Cognitive Neuroscience

Transcription

1 Gazzaniga Ivry Mangun Cognitive Neuroscience FOURTH EDITION Chapter 4 Hemispheric Specialization

2 Science debate: Is it true?

3 Are the followings true? Left and right hemispheres perform different functions. Persons can be divided into left-brained and right-brained. Left-brained persons are analytic and right-brained persons are creative.

4 Different dominant functions in both hemispheres, but both are required to perform many functions

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7 Split-brain research Split-brain research is a neuroscience subfield to investigate the cognitive and behavioral processes of the patients when their corpus callosum connecting the two hemispheres of the brain is severed to some degree by accidental damage or surgery. It is an association of symptoms produced by disruption of or interference with the connection between the hemispheres of the brain.

8 Because our speech center is in the left hemisphere, W.J. can name items presented to his right visual field Speech center

9 A picture was flashed to his left visual field, he responded not to see anything. He responded by pressing the key with his left hand (the right hemisphere controls the left hand), when a light was flashed to his left visual field.

10 The motor apparatus in the left hemisphere which controls the right hand is disconnected to left visual field, right hand do not respond appropriately.

11 The pattern in red on the right is the shape that the patient is trying to create with the blocks given to him. (a) With his right hand (left hemisphere), he is unable to duplicate the pattern. (b) With his left hand (right hemisphere), he is able to perform the task correctly.

12 The central nervous system is laterally specialized: Each of the two cerebral hemispheres performs processes that the other does not.

13 The Wada test, (intracarotid sodium amobarbital procedure, ISAP) This test is named after Japanese Canadian neurologist Juhn Atsushi Wada, is used to establish cerebral language and memory representation of each hemisphere. This test is performed to determine in which hemisphere speech center is located.

14 Wada test procedure The test is conducted with the patient awake. Essentially, a barbiturate (GABAA receptor agonist, which is usually sodium amobarbital) is introduced into one of the internal carotid arteries via a cannula or intra-arterial catheter from the femoral artery. The drug is injected into one hemisphere at a time. The effect is to shut down any language and/or memory function in that hemisphere to evaluate the other hemisphere ("half of the brain"). Then the patient is engaged in a series of language and memory related tests. The memory is evaluated by showing a series of items or pictures to the patient so that within a few minutes as soon as the effect of the medication is dissipated, the ability to recall can be tested.

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16 Strong bias for language lateralization to the left hemisphere Regions of the right hemisphere are also engaged, especially for language tasks that require higher-level comprehension. Language and visuospatial activations in left-handers with typical and atypical language lateralization.

17 Lessons from Split-brain Most cognitive processes are redundant and each hemisphere is capable of carrying out those processes. Two separate, but coexisting brains do not result in split personalities, nor do they fight over control of the body. In short, the individual with the split brain does not feel conflicted. However, the two hemispheres do not represent information in an identical manner.

18 Anatomy of the hemispheres

19 The hemispheres of the brain are distinct yet connected. In the medial view are seen the commissures, the large white matter fiber tracts that connect the hemispheres.

20 The right protrudes in front, and the left protrudes in back. The right is chubbier in the front region, and the left is larger posteriorly in the occipital region.

21 Sylvian fissure or Lateral fissure

22 Sylvianfissure structure is different for both hemispheres.

23 Planum temporale The cortical area at the center of Wernicke s area (understanding of language) was larger in the left hemisphere.

24 Asymmetry in homotopic areas The left hemisphere has greater high-order dendritic branching than that of their homologs in the right hemisphere. Greater long-range connectivity in the language-associated regions of the left hemisphere. cf. homotopic: occurring at the same place upon the body.

25 Visual examination reveals a subtle difference in the sizes of the largest subgroups of layer III pyramidal cells (stained here with acetylthiocholinesterase): in the left hemisphere they are larger (b) compared to the right (a).

26 How do two hemispheres interact each other?

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31 The corpus callosum is the dense fiber tract (white matter) located below the folds of the cortex (250 million axonal fibers). The anterior portion is the genu, the middle portion is the body, and the posterior portion is the splenium.

32 Smaller fibers (~0.4micrometer) are located anteriorly (genu), fitfully grading to larger fibers (5 micrometer) located more posteriorly (splenium).

33 Callosal fiber bundles projecting into the prefrontal lobe (coded in green), premotor and supplementary motor areas (light blue), primary motor cortex (dark blue), primary somatosensory cortex (red), parietal lobe (orange), occipital lobe (yellow), and temporal lobe (violet).

34 The caudal surface of a coronal section of brain roughly through the premotor cortical area. Homotopic callosal fibers (blue) connect corresponding sections of the two hemispheres via the corpus callosum; heterotopic connections (green) link different areas of the two hemispheres of the brain. In primates, both types of contralateral connections (blue and green), as well as ispilateral connections (red), start and finish at the same layer of neocortex.

