The purpose of dreaming still remais largely unexplained. However, researchers have been proposed several theories based on neurobiological findings and behavioral contents of dreams.

The Content of Dreams

Many normal dreams are unpleasant. Calvin Hall (7)) catalogued over 10.000 dreams from normal people, and found that:
64% - were associated with sadness, apprehension, or anger;
18% - were happy and exciting;
1% - were associated with sexual involvement;
Two - Hostile acts by or against the dreamer, such as a murder, attack, or denunciation.

Bizarre Dreams

Crick and Mitchison proposed that the function of dreaming is to eliminate certain undesirable forms of interaction between cells in the cortex that could be prejudicial to the brain (5).. They suggested that dreams could be a mechanism for unlearning or forgetting, where, in this situation, associations are weakened. "We dream to forget," the authors wrote. That is, we dream to reduce fantasy and obsession. This theory predicts that flaws in the ability to process REM sleep, may cause fantasies, hallucinations and obsession. They also believe that the brain needs to get rid of information processed during the awakened state, and that dreams are a channel for the elimination of these informations and for the adjustment of the brain. The responsible agents in this search for balance, would be special structures in the brain.

In studies with neural simulation in computer's network, an advanced technology in computational neuroscience, which is believed to be operated in a way similar to the brain, the authors demonstrated that the nets became overloaded when it was attempted to store an excessive amount of information in them. In this case, the net produced bizarre associations (which may be compared to "fantasies" in dreams), and it tended to return the same result, whichever input data were provided ("obsession"), and could respond to inappropriate input signals, which normally didn't produce response (hallucinations). This theory predicts that flaws in the ability to process REM sleep, can generate fantasies, hallucination and obsession (9).

Emotionals Dreams

Emotions in dreams may reflect the dreamer's personality as well as his or her situation in waking state. Dreams can express worries, wishes, insecurity, ideas of grandeur, jealousy, love, fears, and other feelings or sensations, revealing different aspects of a person's mental state. The evidence of this observation was demonstrated by Rosalind Cartwright (SciA, 47), in a study involving subjects that were separated and divorced. In 70 studied individuals, the content of dreams were strongly related to the way the person was dealing with the crisis in question (the divorce).

Crick and Mitchison (5) suggest that the prevalence of emotion in dreams can only be a characteristic of dreams that are remembered, provided that individuals are awakened by the dreams due to the anxiety associated with them. In these cases, the learning and forgetting processes revert themselves to a positive learning and its recurrence can be explained. This possibility could be tested by observing whether dreams reported by subjects "artificially" awakened during an experiment show a decrease in the proportion of "anxiety-causing" themes.

Dreams With Anti-Social Acts and With Deceased People

In the awakened state, the cortex analyses with precision, impulses arriving from several receptor organs in the sensorial system, coming to a decision and generating an integrated response as, for example, the arm's movement (an action of the effector organ) when picking up a knife. The cortex manifests itself also in the deliberate inhibition of the action (for example, to throw the knife toward a person). For Kleitman (11), in the process of dreaming, the same type of cortical activity process itself in an inferior level of performance. The analysis of phenomena is flawed, the memory mixes the past and the present; the dreamer recognizes a dead person, but accepts its presence without surprises. Consequently, the integration of the cortical response is incomplete and the dreamer is many times lead to commit imaginary anti-social acts. Happily, the impulses of the sleeping cortex die on the way of the effector organs and nothing bad happens. After a sudden awakening, even normal people can become confused and act in a disorganized way for some time.

Corporal Movements During Dreams

Some of the body movements are related to dreams' content . Edward Wolpert, from the University of Chicago, held electrodes to limbs on sleeping s ubjects and registered the electrical action potential (the force applied on an electrically charged particle) of the muscles. The register in one of the subjects showed a sequence of motor activity first on the right hand, then on the left hand, and finally on the legs. Immediately awaken, the subject related that he dreamed he had held a bucket with his right hand, transferring it to the left hand, and then started to walk. Extending this to somnambulism, he speculates that this disturbance can be an extreme expression of such motor deflux to the extremities.

The Evolutive Nature of Dreams

Jonathan Winson (16) suggests that dreams reflect an individual strategy for survival. For him, the nature of REM dreams sustain an evolutive evidence. During the day, animals process information in their brains to be able to walk and move their eyes, with the purpose of feeding, defend themselves against predators, etc. During the night, while processing again those informations during the REM sleep, such re-processing wouldn't be easily separated from locomotion, as this would demand a big revision of brain circuitry. Then, to remain asleep, the locomotion must be suppressed, inhibiting motor neurons (those that promotes the locomotion). The eye movements, on the other hand, don't need to be suppressed because its activities don't disturb the sleep.

Other theories sustain that dreams can reflect a mechanism of memory processing that is inherited from inferior species, in which the important information for survival reprocessed during REM sleep is sensorial (9). According to our ancestral mammals, dreams in humans are sensorial, specially visual. The congenitally blind have auditory dreams, and those who lose their sight gradually lose their ability to dream visually.

Author: Silvia Helena Cardoso, PhD. Psychobiologist, master and doctor in Sciences by the University of São Paulo and post doctoral fellowship by the University of California, Los Angeles. Invited Professor and Associate Researcher of the Center for Biomedical Inofrmatics, State University of Campinas (Unicamp), Brazil.