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U9-Lateralization.docx

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Department
Psychology
Course
PSYC 271
Professor
Monica Valsangkar- Smyth
Semester
Winter

Description
U9: LATERALIZATION, LANGUAGE and the SPLIT BRAIN Mar/22/12 1. Cerebral Lateralization of - The brain is actually 2 separate structures except for the cerebral Function – Intro commisures connecting them - Lateralization of function talks about the differences in function and is studied through split brain patients – who had their L and R hemispheres separated by commissurotomy Left Hemisphere Damage to Aphasia and Apraxia - Aphrasia is an inability to produce or understand language o Broca found that there was a relationship between the inferior prefrontal cortex of the left hemisphere known as Broca’s area - Apraxia (Liepmann) is always associated with left-hemisphere damage even though the symptoms involve both sides of the body. - These patients can’t perform movements when asked to perform them but when performing them naturally and without think about it, they could. - These examples provided evidence for cerebral dominance where one hemisphere (usually the left) is the dominant hemisphere in the control of all complex behavioral and cognitive processes. - Right hemisphere is the minor hemisphere 2. Anatomical Asymmetry - Even though the 2 sides of the brain seem to look the same, there are a number of anatomical asymmetries between the two cerebral hemispheres 1. The RH is larger and heavier than the LH but the LH is more densely packed 2. In the temporal lobe, the planum temporale (location of Wernickes area) is larger on the left while the primary auditory cortex or Heschel’s gyrus is larger on the right because there are 2 on the right side o Comparing neurons in wernicke’s area with the same part of the right hemisphere, the areas of both hemispheres are organized into regularly spaced columns of interconnected neurons and the columns are connected by axons o The L side had longer axons and are farther apart. 3. In the thalamus, the lateral posterior nucleus is larger on the left, while the medial geniculate nucleus is larger on the right 4. Tend to have more temporal-parietal area on the right side because the Sylvian fissure extends further back 5. In Broca’s area there is more surface area on the right and more buried area on the left 6. There is asymmetry in the distribution of some neurotransmitters 7. The RH extends further forward and the LH extends further back Asymmetry and Handedness 8. There is also evidence that the cortical microcircuitry differs between comparable areas of the left and right hemispheres, suggesting that cerebral asymmetry is expressed down to the level of how neurons communicate with one another - While everyone shows some brain asymmetry, a higher proportion of left handers show no asymmetry or a reversal of the direction of anatomical asymmetry o Ex. 60% of Dextrals (right) with L hemisphere lesions and 2% with R hemisphere lesions were aphasic while sinestrals (Left) were 30% and 24% respectively. - Left handers have language functions distributed across hemispheres or in right hemisphere. - Corpus callosum is larger in left-handers, and that might also contribute to the greater interaction between the two hemispheres - Recovery from aphasia in left handers are more rapid and complete in left handers because language functions aren’t as lateralized and less sensitive to damage in one particular brain area. Relationship between Speech laterality and Handedness 1. Nearly all (about 95%) right-handed subjects are left-hemisphere dominant for speech Asymmetry and Gender 2. Most left-handed or ambidextrous subjects (about 70%) are also left- hemisphere dominant for speech 3. Early left-hemisphere damage can cause the right hemisphere to become dominant for speech and the left hand to be preferred. - There are anatomical asymmetries that differ between males and females: 1. Planum temporale: Left is larger than right, more often seen in males than in Q: What is the CC? females 2. Sylvian Fissure: Men have a longer horizontal component in the LH than females/ no difference in RH 3. The posterior portion of the CC is significantly larger in females - The brains of males are more lateralized than the brains of females because male victims of L hemisphere strokes had more deficits on WAIS and 3x more likely to suffer from aphasia. Gender differences in Behavior 1. Women have greater verbal ability than men (fluency, analogy, comprehension) 2. Men have greater visuo-spatial abilities (mental rotation, geometry, map reading) 3. Men have greater mathematical ability (very controversial and has been linked to advantages in music composition and chess) 4. Men show greater aggression Explanation of Sex Differences 1. Hormonal effects on brain function - Hormones have an organizing effect during development and men and women have different levels of hormones can lead to differences in brain structure and functioning. - During menstrual cycle as estrogen levels rise, this was correlated with poorer spatial ability but enhanced articulatory and motor abilities. 