Lesson 1.10 · Unit 1 · CED topic 1.4 (part 2)
Split brains, plasticity, and studying the brain
The two halves of the cerebral cortex look like mirror images, but they do not do identical jobs, and the brain is not fixed once it is built. This lesson covers three ideas the exam tests: the specialization of the hemispheres (shown most dramatically by patients whose hemispheres were separated), the brain's ability to rewire itself after damage or experience, and the tools researchers use to learn how all of this works. It is also a good place to practice evaluating non-experimental research, since much brain research is based on case studies and scans.
Learning objective.
- 1.4.A. Explain how the structures and functions of the brain apply to behavior and mental processes.
Essential knowledge.
- 1.4.A.5. Split brain research, achieved by severing the corpus callosum (often a treatment for severe epilepsy), reveals that the right and left hemispheres of the brain may specialize in different activities and functions.
- 1.4.A.5.i. Areas of the brain that affect language are typically located in the left hemisphere and include Broca’s area (responsible for speech production) and Wernicke’s area (responsible for speech comprehension). Damage to these parts of the brain can lead to aphasia.
- 1.4.A.5.ii. Researchers test for cortex specialization with split-brain patients by showing information in each visual field, taking advantage of the brain’s contralateral hemispheric organization.
- 1.4.A.6. Brain plasticity is the ability of the brain to rewire itself or modify or create new connections throughout development and generally allows for the function of a damaged part of the brain to be assumed by a different part of the brain.
- 1.4.A.7. Research on the brain is done using scans (including EEG and fMRI), case studies, and surgical procedures (such as lesioning) to promote understanding of how the different structures of the brain work and how the brain functions together as a whole.
Hemispheric specialization and the split brain
For most people the left hemisphere is specialized for language, and the right hemisphere takes a larger role in spatial and some perceptual tasks and in the emotional tone of speech. This is called lateralization. The popular idea that people are "left-brained" or "right-brained" is a myth: the hemispheres cooperate on nearly everything, and the corpus callosum constantly passes information between them.
Two areas in the left hemisphere of most people are important for language. Broca's area, in the frontal lobe, is responsible for speech production. Damage causes labored, halting, telegraphic speech, though the person understands much of what is said. Wernicke's area, in the temporal lobe, is responsible for speech comprehension. Damage causes fluent speech that is full of words but often makes little sense, and the person has trouble understanding language. Damage to either area can lead to aphasia, a loss or impairment of language.
The strongest evidence for specialization comes from split-brain research. To control severe epilepsy, surgeons sometimes cut the corpus callosum, which stops seizures from spreading between the hemispheres. These patients usually function normally in daily life, but carefully designed tests show that their hemispheres cannot share information. The tests rely on the brain's contralateral organization: each hemisphere receives information from, and controls, the opposite side of the body. Because of how the optic nerves cross, anything in the left visual field (what you see to the left of a point of focus) goes to the right hemisphere, and anything in the right visual field goes to the left hemisphere. Researchers present information to just one visual field and ask the patient to respond.
Roger Sperry and Michael Gazzaniga tested patients whose corpus callosum had been cut. The patient stared at a dot while an image or word was flashed briefly to one side. When the word was flashed to the right visual field (left hemisphere), the patient could say what it was. When it was flashed to the left visual field (right hemisphere), the patient said they saw nothing, because speech is controlled by the left hemisphere, but the patient's left hand (controlled by the right hemisphere) could pick out the matching object from a group hidden behind a screen. Sperry received a Nobel Prize for this work in 1981.
Reading the research: a set of experiments in which the independent variable was the visual field where the stimulus was shown and the dependent variable was how the patient responded. The participants were a very small number of patients with an unusual medical history, so the findings may not generalize, but the experimental control was strong.
Brain plasticity
Brain plasticity is the brain's ability to rewire itself, modifying existing connections and creating new ones, throughout development. Plasticity is greatest in childhood but continues in adults. It explains why, after damage to one part of the brain such as a stroke, other parts can sometimes take over the function. A young child who has a hemisphere removed to stop severe seizures can often recover remarkable language and movement because the remaining hemisphere takes on both sets of jobs. It also explains why practice changes the brain: learning strengthens certain connections and so physically reorganizes them. A person who loses a sense, such as vision, may find that the cortex area that would have processed it is taken over by another sense, such as touch.
To become a licensed taxi driver in London, a person must memorize the layout of thousands of streets, a process that takes years. The researchers used brain scans (MRI) to compare the brains of 16 licensed taxi drivers with those of 50 other adults who were not taxi drivers. The back part of the hippocampus, which is important for spatial memory, was larger in the taxi drivers, and the amount of time a person had worked as a taxi driver correlated with how large this region was.
Reading the research: a correlational, non-experimental design, so it cannot prove that driving caused the growth, because people with a larger hippocampus might be more likely to become taxi drivers. But the finding that more experience goes with a larger region fits the idea of plasticity.
Ways to study the brain
The brain is difficult to study because it is protected inside the skull and because you cannot ethically cut into a healthy person's brain. The exam names three families of methods.
