From the BBC :
Two key processes are thought to occur when someone recognises a familiar object or scene. First, the brain searches through memory traces to see if the contents of that scene have been observed before. If they have, a separate part of the brain then identifies the scene or object as being familiar. In deja vu, this second process may occur by mistake, so that a feeling of familiarity is triggered by a novel object or scene.
The Leeds team set out to create a sense of deja vu among volunteers in a lab. They used hypnosis to trigger only the second part of the recognition process – hoping to create a sense of familiarity about something a person had not seen before. The researchers showed volunteers 24 common words, then hypnotised them and told them that when they were next presented with a word in a red frame, they would feel that the word was familiar, although they would not know when they last saw it. Green frames would make them think that the word belonged to the original list of 24. After being taken out of hypnosis, the volunteers were presented with a series of words in frames of various colours, including some that were not in the original 24 and which were framed in red or green. Of the 18 people studied so far, 10 reported a peculiar sensation when they saw new words in red frames and five said it definitely felt like deja vu.