Thus, an inhibition center developed below what in humans is the temporal lobe, to turn off much of the functioning of the reptilian brain; and an activation center evolved in the pons to turn on the R-complex, but harmlessly, during sleep.

Carl Sagan
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The idea that the temporal lobe controls our emotions, while the pons controls our imagination, is not new. For hundreds of years, scientists have postulated that humans have two brains: one for rational, linear thought and another for the more creative, non-linear process of dreaming. This theory was revived by neurobiologist Paul MacLean in the 1960’s. He called these two brains “the left brain” and “the right brain.” As MacLean further elaborated on his theory of organ division in humans, he realized that the division of the brain into two halves did not always exist in mammals.

Indeed, many animals, including birds and reptiles, appear to have two hemispheres in which each half controls distinct functions (see image above). MacLean believed that these animals had evolved this way to make it easier for them to deal with confusing or dangerous situations. The side of the brain that was on when they were awake would be more able to handle the situation rationally. The side of the brain that was off would be more able to handle creative thought while asleep. MacLean had some evidence to back up his theory.

He pointed out that some animals are born with one hemisphere that is larger than the other. If they are put into a stressful situation, they can switch which hemisphere is dominant by quickly rotating their heads until the larger hemisphere is facing forward when they are awake. MacLean conjectured that this ability is why some people seem to have better spatial skills when they are sleeping than when they are awake. MacLean also hypothesized that humans do not always need both halves of their brains for different functions.

When one “half” is dominant over the other, it can cause problems with behavior or memory because it dominates either imagination or reason. MacLean suggested that there are periods when people are usually most creative at night when they are sleeping but they may be most dominant when awake during times of stress or during certain emotional situations (e.g., love). It also might explain why creativity seems to be enhanced by sleep deprivation in some people (a condition called REM sleep disorder). Other researchers have found evidence to support MacLean’s theory of duality in humans as well as other animals.

Some studies have shown that people can control one side or another at will during normal waking hours without any apparent ill effects on their behavior or memory (e.g., prefrontal lobotomy in humans). Other research has demonstrated

Source: Dragons Of Eden: Speculations On The Evolution Of Human Intelligence

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