2. Genetic Sex-linkage - Another theory proposes that a major factor in behavioural sex differences in spatial ability is genetic - Recessive gene on X chromosome responsible for differences 3. Maturation Rate - Girls speak sooner than boys so language areas of the brain are faster to mature in females 4. Environment - Boys are expected to be more adventurous and explore their environment, an activity that improves spatial skills. 5. Preferred Cognitive Mode - Men and women use different strategies when solving problems (women tend to solve them verbally) and this may cause observed sex differences in behaviour. 3. The Split Brain - Showed 2 functions of the Corpus Callosum: Cutting the Corpus Callosum 1. Transfer learned information from one hemisphere to the other Myers & Sperry 2. When cut, each hemisphere can function independently - Trained cats to perform visual discrimination of 2 panels – one with a circle on it and one with a square on it with the positions of these shapes varied from trial to trial - The cats learned which symbol to press in order to get food. - M&S thought that there are 2 ways for visual information to move from one eye to the opposite hemisphere: (1) corpus callosum (2) optic chiasm - Wanted to study one hemisphere at a time so they cut both of these and covered one eye with a patch, restricting all incoming visual information to the hemisphere on the same side as the uncovered eye. Results - All cats learned the discrimination with a patch on one eye even though cutting the optic chiasm created an area of blindness - When the patch was transferred to the other eye for intact cats and control with either the OC or CC transected, they performed the task with close to 100% accuracy - When transferred the patch to the experimental cats, the performance dropped for the hemisphere that hadn’t learned the task, but these cats learned the task as if they had never seen it before. 4. Tests of Cerebral Lateralization Sodium Amytal Test - Administered to patients prior to neurosurgery so that the surgeon knows the side of speech lateralization and can take care not to damage the language areas - Sodium amytal is injected into one carotid artery and anaesthetizes the ipsilateral hemisphere and allows the abilities of the contralateral hemisphere to be assessed - Inject on dominant speech hemisphere (left)  muteness or speech errors - Inject on nondominant speech hemisphere (right)  no mutism and a few speech errors. - A sequence of 3 pairs of digits is presented through earphones, the 2 Dichotic Listening Test digits of each pair are presented simultaneously, one digit to each ear - When the subject is asked to report the six digits, there is a slight but consistent tendency to report more of the digits presented to the ear contralateral (opposite side) to the dominant language hemisphere. o Ex. Right-hemisphere specialization for language performed better with the L ear than the R. - Although the sounds from each ear are projected to both hemispheres, contralateral connections are stronger and take precedence when 2 different sounds are simultaneously competing for access to the same cortical auditory centers. Functional Brain Imaging - PET or fMRI techniques have revealed that there is typically more activity in the left hemisphere than the right - If damage is localized to only one hemisphere, any deficits seen may be a Patients with Unilateral Damage result of lateralization of function o Ex. Distinction has been made in the processing of whole objects versus the parts of those same objects . o One part of the brain is involved in determine the overall shape of a triangle, another part processes the specific details (i.e. dots within triangle) - R hemisphere seems responsible for global visual processing - L hemisphere responsible for local visual processing Split Brain Patients - Have a cut in their corpus collosum to reduce the severity of convulsions Q: What does this mean: - 2 Hemispheres are disconnected from each other Commissurotomized patients have 2 - Typical experiment involves the subject seated in front of a computer independent streams of monitor and an object or word would then appear briefly in either the consciousness: however, unlike split- right or left visual field of the monitor and the subject would report what brain laboratory animals, the two they saw. hemispheres of communisurized - All stimuli presented in the left visual field were transmitted to the right patients are not equal? visual cortex and vice versa. - Fine tactual and motor tasks were performed by each hand under a ledge. o Object in right visual field, the subject says comb because the L hemisphere is dominant for language production o Obj in left visual field, the subject can’t report anything because the R hemisphere is unable to speak and L hemisphere can’t see the stimulus. o The R hemisphere still processes the stimulus because if asked to identify that object by touch with their left hand, they are able to do the task. **Think of this as Ms. R Hemisphere who understands a few simple instructions but can’t speak, who receives sensory information from the left visual field and left hand and who controls the fine motor responses of the L hand. Mr. L hemisphere is verbally adept, receives sensory information from R visual field and R hand and controls motor responses of the R hand. 5. Differences in the L and R Left Right