information for minutes (immediate recall), but if he is distracted these are lost. He can also recall events that happened prior to the damage to his brain (long-term memory).
The amnesic syndrome can sometimes result from thiamine deficiency or bilateral occlusion of the posterior cerebral arteries that supply the hippocampus. It also results from surgical removal of one temporal lobe (temporal lobectomy) when the other has been damaged. Temporal lobectomy without prior damage to the other hippocampus does not produce any memory impairment. General symptoms of the amnesic syndrome are indicated in the following case history:
The memory storage process apparently depends upon the integrity of the hippocampus, but we know little about the location of long-term storage of information. Some cases of temporal lobe epilepsy exhibit seizures preceded by auras consisting of memories of past events. Typically, the same past event is played back to the patient just prior to each seizure, serving as a signal that the seizure is impending. During surgery to excise the focus (the pathological cortical area responsible for the seizure), the cortical surface of the temporal lobe is stimulated electrically to locate the site of the focus. When the site is located and stimulated, the memory of the aura is played back again to the patient in an interesting way. When the stimulating current is turned on, the memory begins to be recalled by the patient, and the events in the memory continue in sequence, as they originally happened, until the current is switched off. If, after a moment, the stimulus is turned on again, the memory is picked up again where it left off before. It is as if a tape recorder were being turned on and off. Removal of the focus abolishes the seizures with their auras, but does not eliminate the memory that was part of the aura. It is unknown whether the memories remain because of bilateral storage at conjugate sites (the site of the focus and the same site on the contralateral hemisphere) or whether the memories are actually stored elsewhere. No single site for storage of memory traces has yet been found in the central nervous system.
The nature of the storage process is as much a mystery as its location. It is clear that long-term memory is not a result of activity in reverberating circuits (circuits in which activity is transmitted continuously around a closed-loop pathway), but short-term memory may be. Electroconvulsive shock, deep coma due to overdoses of sedative drugs, and complete cessation of electrical activity, all of which would stop reverberation, do not interfere with already formed long-term memories, but they do destroy short-term memories and disrupt their conversion to long-term memories. Many claims have been made about the involvement of specific, single ribonucleic acids (RNA) or other specific proteins in memory, but in general the claims remain unsubstantiated. It is noteworthy that drugs that block protein synthesis will also block transformation of short-term into long-term memory. Also, a variety of CNS stimulants have been shown to improve learning in animals and restore learned behaviors that have been lost as a result of CNS lesions. The process must certainly involve biochemical changes in cells, perhaps in formation of new or more permanent or more effective synaptic junctions.
It has been known for over 100 years that the prefrontal lobes are involved in determining and regulating personality. Seizures brought on by abnormalities of the prefrontal lobes often involve transient changes in personality, with compulsive behavior and the release of social inhibitions that may persist for several weeks following the seizures. Bilateral removal of the prefrontal lobes results in a reduction in emotional responsiveness and anxiety, but no change in perception. For example, patients with prefrontal lobotomies still report feeling pain, but they say it no longer bothers them. The patients become impulsive and distractible; they lack concern over the consequences of their actions; and they are unable to plan ahead. Following lobotomy, a man urinated in the corner of the room. When told not to repeat this action, he agreed, but shortly was back in the corner again. A woman, who had planned and prepared meals for her family for many years, was unable to do so following a lobotomy, although she was able to perform the individual activities involved when told what to do next.
Prefrontal lobotomy patients sometimes have difficulty postponing gratification. Although they are distractible, they show a tendency to persevere in activities and seem unable to shift their responses to suit a change in environment or circumstance. Their thinking often becomes rigid and concrete with some deficits in abstract reasoning. These symptoms are illustrated in the following case history:
In the 1940s and 1950s, prefrontal lobotomies were frequently performed in mental institutions and prisons on patients whose behavior was considered undesirable and intractable. The procedure made them docile and carefree, but the alterations in their personalities were severe. At one point, it became so fashionable to have a lobotomy performed that it actually became an office procedure. Only a small incision in the orbit above the eye is required. Some physicians did as many as 1000 leukotomies per month during one period. Fortunately, anxiety and intractable behaviors are now controllable on a temporary basis by the use of tranquilizers, and lobotomies are seldom performed. Some states allow them only in cases of uncontrollable, incapacitating anxiety or fear or for relief of the suffering from pain in terminal illness, where it has been shown to a review board that the patient is unresponsive to any other form of treatment.
