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Зміст11.4 The EEG can be used to monitor the activity of the brain
11.4 The EEG can be used to monitor the activity of the brain
11 Some aspects of higher nervous function
11.6 Learning and memory
11.6 Learning and memory
Memory and brain.
Some aspects of higher nervous function
The brain is not simply concerned with controlling movements and regulating the internal environment. It is also concerned with assessing aspects of the world around us so that we may adjust our behavior to assist our survival. Experiences are assimilated into our memory which can be used to determine some future course of action. We communicate with our fellows via language. To achieve all this we require a certain level of awareness of ourselves and our environment that we call consciousness. This chapter will explore some aspects of these very complex phenomena.
11.2 The specific functions of the left and right hemispheres
The cerebral hemispheres are the largest and most obvious structures of the human brain. As discussed in earlier chapters, they receive information from the primary senses (somatic sensations, taste, smell, hearing, and vision) and control coordinated motor activity. The somatosensory and motor pathways are crossed so that the left hemisphere receives information from and controls the motor activity of the right side of the body, while the right hemisphere is concerned with the left side.
The two hemispheres are not symmetrical morphologically or in their functions. It is commonly recognized that people prefer to use one hand rather than another. Indeed most people (about 90 per cent of the population) prefer to use their right hand for writing, holding a knife, or tennis racquet and so on. What is not always appreciated is that most people also have a preference for using their right foot (for example when kicking a ball) and they generally pay more attention to information derived from one eye and one ear. The two halves of the brain are therefore not equal with respect to the information to which they pay attention or with respect to the activities they control.
In the nineteenth century Paul Broca and Marc Dax described an association between the loss of speech and paralysis of the right side of the body in patients who had suffered a stroke. This showed that the left hemisphere controlled speech as well as the motor activity of the right side of the body. Later studies showed that specific deficits in the understanding of language, reading,
and writing were also associated with damage to the left cerebral hemisphere. Other patients with damaged left hemispheres were unable to carry out specific directed actions, such as brushing their hair, although the muscles involved were not paralyzed. Thus, as the clinical evidence accumulated, it became apparent that the two hemispheres must have different capacities. The idea developed that one hemisphere, the left in right-handed people, was the leading or dominant hemisphere while the other was subordinate and lacked a specific role in higher nervous functions. This idea has had to be modified in the light of current evidence and it is now clear that both hemispheres have very specific functions in addition to the role they play in sensation and motor activity.
The association areas of the frontal cortex
Early exploration of the human cerebral cortex showed that while stimulation of some areas elicited specific sensations or motor responses, stimulation of others had no detectable effect. These areas were called 'silent' areas and were loosely considered as association cortex (Fig. 11.1). Over the past century, the functions of these areas has gradually become clearer.
Fig. 11.1 A side view of the human brain, showing the association areas of the frontal, parietal, and temporal lobes.
Compared to the brains of other animals, including those of the higher apes, the frontal lobes of the human brain are
11 Some ospects of higher nervous function
Fig. 11.2 The skull of Phineas Gage, showing the course of the iron bar (left) and the likely extent of the damage to his frontal cortex.
very large relative to the size of the brain as a whole. Indeed, as Fig. 11.1 shows, the frontal lobes are the largest division of the human cerebral cortex. What is their role? One aspect of their function became clear in the nineteenth century when an American mining engineer, Phineas Gage, received a devastating injury to his frontal lobes (Fig. 11.2). He was packing down an explosive charge when it detonated, driving the tamping iron through his upper jaw, into his skull and through his frontal lobes. Remarkably, he survived the accident. Although he was able to live an essentially normal life, his personality had changed irrevocably. He became irascible, unpredictable, and was much less inhibited in his social behavior. So changed was his behavior that his friends remarked that he was 'no longer Gage'.
Later work on monkeys showed that lesions to the frontal lobes appeared to reduce anxiety. This discovery was subsequently exploited clinically in an attempt to help patients suffering severe and debilitating depression. By cutting the fibers connecting the frontal lobes to the thalamus and other areas of the cortex (a frontal leucotomy) it was hoped to reduce the feeling of desperate anxiety and permit patients to resume a normal life. Early results were encouraging in that the affected patients seemed less anxious than before but it later became clear that the procedure could result in severe personality changes which perhaps might have been predicted from the case of Phineas Gage. Epilepsy (abnormal electrical activity of the brain, see Section 11.4) was another undesirable complication that often developed. In recent years this procedure has become replaced by less drastic and more reversible drug therapies.
