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Brain Dysfunction by Location
Because different areas of the brain control specific functions (see 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.
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 (sometimes called executive functions). 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, people may become apathetic, inattentive, and unmotivated. Their thinking becomes slow. If the middle back part of the left frontal lobe (Broca area) is damaged, people may have difficulty expressing themselves in words—an impairment called Broca (expressive) aphasia (see Aphasia).
If the front part is damaged, any of the following may result:
People who lose their inhibitions may be inappropriately elated (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.
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). People may have difficulty recognizing objects by touch (that is, by their texture and shape).
If the middle part is damaged, people cannot tell the right from the left side (called right-left disorientation) and have problems with calculations and writing. They may have problems sensing where parts of their body are (a sense called proprioception).
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. They may also have trouble understanding how objects relate to each other in space. As a result, they may have trouble drawing and constructing things, and they may get lost in their own neighborhood.
If the right parietal lobe is suddenly damaged, people are usually confused. They may ignore the serious nature of their disorder or deny its existence. They may neglect the side of the body opposite the injury. Such people may be unable to dress themselves or to do other ordinary tasks.
In most people, part of the left temporal lobe controls language comprehension. If that part is damaged, memory for words can be drastically impaired, as can the ability to understand language—an impairment called Wernicke (receptive) aphasia (see Table: Testing a Person With Aphasia).
If certain areas of the right temporal lobe are damaged, memory for sounds and music may be impaired. As a result, people may have trouble singing.
Sometimes damage to part of the temporal lobe can cause personality changes such as humorlessness, extreme religiosity, and obsessiveness. People may have an overwhelming urge to write. If seizures involve the temporal lobe, people may smell bad odors that are not there (a type of hallucination). They may not be able to control their feelings or to think clearly.
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.
Seizures that involve the occipital lobe can cause hallucinations involving vision. For example, people may see lines of color when they look in a certain direction.
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