1 Videos The Brain: Teaching Modules. (1997): 5. The Divided Brain (definitely show this short movie to introduce Gazzaniga and corpus callosum resection): Jill Bolte Taylor: My stroke of insight (TED talk)
2 Left hemisphere vs. right hemisphereMorphologically the left hemisphere and right hemisphere are very similar. Connected by corpus callosum. The corpus callosum is the largest and most prominent bundle of axonal projections in the brain. It consists of approximately 200 million axons connecting the left and right cerebral hemispheres. The myelination of the corpus callosum is relatively slow. It is one of the last areas of the brain to undergo myelination, and myelination is not complete until at least the time of puberty (Gbedd JN, 1999; Giedd, JN, 1996). As a result, the brain of a young child is similar to that of a split-brain patient: some types of interhemispheric signal transfer are significantly inferior to that in an adult brain. For example, children have a greater difficulty matching tactile designs between hands than within hands (Hellige, 1993). In addition, children younger than 10 months are not able to recognize a face shown to only one hemisphere with the use of the other hemisphere, which indicates an inability to transfer information about the face over the corpus callosum (Schonen & Mathivet, 1990).
3 Asymmetry in morphology between hemispheresTwo small asymmetries were noticed in the structure of Wernicke’s area: The cortical minicolumns (~40 µm wide vertical columns through the cortical layers of the brain, comprising 80–120 neurons) are wider and the distance between the columns is greater on the left side of the human brain than on the right side. In chimpanzees and rhesus monkeys the cortical minicolumns and intercolumnar spaces are the same size on both sides of the brain. This asymmetry is considered an indication of the greater connectivity of Wernicke’s area in humans.
4 How can the two hemispheres operate with one common goal?
5 Circulating hormones?
6 The Hensel girls are the rarest form of conjoined twins, the result of a single fertilised egg which failed to separate properly in the womb. They have two spines (which join at the pelvis), two hearts, two oesophagi, two stomachs, three kidneys, two gall bladders, four lungs (two of which are joined), one liver, one ribcage, a shared circulatory system and partially shared nervous systems. From the waist down, all organs, including the intestine, bladder and reproductive organs, are shared. While they were born with three arms, one was removed surgically. Although Brittany - the left twin - can't feel anything on the right side of the body and Abigail - the right twin - can't feel anything on her left, instinctively their limbs move as if co-ordinated by one person, even when typing s on the computer. They can ride a bike!
7 Asymmetry in function: Language lateralizationBroca's area and Wernicke's area are located in the left cerebral hemisphere for about 95% of right-handers, but about 70% of left-handers. The right hemisphere can have a robust lexicon (primarily nouns) Usually only the left hemisphere possess mental synthesis necessary to understand syntax and grammar: e.g., “place the notebook under the book”, “the father’s bother”, “the brother’s father”. The right hemisphere controls prosody (intonation, emphasis, and rhythm of speech).
8 Garrett: Brain and Behavior 4eSign Language: Language Areas are similar in Hearing and Deaf Individuals Spoken (left) and signed (right) language processed in same areas of brain Yellow areas were significantly activated and those in red more so. FIGURE LEGEND: (a) fMRI results while hearing subjects read written English. (b) Activation in subjects deaf from birth while processing sign language. Yellow areas were significantly activated and those in red more so. SOURCE: Reprinted with permission from H. J. Neville et al., “Cerebral Organizations for Language in Deaf and Hearing Subjects: Biological Constraints and Effects of Experience,” Proceedings of the National Academy of Sciences, USA, 95, pp. 922–929. Copyright 1998 National Academy of Sciences, USA. Garrett: Brain & Behavior 4e
9 Recall: patients with damage to their left PFC greater problems with task relying on mental synthesis, such as Tower of London. In normal subjects fMRI shows greater activation of the left PFC during task relying on mental synthesis. The more difficult the task the greater the activation of the PFC
10 The two hemispheres use different approach to problem solvingThe left hemisphere uses analytic search strategy. It involves systematic evaluation of problem states which lie on different possible paths linking the starting state and the goal state. These intermediate states and paths are computed by deliberate, predominantly conscious, manipulation of problem elements (mental synthesis). The right hemisphere primarily uses insight, the sudden awareness of the solution to a problem (i.e., the “Aha!” phenomenon) with little or no conscious access to the processing leading up to that solution. In one experiment, in which 38 participants had to think of a single word that could form a compound phrase with three previously presented words aha! solutions were correct 94 percent of the time compared with 78 percent accuracy for analytical solutions. How does the brain generates insights? Processing occurs largely outside a person's awareness, it is all or nothing—either a fully formed neuronal ensemble with an answer fires in synchrony and therefore jumps into consciousness or it doesn't: the sudden awareness of insight solutions to verbal problems corresponds to a burst of high-frequency (gamma-band) oscillatory EEG activity associated with an increase in fMRI signal in the right anterior superior-temporal gyrus. also EEG and functional MRI scans revealed that just before insight takes place, the occipital cortex, which is responsible for visual processing, momentarily shuts down, or “blinks,” so that the neuronal ensemble can overtake the visual neurons.