Detailed psychological testing of patients who have had a frontal leucotomy showed that, although their general intelligence was little affected, they were not as good at problem solving as people with intact frontal lobes. They tended to persevere with failed strategies. They also appeared to be less spontaneous in their behavior. Thus the association areas of the frontal
lobes are implicated in the determination of the personality of an
individual in all its aspects.
Damage to the parietal lobes leads to loss
of higher-level motor and sensory
The parietal lobes extend from the central sulcus to the parieto-occipital fissure and from an imaginary line drawn from the angular gyrus to the parieto-occipital fissure (Fig. 11.1). As with the frontal lobes, most of our knowledge of the functions of the association areas of the human parietal lobes is derived from careful clinical observation. Damage to these parts of the brain is associated with specific deficits known as agnosias and apraxias.
Agnosia is a failure to recognize an object even though there is no specific sensory deficit. It reflects an inability of the brain to integrate the information in a normal way. For this reason the agnosias are regarded as a failure of 'higher-level' sensory performance. An example is visual object agnosia in which the visual pathways appear to be essentially normal but recognition of objects does not occur. If affected patients are allowed to explore the object with another sense, by touch for example, they can often name it. Nevertheless, they may not be able to appreciate its qualities as a physical object. Thus they may see a chair but not avoid it as they cross a room. Another example is astereo-gnosis, in which there is a failure to recognize an object through touch. This disorder is associated with damage to regions in the parietal lobe adjacent to the primary cortical receiving areas of the post-central gyrus.
Apraxia is the loss of the ability to perform specific purposeful movements even though there is no paralysis or loss of sensation. A patient may be unable to perform a complex motor task on command, e.g. waving someone goodbye, but may be perfectly capable of carrying out the same act spontaneously. Other apraxias may result in a patient being unable to use an everyday object appropriately, or he or she may be unable to construct or draw a simple object (constructional apraxia). A deficit in the control of fine movements of one hand can be caused by damage to the premotor area of the frontal lobe on the opposite side. This is known as kinetic apraxia.
The most bizarre effect of lesions to the parietal lobes is seen when the lesion affects the posterior part of the right hemisphere around the border of the parietal and occipital lobes. These lesions lead to neglect of the left side of the body. Affected patients ignore the left side of their own bodies, leaving them unwashed and uncared for. They ignore the food on the left side of their plates and will only copy the right side of a simple drawing (Fig. 11.3). Many of these patients are blind in the left visual field although they are themselves unaware of the fact.
The corpus callosum plays an essential
role in integrating the activity of the two
Sensory information from the right half of the body is represented in the somatosensory cortex in the left hemisphere, and
11.2 The specific functions of the left and right hemispheres
Fig. 11.3 Drawing by a patient suffering from a stroke in the posterior region of the right hemisphere, leading to the neglect syndrome. The model is on the left of the figure and the patient's drawing is shown on the right. Note the patient's failure to complete the left-hand parts of his drawings.
can involve both sides of the brain. In the search for a cure for the severe bilateral epilepsy experienced by some patients, their corpus callosum was cut—the split-brain operation. This had the desired end-result—a reduction in the frequency and severity of the epileptic attacks—but also offered the opportunity of careful and detailed study of the functions of the two hemispheres of the human brain.
The human split-brain operation showed
that each hemisphere is specialized to
perform a specific set of higher nervous
The key to the first experiments on split-brain patients was to exploit the fact that the visual pathway is only partially crossed at the optic chiasm, while speech is located in the left hemisphere. Fibers from the temporal retina remain uncrossed while those from the nasal region of the retina project to the contra-lateral visual cortex, as shown in Fig. 8.23. The effect of this arrangement is that the right visual field is represented in the left visual cortex while the left visual field is represented in the right visual cortex. By projecting words and images onto a screen in such a way that they would appear in either the right or the left visual field, Sperry and his colleagues were able to investigate the specific capabilities of each hemisphere. The subject could then be asked what he or she had seen.
If, say, the word 'BAND' was briefly projected to the right visual field, the patient was able to report that to the investigator (Fig. 11.4), as this word was represented in the visual cortex of the left hemisphere, which also controls speech. If the left visual field had a qualifying word such as 'HAT', the subject was unaware of the fact. If asked what type of band had been mentioned, they were reduced to guessing. If a word such as 'BOOK'
vice versa. Equally, the left motor cortex controls the motor activity on the right side of the body. Despite this apparent segregation, the brain acts as a whole, integrating all aspects of neural function. This is possible because, although the primary motor and sensory pathways are crossed, there are many cross-connections between the two halves of the brain, known as commissures. As a result, each side of the brain is constantly informed of the activities of the other.
