Lesson 1.9 · Unit 1 · CED topic 1.4 (part 1)
The brain: structures and their functions
The brain is about three pounds of tissue that produce every thought, memory, and emotion you will ever have. No single structure does everything, and the exam does not ask you to name parts for their own sake. It asks you to predict what happens to behavior and mental processes when a region is damaged or active: if a person can no longer recognize faces, what region would you suspect? This lesson organizes the brain from the bottom up, from the structures that keep you alive to the cortex that lets you plan, and the next lesson covers specialization of the two hemispheres, plasticity, and how scientists study the living brain.
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.1. The brain stem (including the medulla) generally controls basic functioning such as breathing and heart rate.
- 1.4.A.2. The reticular activating system and the brain’s reward center generally control alertness, some voluntary movement, eye movement, and some types of learning, cognition, and emotion.
- 1.4.A.3. The cerebellum generally controls coordination of muscle movement, balance, and some forms of procedural learning.
- 1.4.A.4. The cerebral cortex is divided into two hemispheres and includes the limbic system (thalamus, hypothalamus, pituitary gland, hippocampus, amygdala), corpus callosum, and the lobes of the cortex.
- 1.4.A.4.i. The occipital lobes generally control visual information processing and are located in the rear of the brain.
- 1.4.A.4.ii. The temporal lobes generally control auditory and linguistic processing and are located on the sides of the brain. 1.A 2.C 2.D 3.A
- 1.4.A.4.iii. The parietal lobes generally control association areas, which process and organize information, and the somatosensory cortex, which processes touch sensitivity. These lobes are located near the back crown of the brain.
- 1.4.A.4.iv. The frontal lobes, located just behind the forehead, generally control linguistic processing, higher-order thinking, and executive functioning, especially in the prefrontal cortex. The motor cortex is located at the rear of the frontal lobes and controls most types of skeletal movement.
The brain stem, the cerebellum, and the reticular formation
The brain stem is the oldest part of the brain, where the spinal cord enters the skull. It controls basic life functions that you do not think about, such as breathing and heart rate. Its lowest part, the medulla, regulates these. Damage to the brain stem is often fatal.
The reticular activating system (RAS) is a network of neurons that runs through the brain stem and up toward the cortex. It controls alertness and arousal, so it helps you stay awake and attentive and filters incoming stimuli, and it plays a role in some voluntary movement, eye movement, and some types of learning, cognition, and emotion. The brain's reward center, a set of pathways that release dopamine when something pleasurable happens, is also part of this deeper circuitry. It motivates you to repeat behaviors such as eating, social contact, and, unfortunately, drug use.
The cerebellum ("little brain") sits at the back of the brain below the cortex. It coordinates the timing and smoothness of muscle movement, balance, and some forms of procedural learning, the learning of skills such as riding a bike or typing. A person with cerebellar damage may appear clumsy or unsteady, as if intoxicated, even though the person can still decide to move.
The limbic system
Between the brain stem and the cortex lies the limbic system, a set of structures involved in emotion, motivation, and memory. The exam includes five:
- The thalamus is the brain's sensory relay station. It receives information from the senses (all except smell) and sends it on to the appropriate part of the cortex.
- The hypothalamus is a small structure that regulates hunger, thirst, body temperature, sleep cycles, and sexual behavior. It also controls the pituitary gland.
- The pituitary gland is the "master gland" of the endocrine system. Under the hypothalamus's direction, it releases hormones that influence growth and other glands. It connects the nervous system to the hormone system.
- The hippocampus is important for forming new explicit memories, those you can consciously recall, such as facts and events. Damage makes it hard to form new memories.
- The amygdala processes emotion, especially fear and aggression. It helps attach emotional significance to events, which is why frightening experiences are remembered so well.
The cerebral cortex
The cerebral cortex is the thin, wrinkled outer layer of the brain, where complex thought, perception, and planning take place. The folds increase its surface area. It is divided into two hemispheres, left and right, connected by a thick band of nerve fibers called the corpus callosum, which lets the hemispheres communicate. Each hemisphere is divided into four lobes.
- The occipital lobes, at the back of the brain, process visual information. Damage can cause blindness even when the eyes are healthy.
- The temporal lobes, on the sides of the brain (above the ears), process auditory information and take part in linguistic processing, including understanding speech.
- The parietal lobes, near the back crown of the brain, contain association areas that process and organize information and the somatosensory cortex, which processes touch sensation (pressure, temperature, pain) from the body.
- The frontal lobes, just behind the forehead, are involved in speaking, higher-order thinking, and executive functioning (planning, judgment, decision making, and self-control), especially in the prefrontal cortex. The motor cortex, at the back of the frontal lobes, controls most voluntary skeletal movement.