Speech and other unilateral functions
Thus far, everything that has been said about the nervous system applies equally to both sides, that is, the nervous system is bilaterally symmetrical. However, there are some cortical functions that are not served equally by both hemispheres. In one cerebral hemisphere, there is a region, called Broca's area, that is involved in speech (Fig. 17-2, lateral aspect of the dominant hemisphere). Broca's area (areas 44 and 45) is located in the frontal cortex just anterior to the face area of the precentral gyrus. Actually, there are two additional areas of the same hemisphere that are involved with speech. These are the inferior parietal lobule, consisting of the angular and supramarginal gyri (areas 39 and 40), and Wernicke's area (area 22), located on the posterior aspect of the superior temporal gyrus just behind the auditory cortex. The location of these three areas in seen in Figure 17-2. These areas are found in only one hemisphere, and their presence defines the dominant hemisphere. The dominant hemisphere is usually the one opposite the favored hand (in most individuals, the left hemisphere), but, in a few cases, it is the hemisphere on the same side as the favored hand (92% of right-handed people are left hemisphere dominant, 50 to 60% of left-handed people are right hemisphere dominant).
Electrical stimulation during neurosurgery in any of these three areas leads to arrest of speech, hesitation or slurring of speech, distortion, repetition, inability to name, misnaming, and vocalization. Vocalization can also be elicited from the motor cortex where it always takes the form of a sustained or interrupted cry.
Various types of speech disorders result from damage to the speech areas. The general term for these disorders is aphasia. Lesions involving Broca's area and adjacent cortex produce a motor or expressive aphasia, in which the patient does not or cannot speak. What words are used are uttered slowly and with great effort. Usually, difficulties also show up in writing words. In expressive aphasia, it appears that the patient has lost his motor programs for making correct and properly timed sequences of movements of lips and tongue, although he apparently still knows what to say. A lesion of Wernicke's area or the inferior parietal lobule produces a receptive aphasia in which the patient cannot understand spoken language or written language (alexia), but is capable of spontaneous speech. Many times, lesions of the inferior parietal lobes of the dominant hemisphere also produce what is known as Gerstmann's syndrome. This syndrome may include one or more of the following symptoms: dysgraphia, a deficit in writing without motor or sensory deficits in the upper extremities; dyscalculia, a deficit in the performance of calculations; left-right confusion; and finger agnosia, errors in finger recognition. It is seldom that aphasias present themselves simply as described above. Usually, a given aphasia is a mixture of expressive and receptive difficulties. A number of classification schemes have been invented, but no one of them can handle all cases. The following case history illustrates aphasia:
The devastating losses in language abilities resulting from even small lesions of speech centers of the dominant hemisphere suggest that a dominant hemispherectomy should abolish language altogether. Surgeons were surprised to find that this procedure does not result in complete aphasia, but in some verbal loss, especially in responses to written commands. Deficits are fewer in writing than in speech and least in responses to verbal commands. Speech abilities are recovered to a great extent over a period of 8 months. Why losses due to hemispherectomy are smaller than predicted is one of the enigmas of neuroscience, yet it should be recalled that the influence of hemispherectomy on movement is also less than expected.
In a few cases of intractable seizures, physicians have elected to section the corpus callosum by itself or in combination with the anterior commissure and the hippocampal commissure in order to stop the spread of the seizures from one hemisphere to the other. In many cases, this procedure has been successful, and it has given the scientist an opportunity to study the abilities of each hemisphere in isolation. The separation of the two hemispheres creates, in some senses, two minds within the same body. The patient, sometimes called a split-brain patient, is able to get around well and can do most, if not all, of the things he did before, but some things he does in a subtly different way. For example, in performing tasks that require the cooperation of both hands, the patient may be severely impaired. If asked to match a pile of blocks by building a new pile next to it, the patient finds one hand putting a block in place, while the other one takes it away or moves it somewhere else. He can only do the task successfully if he sits on one hand, letting the other one do the work. Split-brain patients show no impairment in intellectual functioning. In fact, in the one case where adequate pre- and postoperative testing was performed, the patient showed improvement in standardized memory tests, short-term memory, and learning rate. Apparently, separating the hemispheres does not disrupt these functions. Why they improved in this one case is a matter of conjecture, and whether this is a consistent result of the surgery awaits further tests.
