HAMBURG - Most of us have experienced deja vu at least once in our lives – that uncomfortable feeling that you have been here in this place, in this situation before.
Usually the sensation lasts for just a moment and is quickly followed by the realization that you have not in fact experienced the current situation in the past, and simply cannot know the place you are in.
Nevertheless, there is the feeling that something was there in our memory, either in part or whole.
The sensation of deja vu, meaning "already seen", was given its name in the 19th century, but for much longer the mental phenomenon has both fascinated and frightened those who experience it.
Science has had little to offer by way of explanation, adding to the mystery.
There have been attempts at clearing up the phenomenon, which some people see as evidence for metempsychosis – the belief that after death the soul passes to another body, whether human or animal.
One point of departure for researchers has been that a deja vu experience is linked to an experience in a dream that has been half or totally forgotten.
The French writers on the topic Marc Tadie and his brother Jean- Yves, one a director of a university department of neurosurgery, the other a professor of literature, point to this in their book "Le Sens de la Memoire".
It is characteristic of the experience that one is certain for a moment that one has lived through this experience before but can simply not remember at what point in time.
The description of dreams in works of literature can also point to a way of understanding deja vu experiences.
"In a dream the consciousness can move freely in a space without limits, in which past and future blend," the two Tadies say about descriptions of this kind.
In a contribution last year to the German magazine "Gehirn & Geist" (Brain and Spirit, published by Heidelberg) reference is made to research into memory processes conducted by John D.E. Gabrieli and his team at Stanford University in California.
Their studies indicate that the brain structures of the Hippocampus and the parahippocampal Cortex play differing roles in this process.
While the Hippocampus enables the subject to remember events consciously, the parahipppocampal Cortex can distinguish between accustomed and unaccustomed impulses, and do this without even referring to a concrete experience.
Josef Spatt of the Ludwig-Boltzmann Institute in Vienna has based his hypothesis on this idea, suggesting that deja vu occurs when the Parahippocampus, without the Hippocampus being involved, emits a signal of being accustomed to, or comfortable with, a sensation.
At this moment, a current experience is regarded as well-known, even though it cannot be uniquely placed in time.
Like Spatt, Uwe Wolfradt, who researches self-alienation and memory phenomena at the the Institute of Psychology at the University of Halle-Wittenberg, says there are probably many regions of the brain involved in sensations of deja vu.
"The intensive feelings of self-alienation and unreality, along with the changed feeling about time, appear to indicate a complex series of events in the consciousness," he says.
While the person experiencing deja vu begins to doubt his grasp on reality for a moment, neuro-scientists believe this "little mistake" perpetrated by our consciousness allows them an unaccustomed window onto the processes of the consciousness.
"Perhaps continuing research into deja vu will explain not only how memory errors occur, but also how the brain is able to establish a continuous image of reality at all," Wolfradt says.