Déjà vu… It is really very difficult to study it— the odd feeling you get when you sense you’ve already experienced something that you know you are doing for the first time — in the laboratory. This is mainly because the phenomenon is very rare and difficult to reproduce. There are similarities between déjà vu and the more common experience of seeing a person who seems vaguely familiar to you, Still who’s name, how you know them, and where you previously met escape you.
Unlike déjà vu, scientists are able to test feelings of familiarity in the laboratory. One way they do this is by asking research participants to scan and quickly assess the familiarity of faces or places they have seen before and those that they are seeing for the first time. Such research helped researchers come to understand the familiarity and recollection are two different forms of memory that work together during recognition. While people experience the sense of familiarity rapidly, recollection this requires the recovery of associations prompted by a critical cue, takes longer. For instance, if you begin a casual conversation with that person you know you recognize but cannot remember the place, you might start to uncover details that trigger memories revealing the person’s name and how you know them.
Functional MRI (fMRI) studies of people are asked to judge the familiarity of faces and buildings have revealed and when people try to distinguish familiar from novel faces, changes in activity occur in an area of the temporal lobe which is called the perirhinal cortex. An adjacent area called parahippocampal cortex is shows activity changes when people distinguish familiar buildings from those they are seeing for the first time.
Thus, déjà vu for a face may be the result of messages which is sent from the perirhinal cortex whereas déjà vu for a place may stem from messages relayed from the parahippocampal cortex. Both of these regions send their information to the hippocampus, which supports recollection. So, the full experience of recollection may reflect a combining of converging signals from both perirhinal and parahippocampal areas to the hippocampus.