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Chapter 11&12

Chapter 11 & 12 Notes

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Zachariah Campbell

Chapter 11: Cerebral Asymmetry • Inability to produce or to understand language is called global aphasia. • The single most curious feature of human brain organization is cerebral asymmetry: the left and right cerebral hemispheres have partly separate functions. As described in the preceding Portrait, cerebral asymmetry was especially apparent in M. S.’ s loss of language skills but maintenance of musical skills. Anatomical Asymmetry in the Human Brain • Laterality: the idea that the two cerebral hemispheres have separate functions. • The left hemisphere plays a special role in producing and understanding language and in controlling movement on the right side of the body, whereas the right hemisphere specializes in perceiving and synthesizing nonverbal information, including music and facial expression. And the right hemisphere controls movement on the left side of the body. Four variables complicate the research on laterality: 1. Laterality is relative, not absolute. Both hemispheres play a role in nearly every behaviour; thus, although the left hemisphere is especially important for the production of language, the right hemisphere also has some language capabilities. 2. Cerebral site is at least as important in understanding brain function as cerebral side. The frontal lobes are asymmetrical, but their functions are more similar to each other than they are to those of the posterior cortex on the same side. 3. Laterality is affected by environmental and genetic factors. 4. Laterality is exhibited by a range of animals. Cerebral Asymmetry • Also called Wernicke’s area, the planum temporale lies just posterior to the primary auditory cortex (Heschl’s gyrus) within the Sylvian, or lateral, fissure. On average, in 65 of the 100 brains studied by Geschwind and Levitsky, the planum temporale in the left hemisphere was nearly 1 cm longer than that in the right hemisphere. • In contrast, the neighbouring primary auditory cortex of Heschl’s gyrus is larger in the right hemisphere because there are usually two Heschl’s gyri in the right hemisphere and only one in the left • MRI scans of living brains confirm eight major anatomical differences be-tween the two hemispheres: 1. The right hemisphere is slightly larger and heavier than the left, but the left contains more gray matter relative to white matter. 2. The temporal lobes display a marked structural asymmetry that may provide an anatomical basis for the observed specialization of the left and right temporal lobes in language and in music functions. 3. The asymmetry in the cortex of the temporal lobes is correlated with a corresponding asymmetry in the thalamus. This anatomical asymmetry complements an apparent functional asymmetry in the thalamus, the left thalamus being dominant for language functions. 4. The slope of the lateral fissure is gentler on the left hemisphere than on the right. 5. The frontal operculum (Broca’s area) is organized differently on the left and right. The area visible on the surface of the brain is about one- third larger on the right than on the left, whereas the area of cortex buried in the sulci (ridges) of the region is greater on the left than on the right. 6. The distribution of various neurotransmitters is asymmetrical, in both the cortical and the subcortical regions. The particular asymmetries in the distribution of acetylcholine, gamma- aminobutyric acid (GABA), norepinephrine, and dopamine depend on the structure under consideration. 7. The right hemisphere extends farther anteriorly than does the left, the left hemisphere extends farther posteriorly than does the right, and the occipital horns of the lateral ventricles are five times as likely to be longer on the right as on the left. 8. The details of anatomical asymmetry are affected by both sex and handedness. Neuronal Asymmetry • Arnold Scheibel and his colleagues compared the dendritic fields of pyramidal cells in Broca’s area, the left frontal operculum ( LOP), with those in the facial area of the motor cortex in the left precentral cortex ( LPC) and with homologous regions in the right hemisphere. • Their results show that the neurons in each of these regions have distinct patterns of dendritic branching, • The degree or pattern of branching is important, because each branch is a potential location for the enhancement or suppression of the graded potentials in the dendritic tree. Thus, more branch points allow more degrees of freedom with respect to the final activity of the cell. Genetic Asymmetry • researchers suggest that some of the genes may regulate the production of growth factors that would, in turn, facilitate the development of specific regions in one hemisphere or the other. A provocative idea is that the asymmetrical expression of genes may ac-count for functional properties such as handedness, which to date has no known basis. Asymmetry in Neurological Patients Patients with Lateralized Lesions • In the strongest experimental method for demonstrating the lateralization of function, called double dissociation by Hans- Leukas Teuber, two areas of the neocortex are functionally dissociated by two behavioural tests. Each test is affected by a lesion in one zone but not in the other. • Lesions in the left hemisphere of right- handed patients consistently produce deficits in language functions ( speech, writing, and reading) that are not produced by lesions in the right hemisphere. Thus, the functions of the two hemi-spheres are dissociated. However, performing spatial tasks, singing, playing musical instruments, and discriminating tonal patterns are more disrupted by right- hemisphere than by left- hemisphere lesions. Because right- hemisphere lesions disturb tasks not disrupted by left- hemisphere lesions and vice versa, the two hemispheres are doubly dissociated. • Epileptic seizures may begin in a restricted region of one hemisphere and then spread through the fibers of the corpus callosum ( the commissure) to the homologous location