Because different areas of the brain control specific functions (see Biology of the Nervous System: Viewing the Brain), the location of brain damage determines the type of dysfunction that results. Which side of the brain is affected is also important because the functions of the two halves of the cerebrum (cerebral hemispheres) are not identical. Some functions of the brain are performed exclusively by one hemisphere. For example, movement and sensation on one side of the body are controlled by the hemisphere on the opposite side. Other functions are performed predominantly by one hemisphere, which is said to be dominant for that function. For example, the left hemisphere predominantly controls language in most people. This characteristic is called left-hemisphere language dominance. Damage to only one hemisphere of the brain may cause complete loss of such functions. However, most functions (such as memory) require coordination of several areas in both hemispheres. For such functions to be completely lost, both hemispheres must be damaged.

Specific patterns of dysfunction can be related to the area of the brain that has been damaged.

Frontal Lobe Damage: Generally, damage to the frontal lobes causes loss of the ability to solve problems and to plan and initiate actions, such as crossing the street or answering a complex question. But some specific impairments vary depending on which part of the frontal lobe is damaged.

If the back part of the frontal lobe (which controls voluntary movements) is damaged, weakness or paralysis can result. Because each side of the brain controls movement of the opposite side of the body, damage to the left hemisphere causes weakness on the right side of the body, and vice versa.

If the middle part is damaged, the ability to move the eyes and to perform complex movements in the correct sequence may be impaired. People may have difficulty expressing themselves in words—an impairment called Broca's (expressive) aphasia (see Brain Dysfunction: Broca's (Expressive) Aphasia).

If the front part is damaged, any of the following may result:

• Impaired concentration
• Reduced fluency of speech
• Apathy
• Inattentiveness
• Delayed responses to questions
• A striking lack of inhibition, including socially inappropriate behavior

People who lose their inhibitions may be inappropriately euphoric or depressed, excessively argumentative or passive, and vulgar. They may show no regard for the consequences of their behavior. They may also repeat what they say.

Parietal Lobe Damage: Damage to the front part of the parietal lobe on one side causes numbness and impairs sensation on the opposite side of the body. Affected people have difficulty identifying a sensation's location and type (pain, heat, cold, or vibration).

If the back part is damaged, people cannot tell the right from the left side (called right-left disorientation) and have problems with calculations and drawing.

If the right parietal lobe is damaged, people may be unable to do simple skilled tasks, such as combing their hair or dressing—called apraxia.

If the parietal lobe is suddenly damaged, people may ignore the serious nature of their disorder and neglect the side of the body opposite the injury or even deny its existence. Such people may become confused or delirious and unable to dress themselves or to do other ordinary tasks.

Did You Know... Other areas of the brain can sometimes adapt and take over the functions of a damaged area.

Temporal Lobe Damage: If the right temporal lobe is damaged, memory for sounds and shapes tends to be impaired. If the left temporal lobe is damaged in people with left-hemisphere language dominance, memory for words can be drastically impaired, as can the ability to understand language—an impairment called Wernicke's (receptive) aphasia. Sometimes damage to part of the temporal lobe can cause personality changes such as humorlessness, extreme religiosity, and loss of libido.

Occipital Lobe Damage: The occipital lobe contains the main center for processing visual information. If the occipital lobe on both sides of the brain is damaged, people cannot see, even though the eyes themselves are functioning normally. This disorder is called cortical blindness. Some people with cortical blindness are unaware that they cannot see. If the front part is damaged, people have difficulty recognizing familiar objects and faces and accurately interpreting what they see.

Last full review/revision March 2008 by Juebin Huang, MD, PhD