Déjà-vu is the eerie feeling that something has already happened before. Some people have linked this phenomenon to past lives, dreams, and matrix glitches but science suggests the reality is, sadly, far more mundane: déjà-vu is simply a memory we have forgotten returning without us realizing it.
Déjà-rêvé, however, is much rarer and, so far, has only been found to occur during seizures in epilepsy patients. The word literally translates to “already dreamed” and relates to a recollection of an experience that occurred during sleep.
A team of researchers examined accounts of déjà-rêvé triggered during electrical brain stimulation (EBS) from 23 published studies and seven case notes recorded in epilepsy treatment databases. Their results have been published in the journal Brain Stimulation.
“Electrical brain stimulation makes it possible to trap this phenomenon that is almost impossible to trap in everyday life,” Jonathan Curot, from Toulouse University Hospital, France, who led the study, told New Scientist.
In total, there appear to be three categories of déjà-rêvé. The first, called “episodic-like”, recalls a specific dream that occurred at a specific time. As one patient described:
"Doctor: ‘What did you feel?’ Patient: ‘Something that was in my dreams. […] Well, actually it was with a friend. We read a comic. […] And in fact, it was a nightmare. And then after there is a big beetle coming.’ Doctor: ‘Was it a nightmare you had long ago?’ Patient: ‘Yes. […] I was 3–4 years old.’"
The second is “familiarity-like”. While the patient might not be able to recall the specific dream, they express a vague reminiscence.
“Well, when I started to read, I had a feeling of déjà-rêvé. I don’t know if I have dreamed about it last night. I thought of something I dreamed. And I had the feeling of fear. […] Well I saw a character and I thought I saw him in a dream.”
The third and final is literally a “dreamy state”, during which the patient feels like they are dreaming.
“A funny feeling like passing out… I feel like I dreamed it. Like something I had seen…. Like I am floating.”
The first two categories of déjà-rêvé appear to have been induced by stimulation of the medial temporal lobe, an area of the brain involved in long-term memory, memory formation during sleep, and normal dreaming. The “dreamy state” was more diffuse and generally occurred on the brain's lateral neocortex.
While the paper suggests that déjà-rêvé is a real phenomenon that can be triggered by a zap to the brain, it relies heavily on case studies. It would, therefore, be interesting to see if these results could be replicated in a controlled study.
“It may be disputed that we cannot be sure that déjà-rêvé (episodic-like or familiarity-like) are linked to true previous dreams,” the study authors explained.
“EBS could simply produce a random combination of visual and auditory elements, creating phantasmagoric and unrealistic experiences similar to dreams.”