Sectioning the corpus callosum surgically has some rather interesting effects on sensation in the modalities of somesthesia, audition, and vision. The ability to localize the site of a stimulus to the skin is not impaired in split-brain patients, but if the testing procedure is set up in such a way that each hemisphere is tested separately, then it can be shown that a hemisphere can only localize stimuli on the contralateral body surface. Thermal sensitivity and position sense of a hemisphere are likewise limited to the contralateral side of the body. Complex discriminations of solid shapes with no limit to the number of alternatives are performed only by the contralateral hemisphere, but if the alternatives are easily discriminated (as tested using the contralateral hemisphere) and few in number (e.g., cubes versus spheres), then the ipsilateral hemisphere can discriminate them as well. It is not known by what pathways the sensory information reaches the ipsilateral hemisphere in the split-brain patient. When the discriminations are learned by one hemisphere only, the other hemisphere shows no indication that it "knows" of the learning, that is, there is no intermanual transfer of tactile discriminations.
There is no loss of auditory sensitivity after the brain is split, and the ability to localize the source of a sound is unimpaired. The patient can also respond properly, using either hand, to a verbal command directed at either ear. He cannot, however, fuse two sounds, one presented to each ear. A normal person, presented with "pro" to the right ear and "duct" to the left ear in quick succession, hears "product." The split-brain patient hears "pro" with the left hemisphere and "duct" with the right. The left hemisphere-dominant-patient's verbal report will be "pro" only, but he will write "duct" using his left hand.
Under conditions where the brain has been surgically split, it is possible, provided that the eyes do not move, to direct visual stimuli to each hemisphere independently because of the pattern of crossing of optic tract fibers in the optic chiasm. Objects in the right visual field are seen by the left hemisphere; those in the left visual field are seen by the right hemisphere; and the macula is not represented bilaterally. Following the commissurotomy, objects in the right visual field of a patient with left hemisphere dominant can be correctly named by the patient, those in the left cannot, presumably because of the location of speech areas. On the other hand, a photograph shown to either hemisphere can be correctly pointed out using the contralateral hand. The patient is unable to make same-different discriminations between photographs presented one to each hemisphere.
Split-brain patients have difficulty with stereopsis (depth perception) when an object lies immediately in front of or behind the fixation point. This is because the images fall on the retina so that they are transmitted to different hemispheres, and determining whether to converge or diverge the eyes is impossible. In Figure 17-3, the eyes are fixated on point A; the images of points B and C fall on the temporal retinae and nasal retinae, respectively, and therefore are transmitted one to each hemisphere. So in the case of either point B or C, neither hemisphere would have both images to compare, the basis upon which decisions about convergence and divergence are normally made. When both images of an object fall on the retinae so that they are transmitted to the same hemisphere (as in the case of point D in Fig. 17-3), there is no problem.
The question arises whether the nondominant hemisphere has any verbal capabilities at all. To test this, a word was flashed in the opposite visual field, and the patient was asked to pick the word out of a number of others. The nondominant hemisphere can do this, at least for some kinds of words. It has no difficulty in recognizing nouns and adjectives. Apparently, the nondominant hemisphere does have language skills, but these are not manifested normally, because it has no access to the speech apparatus. They are clearly seen, however, if written responses are required.
The nondominant hemisphere appears to be better at visual form perception than the dominant one, at least for complex forms and faces. This is shown by the following tests. Split-brain patients were asked to copy line drawings that suggested spatial perspective, for example, a cube. Performance was consistently better with the left hand than with the right hand despite the fact that all patients were right handed and had little experience drawing with their left hands. Evidence indicates that the left hand receives its major motor control from the right, in these cases, nondominant hemisphere. Assembling complex puzzles is also much easier for these patients when they use their left hands.
