1.4 The Brain

• Brainstem: the oldest part of the brain, at the top of the spinal cord. Its medulla controls heart rate and breathing, so damage here can be fatal.
• Reticular activating system: a network running through the brainstem that controls alertness and arousal, filtering incoming information. Damage can cause a coma.
• Reward center: brain areas rich in dopamine that make some experiences feel pleasurable and motivate you to repeat them. Many addictive drugs act here.
• Cerebellum: the 'little brain' at the back, below the cortex. It coordinates movement and balance and stores some procedural learning, like how to ride a bike. Alcohol affects it, which is why drunk people stumble.

These structures sit deep in the brain, under the cerebral cortex, and connect emotion, motivation and memory.

• Thalamus: the sensory relay station. It routes information from every sense except smell to the right part of the cortex.
• Hypothalamus: regulates hunger, thirst, body temperature and sexual behavior, and controls the pituitary gland.
• Pituitary gland: the 'master gland,' which releases hormones that direct other glands.
• Hippocampus: helps form new explicit memories of facts and events (2.3).
• Amygdala: processes emotions, especially fear and aggression.

The cerebral cortex is the wrinkled outer layer of the brain, divided into left and right hemispheres. A thick band of axons, the corpus callosum, connects the two hemispheres so they can share information. Each hemisphere has four lobes.

The brain is contralateral: the left hemisphere controls and senses the right side of the body, and the right hemisphere handles the left side. Information from your left visual field goes to your right hemisphere, and from your right visual field to your left hemisphere.

In rare cases of severe epilepsy, surgeons have cut the corpus callosum to stop seizures from spreading. In these split-brain patients, the hemispheres can't share information. If a picture of a key is flashed to the right visual field, it reaches the left hemisphere, which usually controls language, so the person can say 'key.' If it's flashed to the left visual field, it reaches the right hemisphere, so the person can't name it but can pick out the key with the left hand.

Language usually depends on the left hemisphere. Damage to Broca's area causes slow, effortful speech even though the person understands what they hear. Damage to Wernicke's area causes fluent speech that doesn't make sense, along with trouble understanding language. Both are types of aphasia, a language impairment caused by brain damage.

Plasticity is the brain's ability to change by forming new connections or reorganizing. It's strongest in childhood, which is why a young child who loses part of the brain to surgery can often recover much of its function as other areas take over. Plasticity continues throughout life every time you learn.

Researchers use several tools. An EEG records the brain's electrical activity through electrodes on the scalp; it shows timing well but location poorly. An fMRI tracks changes in blood flow to show which areas are active during a task. Case studies examine people with unusual brain damage in depth, like Phineas Gage, a railroad worker whose personality reportedly changed after an iron rod pierced his frontal lobe in 1848. Lesioning means deliberately destroying a small area of brain tissue, usually in animals, to see what changes, which raises serious ethical questions.