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Another common form of mental illness is bipolar disorder, in which a person suffers from extreme bouts of wild, delusional optimism, followed by a crash and then periods of deep depression. Bipolar disorder also seems to run in families and, curiously, strikes frequently in artists; perhaps their great works of art were created during bursts of creativity and optimism. A list of creative people who were afflicted by bipolar disorder reads like a Who’s Who of Hollywood celebrities, musicians, artists, and writers. Although the drug lithium seems to control many of the symptoms of bipolar disorder, the causes are not entirely clear. One theory states that bipolar disorder may be caused by an imbalance between the left and right hemispheres. Dr. Michael Sweeney notes, “Brain scans have led researchers to generally assign negative emotions such as sadness to the right hemisphere and positive emotions such as joy to the left hemisphere. For at least a century, neuroscientists have noticed a link between damage to the brain’s left hemisphere and negative moods, including depression and uncontrollable crying. Damage to the right, however, has been associated with a broad array of positive emotions.” So the left hemisphere, which is analytical and controls language, tends to become manic if left to itself. The right hemisphere, on the contrary, is holistic and tends to check this mania. Dr. V. S. Ramachandran writes, “If left unchecked, the left hemisphere would likely render a person delusional or manic.… So it seems reasonable to postulate a ‘devil’s advocate’ in the right hemisphere that allows ‘you’ to adopt a detached, objective (allocentric) view of yourself.” If human consciousness involves simulating the future, it has to compute the outcomes of future events with certain probabilities. It needs, therefore, a delicate balance between optimism and pessimism to estimate the chances of success or failures for certain courses of action. But in some sense, depression is the price we pay for being able to simulate the future. Our consciousness has the ability to conjure up all sorts of horrific outcomes for the future, and is therefore aware of all the bad things that could happen, even if they are not realistic. It is hard to verify many of these theories, since brain scans of people who are clinically depressed indicate that many brain areas are affected. It is difficult to pinpoint the source of the problem, but among the clinically depressed, activity in the parietal and temporal lobes seems to be suppressed, perhaps indicating that the person is withdrawn from the outside world and living in their own internal world. In particular, the ventromedial cortex seems to play an important role. This area apparently creates the feeling that there is a sense of meaning and wholeness to the world, so that everything seems to have a purpose. Overactivity in this area can cause mania, in which people think they are omnipotent. Underactivity in this area is associated with depression and the feeling that life is pointless. So it is possible that a defect in this area may be responsible for some mood swings.
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Michio Kaku (The Future of the Mind: The Scientific Quest to Understand, Enhance, and Empower the Mind)