Repetition has a strange relationship with the mind. Take the experience of déjà vu, when we wrongly believe we have experienced a novel situation in the past – leaving us with an spooky sense of pastness.

But we have discovered that déjà vu is actually a window into the workings of our memory system.

Our research found that the phenomenon arises when the part of the brain which detects familiarity de-synchronises with reality. Déjà vu is the signal which alerts you to this weirdness: it is a type of "fact checking" for the memory system.

But repetition can do something even more uncanny and unusual.

The opposite of déjà vu is "jamais vu", when something you know to be familiar feels unreal or novel in some way. In our recent research, which has won an Ig Nobel award for literature, we investigated the mechanism behind the phenomenon.

Jamais vu may involve looking at a familiar face and finding it suddenly unusual or unknown. Musicians have it momentarily – losing their way in a very familiar passage of music. You may have had it going to a familiar place and becoming disorientated or seeing it with "new eyes".

It's an experience which is even rarer than déjà vu and perhaps even more unusual and unsettling. When you ask people to describe it in questionnaires about experiences in daily life they give accounts like: "While writing in my exams, I write a word correctly like 'appetite' but I keep looking at the word over and over again because I have second thoughts that it might be wrong."