Originally Posted by denverdoc
Deja vu happens to occur when short term memory is transferred to long term memory w/o indexing.
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This short term/long term mixup is merely someone's conjecture. In fact, deja vu has long been known by neurologists to be a simple-partial seizure in the hippocampus and surrounding brain areas that govern memory:
Anatomical origin of déjà vu and vivid ‘memories’ in human temporal lobe epilepsy :
http://brain.oxfordjournals.org/cgi/...tract/117/1/71
A "simple-partial" seizure is one that is limited to a small area of one hemisphere of the brain, and one during which there is no impairment of consciousness.
A "seizure" is when a group of neurons begins to fire by themselves without a proper stimulus. A seizure may or may not involve muscle convulsions depending on whether the seizure activity spreads to the neurons that control the muscles. Most people are only familiar with the Grand Mal seizure: full body convulsions and loss of consciousness, but that is only one type of seizure (the most serious type.) A deja vu, by itself, is a rather small and insignificant type of seizure.
Oliver Sacks believes deja vu's are also a form of Migraine aura. While it's true that many Migraine sufferers have a lot of deja vu's it is also true that there is a high incidence of co-morbidity of Migraine and seizure disorders. This complicates which disorder to ascribe a symptom to.
Back to the OP: some partial seizures can cause what J. Hughlings Jackson called a "dreamy state". This might possibly cause a situation where you aren't sure if something's an authentic memory or a dream.