in the opposite hemisphere. To prevent the spread of a seizure when medication has failed to impose control, commissurotomy, the surgical procedure of disconnecting the two hemispheres by cutting the 200 million nerve fibers of the corpus callosum, was performed first in the early 1940s by William Van Wagnen, an American neurosurgeon Brain Stimulation • In the early 1930s, Wilder Penfield and his associates at the Montreal Neurological Institute pioneered the use of surgical treatment for epilepsy in patients whose seizures were poorly controlled by drug therapy. The logic of this procedure is to remove the region of cortex where the abnormal neural discharge originates. Because this therapeutic surgery is elective, it can be planned for, and considerable care is taken to ensure that areas of the cortex critical for the control of speech and movement are not damaged. • Applying an electrical current to the cortex of a conscious patient has four general effects, three excitatory and one inhibitory: - Stimulation can produce localized movements, localized dysthesias (numbness or tingling in the skin), light flashes, or buzzing sensations. These effects are normally evoked from primary motor, somatosensory, visual, and auditory areas and pathways, respectively, and are produced by the stimulation of either hemisphere with about the same frequency. - Stimulation can produce what Penfield called “ interpretive” and “ experiential” responses. These uncommon but often highly reliable phenomena include alterations in the interpretation of the patient’s surroundings, such as deja vu, fear, and dreaming states, and the reproduction of visual or auditory aspects of specific earlier experiences. That is, patients report specific “ memories” in response to specific stimulation. These phenomena usually arise from tissue showing epileptogenic discharge, but their occurrence reveals an asymmetry: stimulation of the right temporal lobe produces these phenomena more frequently than does stimulation of the left temporal lobe, suggesting that the right hemisphere has perceptual functions not shared by the left hemisphere. - Stimulation of the left frontal or temporal regions may accelerate speech production. - Stimulation blocks function. This inhibitory effect is most evident in complex functions such as language and memory and is apparent only when current is applied while a patient is actively engaged in these behaviours. Stimulation of the same site in a quiet patient has no discernible effect. Carotid Sodium Amobarbital Injection • Language is usually located in the left hemisphere but, in a small percentage of people, most of them left- handed, language is represented in the right hemi-sphere. In the event of elective surgery, preventing inadvertent damage to the speech zones requires that the surgeon be certain of their location. To achieve certainty in doubtful cases, Jun Wada pioneered the technique of injecting sodium amobarbital into the carotid artery to produce a brief period of anesthesia of the ipsilateral hemisphere. • The Wada test results in an unequivocal localization of speech, because injection into the speech hemisphere results in an arrest of speech lasting up to several minutes; as speech returns, it is characterized by aphasic errors. Injection into the nonspeaking hemisphere may produce no speech arrest or only brief arrest. The advantage of this procedure is that each hemisphere can be studied separately in the functional absence of the other, anesthetised one. Asymmetry in the Visual System • Stimuli in the right visual field travel to the left visual cortex, whereas stimuli in the left visual field project to the right visual cortex. With the use of a special instrument called a tachistoscope, visual information can be presented to each visual field independently. • The simple conclusion to be drawn from the results of more than 50 years of tachistoscope studies is that information presented to only one visual field is processed most efficiently by the hemisphere that is specialized to receive it. Words presented to the verbal left hemisphere, therefore, are processed more efficiently than are words presented to the nonverbal right hemisphere. Asymmetry in the Auditory System • The auditory system is not as completely crossed as the visual, because both hemispheres receive projections from each ear. The crossed auditory connections are more numerous, however, and more rapidly conducting than the ipsilateral projections. • Pairs of spoken digits ( say, “ two” and “ six”) were presented simultaneously through headphones, but one digit only was heard in each ear. The subjects heard three pairs of digits and then were asked to recall as many of the six dig-its as possible, in any order. Kimura noticed that subjects recalled more digits that had been presented to the right ear than had been presented to the left. • This result led Kimura to propose that, when different stimuli are presented simultaneously to each ear, the pathway from the right ear to the speaking hemisphere has preferred access, and the ipsilateral pathway from the left ear is relatively suppressed. Thus, during a dichotic task, the stimulus to the left ear must travel to the right hemisphere and then across the cerebral commissures to the left hemisphere. This longer route puts the left ear at a disadvantage, and words played to the right ear are recalled more accurately. • Patients with damage to the corpus callosum exhibit an almost complete inhibition of words presented to the left ear, even though they can recall words presented to this ear if there is no competing stimulus to the right ear. • The Kimura experiments imply that the left hemisphere is specialized for processing language- related sounds, whereas the right hemisphere processes music- related sounds. There is, however, another interpretation: the asymmetry could be related to the temporal or spectral structure of the sounds— their rhythm and frequency— rather than to language and music themselves. • By blindfolding subjects and requiring them to perform various tasks separately with each hand, for