11 Left hemisphere removal early in lifeLeft hemisphere removed early in life (before 5 YOA): Right hemisphere can take over language functions. Thus, both hemispheres are capable of acquiring any function. It is very interesting that only one hemisphere takes on mental synthesis. Why is the hemispheric functional asymmetry is so interesting? Because the new functions of the left hemisphere evolved recently! NOTES: The right hemisphere contributes prosody to speech; prosody is the use of intonation, emphasis and rhythm to convey meaning. The right hemisphere also is important in understanding information from language that is not specifically communicated by the meaning of the words: when meaning must be inferred from an entire discourse; when the meaning is figurative rather than literal, as in the moral of a story.
12 The Brain: Teaching Modules. (1997): 5The Brain: Teaching Modules. (1997): 5. The Divided Brain (definitely show this short movie to introduce Gazzaniga and corpus callosum resection):
13 Functional differences between hemispheresSplit-brain surgery Corpus callosum is resected Left hemisphere receives visual information from the right visual field only Right hemisphere receives visual information from the left visual field only The verbal IQ remains intact The capacity for problem-solving appears unaffected.
14 Can a hemisphere on its own, integrate words?Flashed “fire” and then “arm” to the right hemisphere. The left hand drew a rifle rather than an arm on fire. Yes: each hemisphere was able to recognize the meaning of the combined word.
15 Corpus callosum resectionSky-scraper Head-stone Are the two hemispheres still able to communicate with each other? Flashed “sky” to one hemisphere, “scraper” to the other. Patient drew a sky atop a comblike scraper, rather than a tall building. One hemisphere drew what it had seen, then the other drew its word. No. Split hemispheres are NOT able to combine the two words.
16 Left hemisphere interpreterEach hemisphere was shown four small pictures, one of which related to a larger picture also presented to that hemisphere. The patient had to choose the most appropriate small picture. The right hemisphere saw the snowstorm and correctly picked the shovel using the left hand that it controls; The left hemisphere correctly picked the chicken using the right hand. Then the patient was asked why the left hand—or right hemisphere—was pointing to the shovel? Because only the left hemisphere retains the ability to talk, the left hemisphere answered. But because the left hemisphere could not know why the right hemisphere was doing what it was doing, it made up a story about what it could see—namely, the chicken. It said the right hemisphere chose the shovel to clean out a chicken shed. Left hemisphere synthesized a story!
17 Can mental synthesis be conducted by one hemisphere or by bothCan mental synthesis be conducted by one hemisphere or by both? - letter-height comparison experiment In a letter-height comparison experiment (letter classification task), a split-brain subject, JW, was asked to classify letters of the alphabet according to the relative heights of their lower case forms (Farah, 1985). For example, the letter ‘a’ and the letter ‘h’ were shown to the subject and the subject was asked to identify which letter was taller. It is generally believed that people perform this task by generating a mental image of the two letters next to each other and examining the image to compare letter heights. In other words, we remember the discrete images of ‘a’ and ‘h’, but we do not store in memory the image of ‘a’ next to ‘h’ – that is the image of ‘a h’. Therefore, in order to compare the heights, we generate a new image of ‘a h’ in the process of mental synthesis. After the image ‘a h’ is synthesized, it can be mentally examined and determined that the letter ‘h’ is taller. In the control experiment, in which the patient needed to compare two letters shown simultaneously on the screen, the patient had no problem. Both separated hemispheres (tested one at a time) were able to identify the taller letter in nearly 100% of trials. When the letters were introduced one at a time with a short pause in between, the left hemisphere performed at 100% but the right hemisphere performed randomly (it was correct 45% times, that is, worse than 50% - the result that would be expected if the answers were generated randomly). The letter-height comparison experiment clearly indicates that, for the split-brain subject JW, only the left hemisphere was capable of mental synthesis. His right hemisphere was incapable of mental synthesis.
18 The left hemisphere is dominant for hypothesis formationThe difference between the two hemispheres in problem-solving is revealed in a probability-guessing experiment. In this paradigm, subjects try to guess which of two events will happen next. Each event has a different probability of occurrence (e.g. a red stimulus might appear 75% of the time and a green 25% of the time) but the order of occurrence of the events is entirely random. There are two possible strategies for responding in this task: matching and maximizing. In the red/green example, frequency matching would involve guessing red 75% of the time and guessing green 25% of the time. Since the order of occurrence is entirely random, this strategy could potentially result in a great deal of error. The second strategy, maximizing, involves simply guessing red every time. That ensures an accuracy rate of 75% since red appears 75% of the time. Animals such as rats and goldfish maximize. Humans match. The result is that non-human animals perform better than humans in this task. The human's use of this suboptimal strategy has been attributed to a propensity to try to find patterns in sequences of events, even when told the sequences are random. Wolford and colleagues (2000) tested the two hemispheres of split-brain patients in this type of probability-guessing paradigm. They found that the left hemisphere used the frequency-matching strategy whereas the right hemisphere maximized. Their interpretation was that the right hemisphere's accuracy was higher than the left's because the right hemisphere approaches the task in the simplest possible manner with no attempt to form complicated hypotheses about the task. The left hemisphere, on the other hand, engages in the human tendency to find order in chaos.
