Deja vu is a French term, literally meaning “already seen”.
The sense of familiarity with the situation, place or experience can be limited to a single sense such as seeing or looking something or it may extend to the combination of some or all of the senses.
The past experience of something, which the observer feels now, may be attributed to a dream or it may give a feeling that such thing or situation has been really done or happened in the past.
Feelings of Déjà vu may persist from few seconds to minutes. However, in some brain disorders such as epilepsy such feelings may persist for hours or even days.
It has been found that about 2/3 of the population experience Déjà vu. It is felt by mostly young people (although such feelings may occur in any age). The experience increases in fatigue, in people with more education, in travel and when someone has liberal attitude.
Déjà vu as a side effect of medicines:
In one study, it was found that concomitant amantadine-phenylpropanolamine treatment against influenza resulted in Deja vu experiences. As the studies were there in which it was found that these medicines caused psychotic symptoms as a result of dopaminergic activity. Based on this, it was hypothesized that Déjà vu occurs as a result of increased dopamine activity. (Dopamine is a chemical compound found in the brain which is involved in the formation of epinephrine).
References and Further Reading:
Brown, A. S. (2008). 2.21 Déjà vu. Learning and Memory: A Comprehensive Reference, 2, 363-375.
Taiminen, T. et. al. (2001). Intense and recurrent déjà vu experiences related to amantadine and phenylpropanolamine in a healthy male. Journal of Clinical neuroscience, 8(5), 460-462.