- Brain scans. An EEG (electroencephalogram) records the electrical activity of the brain through electrodes placed on the scalp. It follows changes in activity on a millisecond scale and is the tool used to study sleep stages, but it shows only the surface and does not show where activity comes from precisely. An fMRI (functional magnetic resonance imaging) measures changes in blood flow (and so activity) in specific regions while a person performs a task, producing detailed images of which areas are active. It has good spatial detail but is slower, taking a few seconds, and is expensive. Other scans, such as the CT and PET scans, show brain structure or metabolic activity.
- Case studies of people with brain damage, like Phineas Gage, show what is lost when a region no longer works.
- Surgical procedures such as lesioning, which means destroying a small area of brain tissue, were used with animals to learn what a region does (for example, lesioning part of the hypothalamus changes eating). Because it destroys tissue, lesioning is done in animals under review by animal care committees, or used therapeutically in humans for a medical purpose, not in healthy people.
Notice the connection to the research methods lessons. A scan study of healthy people comparing activity across tasks is often correlational; studies that compare scans between conditions with random assignment are experimental; and case studies and lesions in animals are non-experimental or experimental, depending on the design. Always ask what was measured and whether the groups were assigned by the researcher.
A split-brain patient stares at a dot. The word "SPOON" is flashed briefly on the left side of the screen. What will the patient say when asked what they saw, and what will they do if asked to pick the object with their left hand from a hidden collection?
Step 1, because of contralateral organization, the left visual field goes to the right hemisphere.
Step 2, speech depends on the left hemisphere in most people. The right hemisphere can see the word but cannot tell the speech centers about it, because the corpus callosum is cut. So the patient will probably say "nothing" or "I didn't see anything."
Step 3, the left hand is controlled by the right hemisphere, which knows what it saw, so the patient will probably choose the spoon.
Step 4, the lesson: the two hemispheres each have their own knowledge when disconnected, and language is typically (not always) a left-hemisphere function.
Patient A says, "Want... water... go... kitchen" slowly and with effort but understands questions. Patient B speaks fluently and quickly, but her sentences are a jumble of real words that do not make sense, and she does not respond appropriately when spoken to. Which patient likely has damage to which area?
Step 1, patient A understands but cannot produce smooth speech, so the problem is speech production, which points to Broca's area in the frontal lobe.
Step 2, patient B produces fluent but meaningless speech and cannot comprehend, so the problem is comprehension, which points to Wernicke's area in the temporal lobe.
Step 3, both are types of aphasia, and both areas are typically in the left hemisphere.
Choose the best method for each question. (a) How does brain activity change second by second when a person falls asleep? (b) Which areas become active when people solve a math problem? (c) What happens to eating when a small region of a rat's hypothalamus is destroyed?
Step 1, (a) requires fast time resolution, so EEG, which records electrical activity continuously.
Step 2, (b) requires locating active regions during a task, so fMRI, which shows detailed images of blood flow.
Step 3, (c) is a question about the effect of removing tissue, so a lesion study in animals, with approval from an animal care committee.
Step 4, in each case, think about what the method allows: scans describe activity (correlation); a lesion in an experiment with a control group can support a cause-and-effect conclusion.
- Lateralization: language is usually in the left hemisphere; Broca's area (speech production) and Wernicke's area (comprehension) damage can cause aphasia.
- Split-brain research cuts the corpus callosum; contralateral organization means left visual field goes to the right hemisphere. "Left-brain" and "right-brain" types are a myth.
- Plasticity: the brain rewires itself, so other regions can take over for damaged ones.
- Methods: EEG (fast, electrical), fMRI (detailed, blood flow), case studies, lesioning.
- Why can a split-brain patient name an object shown to the right visual field but not the left?
Show answer
The right visual field goes to the left hemisphere, which controls speech, so the patient can name it. The left visual field goes to the right hemisphere, which cannot pass the information to the speech centers because the corpus callosum was cut.
- A stroke patient understands what others say but speaks in slow, broken phrases. What area is likely damaged?
Show answer
Broca's area (speech production), in the left frontal lobe.
- Explain how brain plasticity could help a child recover after injury to one part of the brain.
Show answer
The brain can form new connections and reorganize so that another region takes over the function of the damaged one, which is most effective in young children.
- Which method would you use to see which brain regions are active while a person reads: EEG or fMRI? Why?
Show answer
fMRI, because it provides detailed images of where activity (blood flow) occurs, whereas EEG shows only electrical activity at the scalp with little detail about location.
- The taxi driver study found a larger hippocampus in taxi drivers. Why can't the researchers conclude that driving caused the growth?
Show answer
The study is correlational: people with larger hippocampal regions might have been more likely to become taxi drivers (directionality), or some third factor could explain the difference. Only an experiment with random assignment could show cause.
- Why is the claim "I am right-brained, so I am more creative" a myth?
Show answer
Both hemispheres work together on nearly all tasks and communicate through the corpus callosum. Hemispheres specialize in some functions, but people are not dominated by one side.