Hemispheres - Better at language-related tasks - Better at understanding spoken Comparing Functions of L and R - Controls ipsilateral body words and grammar Hemispheres movements - Better at spatial ability - Verbal memory - Process emotional stimuli – - Episodic memory facial emotion recognition and - Acts as the Interpreter musical tasks :hypothetical neuronal - Greater role in nonverbal mechanism that continuously memory assesses patterns of events and tries to make sense of them. - Language, emotion, musical ability... are composed of dozens of different individual cognitive activities and there’s no reason to assume each of them is lateralized in a hemisphere. - Thus, complex cognitive tasks – reading, judging space – should be broken down into their consitutent cognitive processes. Evidence of 2 Independent Streams - When an object is presented to the left hemisphere either by touching of Consciousness with the right hand or viewing something in the right visual field, the split-brain patient could pick out the correct object with the right hand - When object is presented to R hemisphere by touching with L hand or viewing something in the L visual field, the patient could pick out the correct object with the L hand but not the R hand o When an apple picture was flashed in the right visual field of a split-brain patient, the L hemisphere could do one of 2 things: 1. L could say it saw a picture of an apple 2. Patient can reach under the ledge with the right hand and feel out the apple. - If the non-speaking R hemisphere were asked to indicate the object that had previously been presented to the left hemisphere, it can’t. - This is communication between hemispheres via a nonneural route – in Cross Cuing one test, a red or green light was flashed in the left visual field and the Q: How does this work? Was shaking split-brain patient was then asked to name the color: red or green, most his head and frowning the cue to the patients only get 50% right left? - However one subject performed almost perfectly when he initially said the incorrect color using his left hemisphere, his head shook and the patient changed his guess to the correct color because the right hemisphere heard the left’s incorrect guess of the left and was signalled to the left hemisphere that it was wrong by shaking the person’s head. - If a split brain patient is presented with 2 objects at the same time, one on Learning 2 things at once each visual field, they can reach into 2 different bags at the same time and pull out the two objects. - Occurs when the 2 hemispheres are presented with different information about the correct choice and then asked to reach out and pick up the Helping Hand Phenomenon correct object from a collection in full view Q: Why does the right hand act first? - Right hand pick out what left hemisphere saw but the right hemisphere seeing what it thinks is an error being made causes the left hand to grab the right and pull it to the other object. - 6. Experimentation with Split Brain Patients Lateralization of Attention - Used split-brain patients and used a visual search task in which the items Luck, Hillyard, Mangun, Gazzaniga were rectangles made from red and blue squares - When the blue square was placed immediately above the red square, it was a distractor item and when the squares were reversed, the red square was a target item. - Stimuli was presented unilaterally or bilaterally in sets of 2,4, and 8 - The subjects had to decide whether the target item was in display and if so in which visual field - Control – no significant differences between reaction time functions for the unilateral arrays as compared to the bilateral arrays - Split brain patients – faster for bilateral arrays, each hemisphere was able to conduct an independent serial search. Guided Visual Task - Split brain patient searched for a target black circle among distractor black squares, grey circles and grey squares, the reaction times presented in R visual field were faster than for L, suggesting that L hemisphere is specialized for processing stimuli with specific shared features. - When disconnected, the 2 hemispheres may work differently depending on the type of orienting required. - With reflexive/ exogenous orienting there may be independence between the left and right hemispheres and can work in parallel Faces - With endogenous Levy, Trevarthen, Sperry (voluntary) orienting, the hemispheres may actually compete with each other - Chimeric figures test: Visual images are presented to the subject that consist of faces and other patterns that have been split down the center and then recombined with other images - Patients did not realize any discordance and when asked to pick out the face they had seen, they Z Lens always chose the one in their left visual field (RH) - When Archimbaldo paintings were shown, a split-brain patient’s left Q: What relationship does using a Z brain could only see the objects that made up the face while the right lens have to do with hearing speech brain could see the face. in one ear? - Visual completion is when people with scotomas can fill out the missing visual field - The Z lens limits visual input to one hemisphere of split-brain patients - It is a contact lens that is o
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