This superiority of the nondominant hemisphere for form perception also shows up when no fine motor coordination is required by the test. Two photographs, each of a different face, are cut along the vertical midline of the face. The left half of one face is put together with the right half of the other, forming a composite face. If a split-brain patient fixes his gaze on the junction between the two half-faces, the left half-face is seen by the right hemisphere and the right half-face is seen by the left hemisphere. If the composite face is flashed briefly so that the eyes will not have time to move, then neither hemisphere is aware of the half-face in the ipsilateral visual field. The patient, asked to point out which of the original faces he saw in the brief flash, nearly always selects the one, half of which was seen by the right, nondominant hemisphere; if asked to describe the face he saw, he describes the face, half of which was seen by the dominant hemisphere and never reports seeing two halves of different faces.
In spite of what was earlier said about lesions of the inferior parietal lobe of the dominant hemisphere (Gerstmann's syndrome), the calculating ability of the nondominant hemisphere appears to be superior to that of the dominant one. The test is to give the right and left hemispheres addition and subtraction problems and require an answer to be selected from among several alternatives by pointing, the tests not requiring the use of language. Problems can be presented to only one hemisphere at a time by flashing them in only the left or right visual field when the eyes are not moving. The superiority of the nondominant hemisphere, shown in this test in terms of number of correct responses, does not show up in tests requiring verbal responses. This is presumably the reason for the dyscalculia of dominant hemisphere lesions; the ability is normally tested using verbal responses.
The nondominant hemisphere appears to be better at some sorts of tests of creativity involving making associations between concepts, similar to the Miller Analogies Test you may have taken. Still other aspects of creativity seem to be the province of the dominant hemisphere.
Apparently, each hemisphere has a sense of self, recognizing the individual's name and supplying it when requested. Each hemisphere has its own system of subjectively evaluating current events, planning for future events, setting response priorities, and generating personal responses. Both hemispheres are capable of the same emotional responses. A picture of a nude person of the opposite sex evokes the same emotional response whether it is shown to the dominant or nondominant hemisphere of a split-brain patient. The emotional responses and evaluations of people and things by the two hemispheres can also come into opposition with each other. There are reports of a man who shook his wife with the left hand and came to her rescue with the right and of a man who pulled down his pants with one hand and pulled them up with the other.
In split-brain patients, it has been observed that when the two hemispheres agree on evaluations of people and other matters, the patients outwardly appear calm and tractable, but when the two hemispheres disagree, they are outwardly hyperactive, agitated, and aggressive. It would appear that the two hemispheres are also capable of reading differences in each other. In one experiment, words were flashed tachistoscopically to one hemisphere or the other, and the patient, in this case a 15-year-old boy, was asked to "do what the word says." When the word "kiss" was flashed to the right (mute) hemisphere, the patient immediately responded, "Hey, no way, you've got to be kidding." When asked what the word was he responded, "nurse." When "kiss" was flashed to the left (verbal) hemisphere, he responded in the same tone, "No way. I'm not going to kiss you guys." How the verbal hemisphere could read the emotional content of the word in both exposures is unknown, but clearly it did (Gazzaniga MS, LeDoux JE, Wilson DH: Neurol 27:1144-1147, 1977).