19 There are clear functional differences in processing visual informationAbility to recognize object in which parts of the object are presented separately (instead of the house the picture shows its walls, roof, windows, door, chimney)? Right=Yes; Left=No. Ability to memorize objects with very complex shape?
20 The right is holistic, remembers a whole scene. The left breaks a scene into smaller visual details and analyses those details serially. The left hemisphere does not notice if a small detail is missing in the picture (for example, a cow without a tail or a horse with long hanging ears). The left doesn’t pay attention to a missing tail in an elephant. It rushes to identify the main feature - the trunk. The right is happy, left is depressed. The right hemisphere may not recognize an elephant without a trunk at all. The right hemisphere needs a complete elephant.
21 In mice both hemispheres can perceive perceptual groupings. This makes sense: left hemisphere acquired mental analysis, it became super-specialized in separating objects from the background. Now an inducer is the whole object for the left hemisphere. The phenomena is very similar to the Matsuzawa experiment with numbers shown for very short time. Important question: What about other primates? Can their left hemisphere continue to make the judgment after the line around inducers has been drawn? Both hemispheres in a split-brain person can judge whether the illusory rectangles are fat or thin when no line is drawn around inducers. Only the right human hemisphere can continue to make the judgment after the line around inducers has been drawn. In mice both hemispheres can perceive perceptual groupings. This suggests that this capacity has been lost by the left hemisphere.
22 Recently in human evolution:The left hemisphere changed its strategy. In the past the smallest neuronal ensemble was an object (in an object all points move together) The human left hemisphere likely gained the ability to quickly break objects into details: a tail, an eye, a trunk, etc. – the smallest meaningful neuronal ensembles I call this ability mental analysis. Then the left gained the ability to put a word on every small feature the communication system (last two million years) Finally, the left gained the ability to mentally combine those features in any order into new objects and scenes mental synthesis (last one hundred thousand years) In the next lecture we’ll learn about evolutionary driving forces that forced the changes in the left hemisphere.
23 Jill Bolte Taylor: My stroke of insight (TED talk) http://www. tedStop here
24 Conclusions Why is the hemispheric functional asymmetry is so interesting? Because the new functions of the left hemisphere evolved recently! It significantly changed during hominin evolution. The left h. gained the ability to mentally analyze objects into details: a tail, an eye, a trunk, etc. (3 million years ago) Then the left gained the ability to put a word on every such detail (starting from two million years; reached modern ability 600,000 years ago) Finally, the left gained the ability to mentally combine those features in any order into new objects and scenes mental synthesis (last one hundred thousand years)
25 Corpus callosum in childrenThe myelination of the corpus callosum is relatively slow. It is one of the last areas of the brain to undergo myelination, and myelination is not complete until at least the time of puberty (Gbedd JN, 1999; Giedd, JN, 1996). As a result, the brain of a young child is similar to that of a split-brain patient: some types of interhemispheric signal transfer are significantly inferior to that in an adult brain. For example, children have a greater difficulty matching tactile designs between hands than within hands (Hellige, 1993). In addition, children younger than 10 months are not able to recognize a face shown to only one hemisphere with the use of the other hemisphere, which indicates an inability to transfer information about the face over the corpus callosum (Schonen & Mathivet, 1990).
26 Seeing the forest for the treesleft hemisphere preferentially encodes the local features of a stimulus, while the right hemisphere encodes the global configuration. Delis DC, 1986: Unilateral brain-damaged patients and normal control subjects were asked to remember visual hierarchical stimuli consisting of larger forms constructed from smaller forms. The right-hemisphere damaged patients made more errors in remembering the larger forms relative to the smaller forms, whereas the left-hemisphere damaged patients made more errors in remembering the smaller forms relative to the larger forms. These findings are discussed as they relate to hemispheric specialization for visuospatial processing. Excellent introduction that summarizes differences in object recognition between the left and right hemispheres!!! In 2002 Hemispheric asymmetries for simple visual judgments in the split brain
27 Evolutionarily speaking, right hemisphere can be used as a proxy for an animal mind to some extent. Therefore it is interesting to compare function between the two hemispheres. However functions are often mixed. Just as heart can be found over the right in some individuals, so are some cognitive functions can develop in the right hemisphere. Furthermore even the adult brain is a bit malleable. As a result of modification, the brain can learn new tricks. This makes using the right brain as a proxy for animal brain VERY tricky. Discussion can only be conducted “in general.” In any specific individual, general rules can be broken.
28 The difference in visual percept processing reflects the difference in the evolutionary path over the last five million years. While chimpanzees stayed in the relative safety of the treetop canopies, hominids foraging in savannas acquired mental analysis for faster predator identification based on few visual clues. It is possible that if the left hemisphere with its visual analysis was suppressed or otherwise not used in the experiment (for example by showing the display to the right hemisphere only), human subjects matching the numbers using only their right hemisphere could improve their performance and achieve a result that is comparable to that of the chimpanzee.