Mapping the brain: lobes, lesions and the cases that taught us

Transcript

SoheePicture a 62-year-old woman. Intelligent, self-sufficient. She's in the produce section, and she starts eating grapes and strawberries straight out of the bins, before paying.

RyanSomething that would have embarrassed her before the stroke.

SoheeExactly. And she's also having trouble moving her left leg. One event explains both.

RyanWelcome to EPPP Prep, the podcast. I'm Ryan.

SoheeAnd I'm Sohee. That woman is Theona, a case from the OpenStax psychology textbook, and she's our guide today as we map the brain. Which structures do what, and how we learned it, often from people whose brains were injured.

RyanSo, Theona. She had a stroke in the front part of her right hemisphere. Why the left leg?

SoheeBecause each hemisphere controls the opposite side of the body. The left hemisphere controls the right half, and the right hemisphere controls the left half. And the main motor centers sit at the front of the brain, in the frontal lobe.

RyanAnd the grapes?

SoheeThat fits damage to another part of the frontal lobe, the prefrontal cortex, which the textbook links to judgment, reasoning and impulse control.

RyanSo let's build the map. The two hemispheres are separated by a deep groove called the longitudinal fissure, and they're connected by the corpus callosum, a thick band of about 200 million axons that lets the two sides share information.

SoheeEach hemisphere has four lobes. Frontal, parietal, temporal and occipital. Let's take them one at a time.

RyanThe frontal lobe is involved in reasoning, motor control, emotion and language. It holds the motor cortex, for planning and coordinating movement, the prefrontal cortex, for higher-level thinking, and Broca's area, which is essential for language production.

SoheeThe textbook tells the story of Padma, an engineer who damaged Broca's area in a car accident. She completely lost the ability to speak. Nothing was wrong with her mouth or vocal cords. She could follow directions and read, but she couldn't produce words.

RyanAnd probably the most famous frontal lobe case of all, Phineas Gage. In 1848, he was a 25-year-old railroad foreman in Vermont when an iron rod blasted through his skull.

SoheeHe could walk and talk afterward, but people said his personality changed. A well-mannered, soft-spoken man began behaving in odd and inappropriate ways, which fits a loss of impulse control.

RyanHere's a nuance the textbook includes that I love. There's some evidence the changes in Gage were exaggerated. His case landed in the middle of a 19th-century debate about localization, and with very limited information, scientists tended to find support for whatever view they already held.

SoheeA good reminder that a famous case isn't the same as strong evidence.

RyanNext, the parietal lobe, right behind the frontal lobe. It processes information from the body's senses and contains the somatosensory cortex, for touch, temperature and pain.

SoheeAnd the map isn't to scale. Body parts with more surface area and more nerves get more cortex. Fingers take up a lot more space than toes.

RyanThe temporal lobe, on the side of the head, near the temples. Hearing, memory, emotion and some aspects of language. It holds the auditory cortex and Wernicke's area, which is important for speech comprehension.

SoheeAnd this is the distinction the exam returns to again and again. Damage to Broca's area: great difficulty producing language. Damage to Wernicke's area: people can produce sensible language, but they can't understand it.

RyanProduction up front, in the frontal lobe. Comprehension on the side, in the temporal lobe.

SoheeThen the occipital lobe, at the very back. It contains the primary visual cortex, and it's organized retinotopically, meaning where something sits in your visual field relates closely to where it's represented on the cortex.

RyanPause and think. After a stroke, a man speaks in long, fluent sentences, but he can't follow what his wife is saying to him. Which area, and which lobe?

SoheeWernicke's area, in the temporal lobe. The tempting wrong answer is Broca's, because it's the language area people remember first. But Broca's damage hits production. Fluent speech with poor comprehension points to Wernicke's.

RyanNow the hemispheres. There is evidence of specialization, called lateralization, mainly for language. One summary the textbook cites says the left hemisphere is better at forming associations in memory, selective attention and positive emotions, and the right at pitch perception, arousal and negative emotions.

SoheeBut the textbook also cautions that research on hemispheric dominance has produced inconsistent results. It's probably better to think about how the two hemispheres interact than to assign behaviors to one side.

RyanWhich brings us to the split-brain patients. In some cases of severe epilepsy, surgeons cut the corpus callosum to stop seizures from spreading.

