Crick and Mitchison's theory does not explain the enormous incidence in dreams of situations related to fear, anxiety or sexual desire.
There might be an a dvantage in enhancing the memory of situations which induced fear, for the individual would avoid a repeat of the dangerous situation. Events in the future which cause anxiety are usually important events for which we do not feel sufficiently prepared. Remembering them well could be of help. Sex is without doubt the most important drive in natural selection terms, so that a recreation in the mind of potentially successful sexual encounters could enhance fitness. Emotions are the link between cognitive information which the animal acquires during its lifetime and the biological drives which are hard-wired into its genome and which make complicated behaviors which favor the survivor. In this way, situations with possibly important implications for the individual are assigned emotions by the central nervous system. It is thus important that such information be assigned a privileged position in memory. Dreams could play a role in the maintenance of this privileged set of data.
A priori, this could seem to imply a complex cognitive-level mechanism to select dreams which contained fear, anxiety or sexual arousal. However, this is not necessary and is not supported by the evidence for apparently random bursts of activity of the brain stem. The sele ction of emotionally arousing memory contents in dreams could be done in a more general and unspecific manner by the activation of emotional centers in the limbic system and of "startle" networks in the brain stem.10 In contrast to Hobson's pr oposal which describes an "intensification of emotion," I propose that there is selective recall of emotion-arousing memories, suggesting there is a distinctive feature in the way these memories are stored: there is some label of the memories as emotion-a rousing. This is not surprising, since the recall of events which caused the emotion when they happened usually causes emotion itself. Rather than emotion being added to the dreams, I suggest that the activation of emotional centers causes the recall of memories which were already emotional when they were stored. I base my proposal on the fact that dreams are usually related to waking-life emotional situations (concerns, fears, sexual desires), and not with non-emotional or absolutely fictional (non-rel ated to waking-life) situations that are "artificially colored" with emotion. In addition, I can find a function for the first, while I cannot say the same of the second. And, as Hobson admits, mechanisms as complicated as these are unlikely to have evo lved if they do not have a selective advantage.11
This proposal will remain speculative until experiments have been performed to confirm or deny it. Such tests could include the use of drugs that inhibit the functioning of the sy stems underlying emotion and verifying whether dreaming is reduced or if the topics of dreams change significantly, and the search for a lesion in rat brains - or a human disorder - that will impair the subjects' ability for the recall or storage of emot ion-associated conditioned responses (e.g. anger or fear) without disabling the ability to display emotional behavior itself or the learning or recalling abilities of non-emotion related memories.
Crick and Mitchison suggest that the prevalence of emo tionally arousing themes in dreams may be a characteristic only of dreams that are remembered, because they awake the subject due to the anxiety associated with them. In these cases, the learning process reverts from reverse learning to positive learning, and so their recurrence could be explained. This possibility could be tested by observing whether the dreams reported by subjects "artificially" awakened during an experiment show a decrease in the proportion of "anxiety-causing" themes.
In the selec tion of memories for dreams, it is obvious that other factors, apart from emotionality, intervene: one such example is the age of the memory. This is functionally advantageous too: recent events which aroused emotion are more likely to happen again in the near future than distant ones, and so intensifying their memory is likely to be more useful to the individual.
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