This is a stunning thread. It points to several possibilities really fascinating to me.
First, I think it says something interesting about the Hypnic Jerk (aka Myoclonic Jerk and Hypnagogic Jerk) which is discussed in an excellent previous thread. As noted there, imagery is associated more often than not with these events, which occur just as people fall asleep, and lobstah’s experience, and some of the answerers’ here, both of which I think we can reasonably equate to the imagery part of the Hypnic Jerk, suggest that previous dreams could be the source of this often surprisingly complex and involved material, which is very like a dream but occurs outside of REM sleep.
It also gives strong support to the idea that dreams and the content of dreams are an important part of cognition, rather than a trivial epiphenomenon; otherwise, why would a mechanism exist which seems so clearly aimed at resolving the action or conflict of a previous specific dream– a mechanism which involves the extra step of transferring dream content to a distinct state of the brain?
If we consider stauf’s introduction of the problem of deja vu, things could get really interesting. A paper just published in Nature, vol 443, p 287, discussed herehere and here and nicely explored in a recent Metafilter thread, demonstrates that stimulating part of the temporoparietal lobe of the left side of the brain can produce a feeling of being followed and even a feeling of being embraced by a shadowy person. According to the study, the actions of the shadow person mimic the actions of the subject, which implies that this region of the brain can make one’s own doubled actions seem to come from outside as the actions of another person. The paper also suggests this region of the brain as the source of the schizophrenic’s feeling of being followed and watched.
In the thread stauf links to, zardoz, the OP, asks about certain days during which he has the experience of deja vu "dozens of times," and goes on to say "actually it’s not quite deja vu, it’s more linked to my dreams last night… it’s hard to explain…"In answer, nanojath mentions that repeated deja vu’s can be an "early warning sign of schizophrenia" and cites an article which mentions this in passing. Other answerers report similar experiences to zardoz’s, with a particular emphasis on being especially tired during the times they (the repeated deja vu’s) tend to occur.
So, perhaps the part of the brain stimulated in the Nature study normally becomes active just as we fall asleep and can help to open a pipeline for material from unresolved dreams. When it becomes active when we are awake, perhaps it can produce feelings of deja vu, which may or may not seem to the person experiencing them as being associated with a previous dream. For me, it’s hard not to conclude that this is the root of the deja vu phenomenon. As I reread the Metafilter thread, I notice that blackleotardfront characterizes the brain stimulation events as "proprioceptive ‘deja vu.’"
The demonstrated connection with paranoia and a sense of being watched, which argues for its activation in schizophrenia, taken together with the connection to the onset of sleep indicated in this thread, could also then explain the fact that sleep deprived individuals almost always ultimately develop some degree of paranoia, a sense of being watched and a sense of alienation from themselves, as well as the development of paranoia and the ‘gang illusion’ in the course of ampthetamine intoxication.
If we posit activation of the brain region in question during dreams, which is at least an intriguing possibilty based on lobstah’s and others’ accounts, we could explain, very speculatively, how your own impulses and sensations can seem to come to you as those of the characters in your dreams, instead.
And finally (please pardon me for the length of this answer), I can’t help thinking all this, properly construed, is the key to narcolepsy.
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