35 Interhemispheric communication: cooperation or competition? Cooperation Perceptual categorization (right) vs. sementic categorization (left) Competition Primary mode of corpus callosum: inhibition Processing delays inherent in transcollosal communication (175mm, 6.5m/s = 30ms) may limit the extent to which the two hemispheres can cooperate.

36 Synchronization between cortical neurons in two hemispheres

37 Synchronization between cortical neurons in two hemispheres (a) When receptive fields (1 and 2) on either side of fixation are stimulated by two separate light bars moving in different directions (as indicated by the arrows), the firing rates of the two cells are not correlated. (b) In animals with an intact corpus callosum, cells with spatially separate receptive fields fire synchronously when they are stimulated by a common object, such as a long light bar spanning both fields. (c) In animals whose corpus callosum has been severed, synchrony is rarely observed.

38 Split brain research for interhemispheric communications

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40 Callosal section for epileptic seizure

41 The split-brain patient reports through the speaking hemisphere only the items flashed to the right half of the screen and denies seeing left-field stimuli or recognizing objects presented to the left hand. Nevertheless, the left hand correctly retrieves objects presented in the left visual field, about which the patient verbally denies knowing anything.

42 MRI scan showing that the splenium (arrow) was spared in the split-brain procedure performed on this patient. As a result, visual information can still be transferred between the cerebral hemispheres.

43 Schematic representation of split-brain patient J.W. s naming ability for objects in the left visual field at each operative stage.

44 Functional consequences of the split brain procedure Visual or tactile information presented to one half of the brain was not available to the other half. Verbal IQ and problem solving capacity is intact after callosotomy. Visual, tactile, and auditory sensory information is transferred by posterior half of callosum. The anterior part of callosum transfers semantic information about the stimulus.

45 Letter priming: response latency between compatible (h-h) versus incompatible (t-h) trials The latencies for both types of trials are much longer for the left visual field (right hemisphere).

46 The right hemisphere is capable of understanding language but not syntax. When presented with a horse stimulus in the left visual field (right hemisphere), the subject maintains through the left hemisphere that he saw nothing. When asked to draw what goes on the object, the left hand (right hemisphere) is able to draw a saddle.

47 The capacity of the right hemisphere to make inferences is extremely limited. Two words are presented in serial order, and the right hemisphere (left hand) is simply required to point to a picture that best depicts what happens when the words are causally related. The left hemisphere finds these tasks trivial, but the right cannot perform the task.

48 Data from three patients show that the right hemisphere is more accurate than the left in recognizing unfamiliar faces.

49 Emotional prosody Emotional Prosody is characterized as an individual's tone of voice in speech that is conveyed through changes in pitch, loudness, timbre, speech rate, and pauses which is different from linguistic and semantic information. It can be isolated from linguistics and interacts with verbal content (i.e. sarcasm). It is perceived or decoded slightly worse than facial expressions but accuracy varies with emotions. Anger and sadness are perceived most easily followed by fear and happiness, with disgust being the most poorly perceive.

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51 The brain in vocal emotion Language can be split into two components: the verbal and vocal channels. The verbal channel is the semantic content made by the speaker's chosen words. In the verbal channel, the semantic content of the speakers words determines the meaning of the sentence. The way a sentence is spoken however, can change its meaning which is the vocal channel. This channel of language conveys emotions felt by the speaker and gives us as listeners a better idea of the intended meaning. Nuances in this channel are expressed through intonation, intensity, a rhythm which combined for prosody. Usually these channels convey the same emotion, but sometimes they differ. Sarcasm and irony are two forms of humor based on this incongruent style.

52 Emotional prosody: The brain in vocal emotions Neurological processes integrating verbal and vocal (prosodic) components are relatively unclear. However, it is assumed that verbal content and vocal are processed in different hemispheres of the brain. Verbal content composed of syntactic and semantic information is processed the left hemisphere. Syntactic information is processed primarily in the frontal regions and a small part of the temporal lobe of the brain while semantic information is processed primarily in the temporal regions with a smaller part of the frontal lobes incorporated.

53 Emotional prosody: The brain in vocal emotions In contrast, prosody is processed primarily in the same pathway as verbal content, but in the right hemisphere. Neuroimaging studies using functional magnetic resonance imaging (fmri) machines provide further support for this hemisphere lateralization and temporo-frontal activation. Some studies however show evidence that prosody perception is not exclusively lateralized to the right hemisphere and may be more bilateral. There is some evidence that the basal ganglia may also play an important role in the perception of prosody.

54 Visuospatial task: self vs. familiar other The image on the far left contains 10% M.G. and 90% J.W. and changes in 10% increments from left to right, to 90% M.G. and 10% J.W. on the far right. The two original photographs of M.G. and J.W. pictured above and these nine morphed images were presented to each hemisphere randomly.

55 The proportion of yes responses to recognition judgments are plotted on the y-axis as a function of the percentage of the individual contained in the image and the cerebral hemisphere to which the image was presented.