The largest of the commissures is the vast number of fibers that connect the two cerebral hemispheres, known as the corpus callosum (see Chapter 6, Figs 6.2 and 6.3). Damage to the corpus callosum was first reported for shrapnel injuries during the First World War. Amazingly, soldiers with these injuries showed remarkably little by way of a neurological deficit that could be attributed to the severance of so large a nerve tract.
Experimental work has shown that most of the nerve fibers that traverse the corpus callosum project to comparable functional areas on the contralateral side. It was found subsequently that epileptic discharges can spread from one hemisphere to the other via the corpus callosum and that major epileptic attacks
Fig. 11.4 The response of a 'split-brain' patient to words projected onto the left and right visual fields.
the left visual field, they then made a suitable choice with their left hand (i.e. the hand that is controlled from the right hemi-
totally lateralized to the left hemisphere, the right hemisphere was aware of the environment, was capable of logical choice, and possessed simple language comprehension.
In one patient a picture of a nude woman was projected onto the left visual field, there was a strong emotional response. The patient blushed and giggled. When asked what she had seen she replied 'nothing just a flash of light'. When asked why she was laughing, she could not explain but replied, 'Oh Doctor, you have some machine!' From this observation it would appear that emotional reactions begin at a lower level of neural integration than the cortex and involve both hemispheres.
Further studies showed that the right hemisphere could identify objects held in the left hand by their shape and texture
cluded that, far from being subordinare to the left hemisphere, the right hemisphere was conscious and was better at solving
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summarized in Fig. 11.5.
In addition to their role in motor control, the frontal lobes appear to play a significant part in shaping the personality of an individual. Lesions in the parietal lobe result in defects of sensory integration known as agnosias and in an inability to perform certain purposeful acts (apraxia). Severance of the corpus callosum, which interconnects the two hemispheres, has shown that the two hemispheres have very specific capabilities in addition to their role in sensation and motor activity. Speech and language abilities are mainly located in the left hemisphere, together with logical reasoning. The right hemisphere is better at solving spatial problems and nonverbal tasks.
Fig. 11.5 The funcrional specialization of rhe two cerebral hemispheres. (Note that the areas of rhe cortex associated with the primary senses and those involved in motor control are not represenred in this diagram).
Many animals have some means of communicating with their fellows. What distinguishes human communication is its range and subtlety of expression. Humans use language. The production of sound that has no specific meaning is called vocalization. A language consists of a specific vocabulary and a ser of rules of expression (syntax). While most communication is by means of speech, expression in a given language is independent of the mode of communication. This text is written in English and obeys its grammatical rules but the written word is only the same as the spoken word in irs meaning. The representation of a word on the page is arbirrary.
Speech is a very complex skill. It involves knowledge of the vocabulary and grammatical rules of at least one language. It requires very precise motor acts to permit the production of specific sounds in their correct order. It also requires precise regularion of the flow of air through the larynx and mouth. For these reasons it is perhaps not surprising to find that large areas of the brain are devoted to speech and its comprehension. Because it is specifically a human characteristic, our knowledge of the systems that govern the production and comprehension of speech has largely come from careful neurological observations.
The principal areas of the brain
controlling speech are located in the
frontal and temporal lobes
Earlier in this chapter, the work of Broca and Dax was cited as evidence for the lateralization of speech to the left hemisphere. This was based on the study of patients who had difficulty in producing speech following a stroke. Such disabilities are known
Fig. 11.6 A diagram of the left hemisphere to show the location of the principal regions involved in the control of speech. 1 & 2 indicate the postulated connections between the primary visual and auditory receiving areas, and the language areas of the temporal lobe and angular gyrus. 3 indicates the connection between the angular gyrus and Broca's area via the arcuate fasciculus.
as aphasias. Broca studied a number of patients and noted that they had difficulty in producing speech but appeared to be quite capable of understanding spoken or written language. Characteristically their speech is slow, halting, and telegraphic in quality. Such patients are able to name objects and describe their attributes but have difficulty with the small parts of speech that play such an important role in grammar (e.g. 'if, 'is', 'the', and so on). They also have difficulty in writing. Since these patients apparently have a good understanding of language, this type of aphasia is sometimes called expressive aphasia. It is also known as Broca's aphasia.