SoheeAfterward, if a picture is shown in the patient's left visual field, they can't name it, because that information reaches only the largely nonverbal right hemisphere. But they can recreate it with their left hand, which the right hemisphere also controls.

RyanThat's a favorite exam scenario, so it's worth tracing the wiring once slowly: left visual field, right hemisphere, left hand.

SoheePause and think. A split-brain patient sees a picture of a key flashed to her left visual field. Can she say what it was? Can she draw it?

RyanShe can't name it, because the image reached only the largely nonverbal right hemisphere. But she can draw it with her left hand, since the right hemisphere controls that hand. And once her left hemisphere sees the drawing, she can name it.

SoheeLet's go beneath the cortex. The thalamus is the sensory relay. Every sense except smell is routed through the thalamus before going on for processing.

RyanAnd smell goes straight to the limbic system, which helps explain why a smell can stir emotions the way other senses can't.

SoheeThe limbic system processes emotion and memory. Three key structures. The hippocampus is essential for learning and memory. The amygdala is involved in our experience of emotion and in tying emotional meaning to memories.

RyanAnd the hypothalamus regulates homeostatic processes like body temperature, appetite and blood pressure. It's also the interface between the nervous system and the endocrine system, and it's involved in sexual motivation and behavior.

SoheeThe hippocampus brings us to Henry Molaison, known as H.M. In 1953, at age 27, he had surgery to remove his hippocampus and amygdala to control severe seizures.

RyanThe seizures improved, but he lost the ability to form many kinds of new memories. He couldn't learn new facts or remember new faces. Yet he could learn new skills. He just had no memory of learning them.

SoheeAnd the textbook explains why. Procedural memory, how to perform tasks, is thought to be associated with the cerebellum, and H.M.'s cerebellum was intact.

RyanSo the exam distinction is: hippocampus for consolidating new explicit memories, cerebellum for procedural memory. Lose the hippocampus, and new facts don't stick, but skills still can.

SoheeLet's finish with the midbrain and hindbrain. The reticular formation, centered in the midbrain, regulates the sleep-wake cycle, arousal, alertness and motor activity.

RyanThe substantia nigra and the ventral tegmental area are also in the midbrain. Both contain cells that produce dopamine, both are critical for movement, and their degeneration is involved in Parkinson's disease. They're also involved in mood, reward and addiction.

SoheeIn the hindbrain, the medulla controls automatic processes like breathing, blood pressure and heart rate. The pons connects the hindbrain to the rest of the brain and helps regulate brain activity during sleep.

RyanAnd the cerebellum, the little brain, handles balance, coordination, movement and motor skills, plus that procedural memory we talked about.

SoheeOne more idea that matters for the clients you'll see. Neuroplasticity. The nervous system can change and adapt, through new synapses, pruning of unused ones, changes in glia, and even new neurons. It's greatest when we're young, but it continues later in life.

RyanThe textbook's example is the reporter Bob Woodruff, who had a traumatic brain injury and, with intensive cognitive and speech therapy, made a remarkable recovery.

SoheeSo, the traps. Broca versus Wernicke. Hippocampus versus amygdala. Cerebellum for skills. Thalamus relays every sense except smell. Medulla for breathing and heart rate. And remember that control is crossed, left brain, right body.

RyanQuick recap. Frontal lobe: movement, reasoning, impulse control, and Broca's area for speaking. Parietal: touch, temperature, pain. Temporal: hearing, memory, and Wernicke's area for understanding. Occipital: vision.

SoheeLimbic system: hippocampus for new memories, amygdala for emotion, hypothalamus for homeostasis and the link to hormones. Midbrain: reticular formation for arousal, substantia nigra for dopamine. Hindbrain: medulla, pons, cerebellum.

RyanAnd the cases: Theona's right frontal stroke, Padma and Broca's area, Gage and impulse control, with a grain of salt. H.M. and the hippocampus. The split-brain patients and the corpus callosum.

SoheeIf you just held that whole map in your head for twelve minutes, that's real work. And it's neuroplasticity in action, by the way. Your brain is literally changing as you study.

RyanTry sketching the map from memory before your next practice set. It sticks better than rereading. Thanks for listening to EPPP Prep.

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