Scientific American have a summary of the latest explanations of deja vu and to summarize the summary, we still don’t really know.
Or, at least, there are three plausible partial explanations, none of which is the full story nor rules out effect of the others and none of which can easily be proven or disproved. They are known (in some circles) as the Marcel Proust, the Charlie Chaplin and the Homer Simpson.
Rather pleasingly, all three are in tune with the (non-bonkers) folk wisdom explanations that it is “second sight” (in a non-bonkers sense).
The first contributor to a deja vu experience might be quite simply that you have seen or otherwise experienced something extremely similar before, and the latent memory triggers the feeling of familiarity. I certainly felt this type of deja vu before. Returning wearily home on SilverLink train, not long after a heavy weekend, I had a mild wave of deja vu as I stared vacantly across the ailse at nothing in particular. It was rather nice, and wanting to recreate it I stared more intently, that didn’t work so I tried staring less intently but that didn’t work either but it made me realise that i had seen a very similar sight many times before. No wonder the view was reminscent of something.
Secondly, we might have recognition of something that was previously only on the periphery of our attention, and just like in the silent movies we do a double-take but here’s the clever bit without doing the double-take part in that we don’t realise this has happened… instead we get that spooky deja vu feeling all over again. Cool. Can’t say, I noticed experiencing it in this way but then that’s the whole point, if I’d noticed it wouldn’t be deja vu! Great idea but very hard to test.
Thirdly, we have Robert Efron’s neurological explanation that it is caused by bits of your brain working slower than they are supposed to.. He speculated that the left lateral or medial temporal loaf (that’s temporal = temples) is where your conscious experiences are sorted into correct temporal order (that’s temporal = time [as if this stuff wasn’t complicated enough already, already!]). You get a set of visual information from each of your hemispheres. If for some unexplained reason, the stuff coming from way out in right field dawdles along the way it jumbles up the timing and see everything twice or so we think. It’s another nice idea and some tests are possible, temporal lobe epileptics report more deja vu that the rest of us. And if I were to cut your head open and poke your left temporal cortex with high-voltages you’d get a very strange feeling.
(And it occurs to me that in some sense this is a consequence of binocular vision, so congenital cortical cyclopses shouldn’t experience it.)
So there you go… and if you had read the article you’d have seen all this already, I only mention it because I haven’t yet found any research on deja vu and recreational drugs.
I have on occasion and mostly in the interests of science taken doses of mind-altering substances.. (though like in all good trials sometimes they sell me placebos) and I have noticed that in the few days after taking an ecstacy tablet I experience a (statistically??) significant increase in deja?vu. Usually very mild, (like that time on the train, remember? ) but deja vu all the same. I’ve conducted an informal survey of other experimentalists and sadly none report anything similar but purely on my own anecdotal experience i’m convinced there must be some link. Think I might have to wade through a few of those daunting 1000 page neuro-physiology textbooks to come up with a possible explanation.