AP Psychology 1.4: The Brain

The structures low in the brain run jobs you rarely notice. The brain stem, which includes the medulla, generally controls basic functioning such as breathing and heart rate. You can hold your breath on purpose for a while, but once you stop trying, breathing carries on without you, and your heart keeps beating all night while you sleep.

Running up through the brain stem is the reticular activating system. Together with the brain's reward center, it generally controls alertness, some voluntary movement, eye movement, and some types of learning, cognition and emotion. When a sudden crash snaps a dozing student wide awake, alertness is the job in play. When something feels good and you find yourself wanting to do it again, the reward center is part of why that feeling teaches you anything.

At the back of the brain, below the rest, sits the cerebellum. It generally controls coordination of muscle movement, balance, and some forms of procedural learning: skills learned by doing, such as typing, until they run without step-by-step thought. Catching a ball and staying upright on an icy sidewalk both lean on it.

Notice the word generally in each of these. Brain structures share work, so a structure is linked to a job without being the only place that job happens.

The cerebral cortex is the brain's wrinkled outer layer, and it is divided into two halves, the left and right hemispheres. Along with it come the corpus callosum, a thick band of fibers joining the two hemispheres, and the limbic system, a group of structures made up of the thalamus, hypothalamus, pituitary gland, hippocampus and amygdala. Some textbooks draw the limbic system smaller and leave out the thalamus or the pituitary gland; the AP course lists all five, so count all five here. Later topics come back to several of these.

Each hemisphere has four lobes, and each lobe generally leans toward certain work.

• The occipital lobes, at the rear of the brain, handle visual information.
• The temporal lobes, on the sides, handle auditory and linguistic processing.
• The parietal lobes, near the back crown, hold the somatosensory cortex, which processes touch, and association areas, which process and organize information.
• The frontal lobes, just behind the forehead, handle linguistic processing, higher-order thinking and executive functioning, such as planning ahead and holding back an impulse. That work centers on the prefrontal cortex, at the very front. The motor cortex, at the rear of the frontal lobes, controls most types of skeletal movement.

To decide where to look, name the job first. Seeing points to the occipital lobes, hearing to the temporal lobes, touch to the parietal lobes, and moving or planning to the frontal lobes.

The corpus callosum lets the hemispheres share information. Severing it has been used to treat severe epilepsy, and research with those patients, called split-brain patients, shows that the right and left hemispheres may specialize in different activities and functions.

The best-known specialization is language, whose areas are typically in the left hemisphere. Broca's area is responsible for speech production, and Wernicke's area is responsible for speech comprehension. Damage to either can lead to aphasia, a loss of language ability. After damage to Broca's area, a person tends to understand speech but struggle to produce it. After damage to Wernicke's area, speech may flow but make little sense, and understanding others is hard.

To test specialization, researchers use the brain's contralateral organization: each hemisphere mostly handles the opposite side. The right hemisphere feels and moves the left side of the body, and the left hemisphere the right. In vision, everything to the left of the point you are looking at, your left visual field, goes to the right hemisphere, and your right visual field goes to the left hemisphere. That is true for both eyes, not one eye per side.

So a researcher has a split-brain patient stare at a dot and flashes a word beside it. A word flashed to the right of the dot reaches the left hemisphere, where the language areas usually are, and the patient reads it aloud. A word flashed to the left reaches only the right hemisphere.

Brain plasticity is the brain's ability to rewire itself, modifying or creating new connections throughout development. It is also why a person can regain an ability after brain damage: the function of a damaged part can be assumed by a different part. The damaged tissue does not grow back as it was. Other areas take on the work, often slowly and with practice.

Researchers study how the brain works in several ways.

• An EEG records the brain's electrical activity through electrodes on the scalp. It shows when activity changes, moment by moment, but only roughly where.
• An fMRI scan tracks changes in blood flow to show which areas grow more active during a task.
• A case study looks in depth at one person, such as someone with damage to one area. It can show what is possible, but one person may not be typical.
• Lesioning is a surgical procedure that destroys a small area of brain tissue, usually in animals, so researchers can watch what changes. Because the researcher chooses the area, it can show what that area is needed for.

A scan showing an area active during a task fits the idea that the area does the work, but it does not show the task depends on it. Damage, whether from an injury or a lesion, shows what happens without the area.