Broca was able to examine the brains of several aphasic patients at post-mortem. He discovered that there was extensive damage to the frontal lobe of the left hemisphere, particularly in the region that lies just anterior to the motot area responsible for the control of the lips and tongue (Fig. 11.6). This is now known as Broca's area. Patients with Broca's aphasia do not have a paralysis of the lips and tongue. They can sing wordlessly. What they have lost is the ability to use the apparatus of speech to form words, phrases, and sentences.
Another type of aphasia, discovered by Carl Wernicke, was characterized by free-flowing speech that had little or no informational content (technically known as jargon). Patients suffering from this kind of aphasia tend to make up words (neologisms), e.g. 'lork', 'flieber', and often have difficulty in choosing the appropriate words to describe what they mean. They have difficulty in comprehending speech and this type of aphasia is called receptive aphasia or Wernicke's aphasia after its discoverer. Wernicke's aphasia is associated with lesions to the posterior region of the temporal lobe adjacent to the primary auditory cortex and the angular gyrus (Fig. 11.6). Patients suffering from Wernicke's aphasia have difficulty with reading and writing.
The two brain regions responsible for speech are interconnected by a set of nerve fibers called the arcuate fasciculus. If these fibers are damaged, anorher rype of aphasia occurs, called
conduction aphasia. This is typified by the speech characteristics of a Wernicke's aphasic but comprehension of written and spoken language remains largely intact.
Electrical stimulation of the speech areas of the cerebral cortex of conscious human subjects does not lead to vocalization. However, if a subject is already speaking when one of the areas of cortex concerned with language is stimulated, their speech may be interrupted or there may be an inappropriate choice of words. Stimulation of the supplementary motor area on the medial surface of the left hemisphere may lead to speech which is limited to a few words or syllables that may be repeated, e.g. 'ba-ba-ba-'. Unlike lesions to Broca's area and Wernicke's area, lesions to the supplementary motor area do not result in permanent aphasia.
Although the areas that control speech production are located in the left hemisphere, the right hemisphere has a very basic language capability. More significantly, the posterior part of the right cerebral hemisphere seems to play an important role in the interpretation of speech. Unlike the written word, spoken language has an emotional content that reveals itself in its intonation. Consequently, many patients who have damage to their left temporal lobe are able to understand the intention of something said to them even though their comprehension of individual words and phrases is poor. Conversely, patients with damage to their right hemispheres will often speak with a flat monotone.
Fig. 11.7 Variations in local blood flow in a conscious human subject engaged in various language-related tasks.
These observations suggest an organization of the pathways that control speech similar to that shown in Fig. 11.6. According to this model, speech is initiated in Wernicke's area and is passed to Broca's area via the arcuate fasciculus for execution. The neural pathway involved in naming an object that has been seen is also shown in Fig. 11.6 and disconnection of the visual association areas from the angular gyrus will lead to word blind-
188 II Some aspects of higher nervous function
ness or alexia. Word deafness (auditory agnosia) occurs when lesions disconnect Wernicke's area from the auditory cortex. Comprehension of spoken, but not written, language is impaired.
Recent studies with imaging techniques have revealed some other areas of the cortex that are also involved in language (Fig. 11.7). As described above, normal speech involves Wernicke's area, Broca's area, and the premotor area. Reading aloud also involves the visual cortex and a region close to the end of the lateral fissure known as the angular gyrus. The angular gyrus interprets visual information that is then converted to speech patterns by Wernicke's area before being transmitted to Broca's area. Silent reading involves the visual cortex, the premotor area, Broca's area, but not the auditory cortex or Wernicke's area, while silent counting involves the frontal lobe.
Perhaps not surprisingly, careful anatomical examination has found that the upper aspect of the left temporal lobe (the planum temporale) of most brains is larger than that of the right. Despite these important insights, it is important to remember that knowing which parts of the brain are involved in the production of speech and the comprehension of language is not the same as knowing how the brain encodes and executes the patterns of neural activity responsible for speech.
In the majority of people, speech is controlled from the left hemisphere
It is of some importance that a neurosurgeon is aware of which hemisphere controls speech before embarking on an operation. This is established by the Wada test, which temporarily anesthetizes one hemisphere. The patient lies on their back while a cannula is inserted into the carotid artery on one side. The patient is then asked to count backwards from 100 and to keep both arms raised. A small quantity of a short-lasting anesthetic is then injected. As the anesthetic reaches the brain, the arm on the side opposite that of the injection falls and the patient may stop counting for a few seconds or for several minutes, depending on whether speech is localized to the side receiving the injection.