The dominant hemisphere clearly has the major role in complex motor functions as indicated by the existence of a preferred hand, but the nondominant hemisphere is perfectly capable of controlling its half of the body on its own. There is even some indication of ipsilateral control of muscles other than finger muscles; however, it is not known to what extent this control is the result of cross-cuing, that is, using small compensatory changes in the midline postural muscles as indicators of motor performance, and to what extent there is direct control by sensory feedback to the ipsilateral hemisphere. In some surgical cases, an entire hemisphere has been removed down to the thalamus, and in these cases both sides of the body move about equally well. The two hemispheres do compete with each other in split-brain patients. Such a patient can easily build a simple jigsaw puzzle with either hand, but has great difficulty using both hands, because as one hand puts the pieces in place, the other removes them. Recall that complex puzzles are only constructed with ease by the hand contralateral to the nondominant hemisphere because of its spatial abilities. The two hemispheres normally work together, communicating with each other by way of the commissures, to produce normal behavior, both in terms of movement and sensation and in terms of covert processes like calculating and reasoning. It is only when one hemisphere is not functioning or when the two are functioning independently that we can see that they are not equal. There are a number of known cases of callosal agenesis, cases in which the corpus callosum fails to develop. The same tests used to study split-brain patients have also been applied to patients with callosal agenesis. They have little deficit in visual or tactile tests of the sorts already described, but they do have deficits in certain spatiomotor tests, e.g., tactile maze learning, regardless of whether the tests required interhemispheric transmission. They also had difficulty with certain purely motor tasks, yet the investigators concluded that they have "fewer disconnection symptoms" than split-brain patients. It is possible that use of other undamaged commissures may explain the differences between the results of agenesis and surgical section. The differences should not be entirely surprising; the results of damage to the brain early in life are often different from those of damage occurring in adults.
It is not known how this asymmetry in the cerebral hemisphere comes about. Some investigators believe that the dominance of one hemisphere develops as a result of experience in early life. One theory links the development of language skills to reinforcement of handedness by parents, but the existence of left dominant, left-handed people seems to contradict this theory. Other investigators maintain that the asymmetry is part of the heredity of the individual and that it merely shows itself gradually as behavior develops. Clearly, there are some asymmetries in handedness and in neural responses to speech sounds in infants. These would appear to be innate. Yet, some verbal asymmetries have not been found in boys until 5 to 7 years of age and in girls until 9 to 11 years of age. Whether this late appearance is innate or learned, the difference in time of onset in the sexes may help to explain the greater resistance of girls to permanent damage to language function caused by early brain trauma. It may be that either hemisphere can become dominant, but only one does under normal circumstances.
There are clear structural differences between the two hemispheres that may underlie these differences in hemispheric functions. There are, of course, the larger "speech areas" in the dominant hemisphere. In addition, auditory primary sensory and association areas appear to be larger in the dominant hemisphere than in the nondominant, whereas the temporoparietal cortex appears to be larger in the nondominant hemisphere. In right-handed people, most of whom are left hemisphere dominant, the frontal lobes are more often wider in the right and the occipital lobes more often wider in the left hemisphere. There are many more structural asymmetries in the brain, but we do not know which may be associated with dominance. Structural asymmetries have also been found in fossil human and protohuman brains, in all of the living great apes, and in some of the lesser apes and monkeys. Again, the relationship of these asymmetries to language is unknown; in fact, we are only just becoming aware of the extent of language capabilities in animals.
Such asymmetries are not unique to the brains of mammals. Birds also show asymmetries in areas associated with the production of song, and there is a clear hemispheric dominance in the control of song. The lateral asymmetry in birds is maintained down to the motoneurons that innervate the syrinx. This dominance is interesting in light of the fact that the bird's brain is so different in structure from the mammalian brain.
Consciousness, in terms of the state of arousal, is determined by subcortical structures, whereas the content may be more the province of the cortex. Sleep is controlled by brain-stem, hypothalamic and subthalamic mechanisms, but the cortex may also play a role in its regulation. Emotional behaviors, including both sensory and motor aspects, seem to be a function of the limbic system. Learning is a property of the entire nervous system, but instrumental or operant conditioning and discrimination learning can be severely impaired by cortical lesions. Memory consists of both short-term and long-term events. Most damage to memory involves interferences with conversion of short-term to long-term memory, a function that involves the hippocampus. Memory traces may be stored bilaterally, and they can be stimulated during brain surgery causing them to be played back. Personality is determined, at least to some extent, by the prefrontal lobes. Lobotomies produce asocial behavior, distractibility, perseveration, inability to plan ahead and inability to postpone gratification. The two cerebral hemispheres are not equivalent. One is dominant by virtue of its possession of speech centers in Broca's, Wernicke's, and the inferior parietal areas. The nondominant hemisphere, is superior in calculation and recognition of spatial relations. Each hemisphere controls movements of and receives sensory information predominantly from the opposite side of the body. It normally shares its knowledge and control by way of the commissures.