INSIDE THE HUMAN BRAIN.

How do scientists find out how the brain works? They use a number of different procedures that map out parts of the brain.

A relatively new technique called a PET scan (positron emission tomography) shows which parts of the brain are active while a person performs certain tasks. Doctors first inject a harmless radioactive glucose (sugar) solution into the patient's arm. The glucose soon enters the brain, which absorbs the solution. As the patient performs a particular task (for example, looks at a picture), the part of the brain that's involved in the task consumes large amounts of glucose. Sensors detect the consumption rate and display the active region on a computer screen.

Surgeons sometimes use a more invasive method of brain mapping, electrically stimulating different sections of the brain during surgery. The brain is not sensitive to pain, so a patient can be awake during this procedure. As a surgeon stimulates one part, the patient could say what he or she is experiencing.

Regardless of the procedure used, scientists have gained tremendous insight as to how the brain works.

The cerebral cortex is a thin layer, less than 1/3 of a centimeter, that forms the outermost part of the cerebrum. The cortex is folded with many grooves and ridges, which increases the surface area of the cortex and the number of nerve cells it contains.

The cerebrum makes up about 85 percent of the brain's total weight. (The human brain weighs about 1.4 kilograms.) It consists of two hemispheres, the right and left. Each hemisphere, in turn, is comprised of four sections, or lobes: frontal, temporal, parietal, and occipital lobe.

The brain consists of up to 100 billion neurons, or brain cells.

Threadlike extensions called dendrites reach out to make connections and communicate with other neurons.

The motor cortex controls voluntary muscle movements.

The frontal lobe controls reason, emotion, judgement, and voluntary movement.

The temporal lobe is the center for hearing and memory.

The occipital lobe controls the vision centers.

The parietal lobe contains important sensory centers.

Ventricles hold a clear liquid called the cerebrospinal fluid, which forms a thin layer that cushions the brain's soft tissues against the skull's hard bones.

The corpus callosum is a bundle of nerves that connects the two halves of the brain. In some epilepsy cases, surgeons sever this connection so that the left and right hemispheres are unable to communicate with each other.

The cerebellum controls balance, posture, and coordination of movement.

The brain stem connects the spinal cord to the cerebrum.

The aura mater is the tough outermost membrane that protects the brain.

Even though the brain's hemispheres are virtually identical, certain areas (like the language center, (for example) may be larger in one hemisphere.