pathways to the frontal lobes of the cortex, where feelings are monitored and interpreted. These two brain structures next influence the hypothalamus, which transmits the messages that trigger appropriate physical responses.
The latest research suggests that different parts of the brain may process emotions differently. Scientists have found that the frontal lobes of the left hemisphere display more electrical activity when subjects experience positive emotions such as enthusiasm or happiness, and the frontal lobes of the right hemisphere display more electrical activity when the subjects experience negative emotions such as disgust or sadness.
While lower animals are guided by primitive instincts and may experience only rudimentary feelings, most mammals probably do have a richer range of emotional feelings. Evidence of the limbic system's so-called pleasure center was discovered in 1953 by scientists at McGill university, when a rat incessantly self-stimulated an area of the brain called the septum in the front part of the hypothalamus. Further tests revealed that not just rats, but rabbits, dogs, dolphins, monkeys, and even humans experienced intense pleasure when this part of the brain is stimulated.
The limbic system controls not just pleasure but also a wide variety of emotions with many subtle shades. Separate emotions do not exist in individual, isolated areas of the brain, however; instead, emotional reactions appear to develop from a wide range of neural pathways throughout the limbic system, working together to produce a symphony of human emotion.
encephalitis An often-fatal inflammation of the brain that can cause damage on both sides (especially the medial temporal lobe) and the orbital frontal lobe. Encephalitis may be caused by several different viruses, but the herpes simplex virus is most commonly the cause. Many times, the meninges (membranes that cover and enclose the brain) are also affected.
While an encephalitis attack may be so mild that the patient does not notice anything amiss, more often it is a serious condition.
In addition to the herpes virus, encephalitis may be caused by a virus transmitted to humans by mosquito bites, causing St. Louis encephalitis; other cases may be caused by infection with HIV virus, the organism responsible for aids. On rare occasions, the condition may follow viral infections such as measles or mumps.
Symptoms often begin with headache, fever, and prostration that leads to hallucinations, confusion, paralysis, disturbed behavior, and problems in speech, memory, and eye movement. The amnesia in patients with this disorder is probably caused by the destruction of the hippocampus and amygdala. Some patients may also suffer damage to the front lobes and in extreme cases show symptoms of the Kluver-Bucy syndrome, a condition causing a range of symptoms including amnesia, visual problems, and altered sexual behavior. There may be a gradual loss of consciousness and sometimes coma; seizures may also occur.
If the meninges area also is inflamed, the neck usually becomes stiff and the eyes are abnormally sensitive to light.
A diagnosis can be determined from symptoms plus results of brain scans, and eeg (electroencephalogram), and a lumbar puncture (a sample of spinal fluid). Blood tests may also be necessary.
The antiviral drug acyclovir is an effective treatment for encephalitis caused by the herpes simplex virus. However, there is no treatment for the disease caused by other viruses. Some patients die, and others have brain damage, behavioral problems, and persistent epilepsy.
encephalitis lethargica An epidemic form of encephalitis (inflammation of the brain) that has not been seen in major outbreaks since the 1920s. Occasional isolated cases do still occur.
About 40 percent of all patients died during the epidemics; those who survived developed posten-cephalitic parkinsonism, a movement disorder
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