example, investigators can identify differences in each hand’s efficiency— differences that can be taken to imply functional asymmetry in cerebral organization. One line of somatosensory research compares the performance of the left and right hands in the recognition of shapes, angles, and patterns. The left hand of right- handed subjects is superior at nearly all tasks of this type. Both blind and sighted subjects read Braille more rapidly with the left hand (Rudel et al.). Some children are fluent readers with the left hand but are totally unable to read with the right. Because Braille patterns are spatial configurations of dots, this observation is congruent with the pro-posed right- hemisphere role in processing spatial information that is not shared by the left hemisphere. • A second type of somatosensory test employs an analogue of the dichotic- listening procedure, the dichaptic test. Subjects feel objects, then look at an array of objects and select those that they previously touched. Asymmetry in the Motor System • Neuroscientists have long known that left- hemisphere lesions can produce apraxia— severe deficits in copying sequences of movements. The logic of studying asymmetry in intact sensory systems makes it seem reasonable to look for asymmetries in motor control. • Two different types of experiments have been devised to assess motor asymmetries: (1) direct observation and ( 2) interference tasks. Direct observation • The primary interest in most imaging studies is the localization, rather than the lateralization, of functions. Because both hemispheres are scanned, however, left– right differences in cerebral activation can be assessed during a large range of behavioural measures. Neuroimaging and Asymmetry • The changes in cerebral perfusion during cognitive tasks that underlie fMRI result in alterations of blood- flow velocities in the feeding basal arteries. The changes in blood flow in these arteries can be measured with the use of a procedure known as functional transcranial doppler ultrasonography ( fTCD). Stefan Knecht and colleagues have shown that the changes in blood- flow velocity in the basal arteries can be used to identify the language- dominant hemisphere. Theoretical Arguments: What Is Lateralized? • In right- handed people, the left hemisphere has a greater role in language and in the control of complex voluntary movements than does the right hemi-sphere, and the right hemisphere has a greater role in the control of certain visuospatial and nonverbal abilities. • An enormous number of proposals have been made on what is lateralized in the brain. At the broadest level, these theories fall into two groups: specialization theories propose unique functions for each hemisphere, and interaction theories propose cooperation between the two hemispheres. Specialization Models • Although the left hemisphere mediates verbal function, it is specialized not for verbal function itself but rather for certain kinds of motor function, both verbal and nonverbal. Kimura’s argument is based on two premises: 1. Lesions of the left hemisphere disturb the production of voluntary movement— an impairment correlated with disturbance in speech. 2. Verbal communication among humans evolved from a stage that was primarily gestural, though with vocal concomitants, to one that is primarily vocal but that retains the capacity for gestural communication. Because the neurological control of speech and language thus evolved out of a manual system of motor control, the left hemisphere is specialized not for language itself but rather for motor control. • The left hemisphere operates in a more logical, analytical, computer- like fashion, analyzing stimuli input sequentially and abstracting the relevant details to which it attaches verbal labels. The right hemisphere is primarily a synthesizer, more concerned with the overall stimulus configuration, and organizes and processes information as gestalts, or wholes. • To account for these differences, Semmes argued that a person with a small lesion in the right hemisphere exhibits no deficits, because specific functions are not localized in discrete regions in the right hemisphere, the functions being diffusely represented. A large lesion of the right hemisphere produces many more deficits than would be predicted from the total of smaller lesions because an en-tire functional field is removed. A large lesion of the left hemisphere produces many deficits simply because many small focal regions have been destroyed; that is, in the left hemisphere, the total is equal to the sum of the parts. Interaction Models • All interaction models have in common the idea that both hemispheres have the capacity to perform all functions, but they do not. The specific reasons “ why not” have spawned debates, experiments, and models. Three versions of the interaction model are: 1. The two hemispheres function simultaneously but work on different aspects of processing. This version is a direct analogue of the multiple-channel idea of sensory processing but takes it one step further, proposing that the two hemispheres represent a class of sensory channel. Although simultaneous processing is generally appealing as a model, this hypothesis has yet to offer a satisfactory explanation of how information is combined into a single percept or behaviour. 2. An entire group of interaction models proposes that, although the two hemispheres have the capacity to perform a given function, they inhibit or suppress each other’s activity. 3. Interaction models based on information processing suggest either that the two hemispheres receive information preferentially and thus perform different analyses simultaneously or that some mechanism enables each hemisphere to “ pay attention” to specific types of information, thus leading to different hemispheric analyses Preferred Cognitive Mode • From the preceding theoretical arguments, we can speculate that individual differences in the behaviour of normal subjects result, at least in part, from individual differences in how the cerebral hemispheres are organi
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