56 Voluntary expressions that can signal intention have their own cortical networks in humans.

57 The neural networks for spontaneous expressions involve older brain circuits and appear to be the same as those in chimpanzees. (inset) The location of the section that has been overlaid onto each face.

58 The patient in the upper row suffered brain damage to the right hemisphere. (a) The lesion did not interfere in spontaneous expression but (b) it did interfere with voluntary expression. (c) This Parkinson s disease patient has a typical masked face. Because Parkinson s disease involves the part of the brain that controls spontaneous facial expression, the faces of these patients, when they are told to smile (d), light up because the other pathway is still intact. Spontaneous smile Voluntary smile

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63 Split brain patient P.S. His left hemisphere had seen a chicken claw and his right hemisphere had seen a snow scene. When asked to point to a picture associated with the image he had just seen, his right hand (guided by his left hemisphere) pointed to the chicken (to go with the claw), and his left hand pointed to the shovel ( to clean out the chicken shed ).

64 (a) Drawing of the LVF word Toad (ipsilateral to the drawing hand). (b) Drawing of the RVF Saw (contralateral to the drawing hand). (c) Drawing combining both words: Scraper and Sky (ipsilateral + contralateral).

65 Cross-integration of spatial information. (a) On within-field trials, the eye moved to the stimulus that was surrounded by the probe. (b) On between-field trials, the eye moved to the corresponding stimulus in the other hemifield.

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67 As more items are added to a set, for split brain patients the increase in reaction time for bilateral arrays is only half as fast as when all objects are confined to one side.

68 Hierarchical structure Global and local representations. We represent information at multiple scales. At its most global scale, this drawing is of a house. We can also recognize and focus on the component parts of the house.

69 Navon Letter task Navon task uses local and global stimuli to investigate hierarchical representation. Each stimulus is composed of a series of identical letters whose global arrangement forms a larger letter. The participants task is to indicate whether the stimulus contained an H or an L. When the stimulus set included competing targets at both levels (b), the participants were instructed to respond either to local targets only or to global targets only. Neither target is present in (e).

70 The left hemisphere is more adept at representing local information and the right hemisphere is better with global information.

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72 Extreme failures of hierarchical processing following brain damage. Two patients were asked to draw the two figures shown in the left column of each panel. The patient with right-hemisphere damage was quite accurate in producing the local element the Z in (a) or the square in (b) but failed to arrange these elements into the correct global configuration. The patient with left-hemisphere damage drew the overall shapes but left out all of the local elements. Note that for each patient, the drawings were quite consistent for both the linguistic (a) and the nonlinguistic (b) stimuli, suggesting a task-independent representational deficit.

73 Theory of mind: Ann and Sally experiment Theory of mind is on the right hemisphere.

74 The attribution of beliefs is located in the right hemisphere s temporal parietal junction.

75 Competing messages are presented, one to the left ear and one to the right ear. Auditory information is projected bilaterally. Although most of the ascending fibers from the cochlear nucleus project to the contralateral thalamus, some fibers ascend on the ipsilateral side.

76 Participants are asked either to report the stimuli or to judge whether a probe stimulus was part of the dichotic message. Comparisons focus on whether they heard the reported information in the right or left ear, with the assumption that the predominant processing occurred in the contralateral hemisphere. With linguistic stimuli, participants are more accurate in reporting the information presented to the right ear.

77 Participants listened to a dichotic message in which each ear was presented with a series of letters sung to short melodies. When given a recognition memory test, participants were more accurate on the letters task for stimuli heard in the right ear. In contrast, a left-ear advantage was observed when the participants were tested on the melodies.

78 애국가 1. 동해물과백두산이마르고닳도록하느님이보우하사우리나라만세무궁화삼천리화려강산대한사람대한으로길이보전하세 3. 가을하늘공활한데높고구름없이밝은달은우리가슴일편단심일세무궁화삼천리화려강산대한사람대한으로길이보전하세 2. 남산위에저소나무철갑을두른듯바람서리불변함은우리기상일세무궁화삼천리화려강산대한사람대한으로길이보전하세 4. 이기상과이맘으로충성을다하여괴로우나즐거우나나라사랑하세무궁화삼천리화려강산대한사람대한으로길이보전하세

79 What determines hemispheric asymmetry or handedness?

80 This organization indicates that there is little overlap in the regions of space seen by each eye, and thus the visual input to the left hemisphere is independent of the visual input to the right hemisphere. This anatomical segregation would be expected to favor the emergence of hemispheric asymmetries.

81 A picture was flashed to his left visual field, he responded not to see anything. He responded by pressing the key with his left hand (the right hemisphere controls the left hand), when a light was flashed to his left visual field.

82 Functional asymmetries in manual coordination are sometimes attributed to the prenatal environment of the fetus. The position of the fetus in the uterus is thought to influence prenatal vestibular experience. Most fetuses are oriented with the right ear facing outward, resulting in different vestibular signals between two hemispheres. At birth, the left side of the body is more stable, freeing the right hand for exploration.