The sensory and motor cortex strips lie next to each other, separated by the central groove that divides the frontal and parietal lobes. Parts of the body with more sensitivity (the fingertips, the lips) or finer control (the fingers, the tongue) have more cortex devoted to them than parts such as the back.
| Region | Main functions | Example of damage effect |
|---|---|---|
| Brain stem (medulla) | Breathing, heart rate | Loss of vital functions |
| Reticular activating system | Alertness, arousal | Coma or inability to stay alert |
| Cerebellum | Coordination, balance, procedural learning | Clumsy, unsteady movement |
| Thalamus | Relays sensory information | Disrupted sensation |
| Hypothalamus | Hunger, thirst, temperature, controls pituitary | Disrupted eating or body temperature |
| Hippocampus | Forming new explicit memories | Trouble forming new memories |
| Amygdala | Fear, aggression, emotional memory | Reduced fear response |
| Occipital lobe | Vision | Vision loss, trouble recognizing objects |
| Temporal lobe | Hearing and language | Trouble understanding sounds or speech |
| Parietal lobe | Touch, association | Trouble with touch and spatial tasks |
| Frontal lobe | Executive functions, speech, movement | Poor planning, impulse control, weak movement |
Phineas Gage was a 25-year-old railroad construction foreman in Vermont. In 1848 an explosion drove an iron tamping rod, about 3 feet 7 inches long and an inch and a quarter across, up through his left cheek and out the top of his skull, destroying much of his left frontal lobe. Gage survived and could walk, talk, and work, but accounts by his doctor, John Harlow, described a change in personality: he had been responsible and well liked, and afterward was said to be impulsive and rude. The case suggested that the frontal lobes are involved in planning and regulating behavior.
Reading the research: a case study, a non-experimental method. It is valuable because it is a rare "natural experiment" in which a specific region was damaged. But the evidence is limited: it is a single case, observations were reported by one doctor and were not measured with tests, and later accounts exaggerated the story, so the conclusions must be drawn with care. Case studies suggest hypotheses that other methods can test; they cannot establish cause and effect, and one person may not represent others.
James Olds and Peter Milner implanted tiny electrodes in the brains of rats, in a region that is part of the reward circuitry, and let the rats press a lever to deliver a brief electrical stimulus to themselves. Some rats pressed the lever hundreds or even thousands of times an hour and chose it over food. The researchers concluded that the stimulation was intensely rewarding and that the brain has a reward center.
Reading the research: an experiment with animals; the independent variable was the placement of the electrode and the dependent variable was the rate of lever pressing. It identified a reward circuit that involves dopamine, which is also implicated in addiction. Because the subjects were animals, the research would be reviewed by an institutional animal care committee, and it is not certain that the same experience occurs in humans.
For each patient, name the likeliest damaged region. (a) She can see normally but cannot recognize objects by their appearance, and a scan shows damage at the back of the head. (b) He cannot form new memories of events, although he remembers his childhood. (c) After a stroke, she walks with a jerky, unsteady gait and cannot touch her nose smoothly, but her thinking is intact. (d) After an accident, he is easily angered, plans poorly, and ignores consequences, though he can move and see normally.
Step 1, (a) the back of the head holds the occipital lobe, which processes vision. Damage there disrupts the processing of visual information.
Step 2, (b) trouble forming new explicit memories with old memories intact points to the hippocampus.
Step 3, (c) poor coordination and balance with intact thinking points to the cerebellum.
Step 4, (d) impulsivity and poor planning are executive functions, so the prefrontal cortex of the frontal lobe.
A neurologist reports on a patient whose left temporal lobe was damaged in a stroke. After the stroke, the patient could speak but did not understand words spoken to her. The neurologist concludes that the temporal lobe is "the" center for understanding language. Evaluate the conclusion.
Step 1, the design is a case study of one person. It does not use an IV or random assignment, so it cannot prove that the damage caused the problem.
Step 2, what the case supports: it fits what is known about the temporal lobes' role in auditory and linguistic processing, so it is consistent with that function.
Step 3, limits: with one patient, the finding may not generalize, and the stroke could have damaged other regions that were not examined. Language involves a network of areas, including the frontal lobe.
Step 4, a better conclusion: the temporal lobe is important for understanding speech. Studies using brain scans of many people would give stronger evidence.
- Brain stem (medulla): breathing, heart rate. RAS: alertness. Cerebellum: coordination, balance, procedural learning. Reward center: dopamine and motivation.
- Limbic system: thalamus (relay), hypothalamus (hunger, thirst, temperature; controls pituitary), pituitary, hippocampus (new explicit memories), amygdala (fear, emotion).
- Lobes: occipital (vision), temporal (hearing, language), parietal (association, somatosensory cortex, touch), frontal (executive function, speech, motor cortex).
- Predict function from damage. A case study (Phineas Gage) can suggest hypotheses but cannot show cause and effect.
- Which part of the brain is most associated with each: (a) breathing, (b) balance, (c) fear, (d) relaying sensory information.
Show answer
(a) brain stem (medulla); (b) cerebellum; (c) amygdala; (d) thalamus.
- A person's vision is lost though her eyes are healthy. Which lobe is most likely damaged, and where is it located?
Show answer
The occipital lobe, at the back of the brain.
- What is the function of the hypothalamus, and what is its relationship to the pituitary gland?
Show answer
The hypothalamus regulates hunger, thirst, body temperature, and related drives, and it controls the pituitary gland, which in turn releases hormones that influence other glands.
- In what way is the case of Phineas Gage evidence about the frontal lobes, and what are two limitations?
Show answer
After damage to his frontal lobe his behavior reportedly became impulsive, so the frontal lobe is involved in regulating behavior. Limitations: it is a single case, and the reports of his change were subjective and later exaggerated, so it cannot show cause and effect.
- Where is the motor cortex, and what is the somatosensory cortex?
Show answer
The motor cortex is at the rear of the frontal lobes and controls voluntary movement. The somatosensory cortex is in the parietal lobe and processes touch sensation.
- Explain why Olds and Milner's rats pressed the lever so many times. What brain system does this implicate?
Show answer
The stimulation activated the brain's reward center (a dopamine-related circuit), which made the behavior intensely rewarding, so the rats repeated it.