A short but interesting article at Scientific American discussing what is going on in the brain when we experience déjà vu:
Although scientists have not pinpointed exactly what goes on in the brain when a person experiences déjà vu, they can make good guesses based on models of memory. All theories of memory acknowledge that remembering requires two cooperating processes: familiarity and recollection. Familiarity occurs quickly, before the brain can recall the source of the feeling. Conscious recollection depends on the hippocampus and prefrontal cortex, whereas familiarity depends on regions of the medial temporal cortex.
…The best evidence for a neural mechanism for déjà vu, which around 60 percent of people experience at least once, comes from studies of patients who experience it chronically. In 2005 cognitive neuropsychologists at the University of Leeds in England described two patients with recurring and persistent feelings of déjà vu. The patients refused to read a newspaper or watch television because they felt as if they had already seen it all before. They found it difficult to shop for groceries because they thought they had just purchased those items. The researchers discovered that these patients had damage to their frontal and temporal regions. Harm to these areas likely caused the patients’ familiarity circuitry to fire frequently, even when they were in a novel situation. In undamaged brains, déjà vu likely occurs because of processing errors in these same regions.
Full article at Sci-Am.