This procedure reveals which hemisphere controls speech in a person who has no aphasia. In 95 per cent of right handers
Table 11.1 The distribution of handedness and the location of speech in the cerebral hemispheres
The table shows the location of speech as a percentage of the total adult population. Thus 85.5% of the population ate tight handed and have theif speech localized to their left hemispheres, and so on.
speech is controlled from the left hemisphere. This is also true of about 70 per cent of left handers. Speech is localized to the right hemisphere in about 15 per cent of left handers and the remainder show evidence of speech being controlled from both hemispheres (Table 11.1).
Speech is localized to the left hemisphere in the majority of the population irrespective of whether they are right or left handed. The neural patterns of speech originate in the temporal lobe in Wernicke's area which is adjacent to the auditory cortex. The neural codes for speech pass via the arcuate fasciculus to Broca's area for execution of the appropriate sequence of motor acts. Damage to the speech areas results in aphasia. Damage to Broca's area in the left frontal lobe results in expressive aphasia. Although patients with damage to Wernicke's are able to speak fluently, they have poor comprehension of speech and their speech lacks clear meaning. This is known as receptive aphasia.
The brain is constantly involved in the control of a huge range of activities, both when awake and during sleep. Its activity can be monitored indirectly by placing electrodes on the scalp. If this is done in such a way as to minimize electrical interference from the muscles of the head and neck, small oscillations are seen that can reflect the overall activity of the brain (Fig. 11.8). These electrical oscillations are known as the electroencephalogram or EEG. For normal subjects the amplitude of the EEG waves ranges from 10 μ-v to over 100 μV.
The activity of the EEG is continuous throughout life but is not obviously related to any specific sensory stimulus. It reflects the spontaneous electrical activity of the brain itself. The appearance of the EEG varies according to the position of the electrodes, the behavioral state of the subject (i.e. whether awake or asleep), the subject's age, and whether there is any organic disease.
The specific state of the EEG is classified by the frequency of the electrical waves that are present. When a subject is awake and alert, the EEG consists of high-frequency waves (20—50 Hz) which have a low amplitude (about 10-20 /iV). These are known as beta waves and they appear to originate in the cerebral cortex. As a subject closes his (or her) eyes, this low-amplitude, high-frequency pattern gives way to a higher-amplitude, lower-frequency pattern known as the alpha rhythm. The alpha waves have an amplitude of 20-40 μV and contain one predominant frequency in the range of 8—12 Hz. As the subject becomes more drowsy and falls asleep, the alpha rhythm disappears and is replaced by slower waves of greater amplitude known as theta waves (40—80 μV and 4—7 Hz). These are interspersed with brief periods of high-frequency activity known as 'sleep spindles'. In very deep sleep the EEG waves are slower still (delta waves, which
Fig. 11.8 Typical stretches of the EEG for various stages of awareness. The top panel (a) shows the changes in the EEG as a subject falls asleep. The lower panel (b) shows alpha block following opening of the eyes. Note the sudden loss of the alpha waves when the eyes open and their resumption when the eyes close again. The two lines in part (b) are of a continuous stretch of record from the occipital region of the left hemisphere. Note that during deep sleep the EEG changes to a pattern similar to that seen in the normal awake state but which has characteristic 'sawtooth' waves. This is the REM phase of sleep.
have a frequency of less than 3 Hz) and they have a relatively high amplitude (100-120/xV). The characteristics of the principal EEG waves are summarized in Table 11.2.
Alpha waves are best seen over the occipital region and they have a characteristic appearance, slowly growing in amplitude to a maximum and then slowly declining ('waxing and waning'). The alpha rhythm is disrupted when subjects concentrate their attention on a problem or when they open their eyes (Fig. 11.8b). This is known as 'alpha block'. The alpha waves appear to be driven by feedback between the cerebral cortex and the thalamus. Theta waves are believed to originate in the hippocampus, while the delta waves probably originate from activity in the brainstem.
Table 11.2 The characteristics of the principal waves of the EEG
Note that the higher the frequency, the lower the amplitude, i.e. the higher frequencies are less synchronized than the low frequencies.
Although detailed interpretation of the EEG is fraught with difficulties, it has proved to be of great practical value in the diagnosis and localization of organic brain disease. For this purpose, many pairs of electrodes are placed over the scalp and the pattern of activity between specific electrode pairs is scrutinized for the presence of abnormal activity. Its greatest use is in the diagnosis of epilepsy, which may be defined as a disorder of
Fig. 11.9 The change in the EEG that can be seen during an epileptic attack, (a) The EEG has a preponderance of delta-wave activity, which is replaced by large spikes during a convulsive seizure, (b) The change in the EEG of another patient during a petit mal seizure. Note the characteristic spike and wave complex.
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