A "Deja vu" means "already seen" in French.
The exact cause of this phenomenon is still unknown, but there are a few hypotheses to explain this.
One popular theory is that deja vu occurs when a stimulus erroneously or insufficiently activates an existing memory.
Such stimulus may elicit only an impression of familiarity without complete successful recall.
This hypothesis is supported by a recent virtual reality study, which showed that when a scene resembled a previously viewed but forgotten scene, the viewers tended to report higher familiarity and deja vu ratings than when they saw a novel scene.
It is possible that deja vu occurs when a few brain areas interact with each other closely, particularly the areas involved in memory formation (such as the rhinal cortices and hippocampus) and emotion (such as the amygdala).
Evidence supporting this idea came from a recent study of epileptic patients, who have neurological diseases characterised by epileptic seizures.
To localise the source of the seizures, it often requires implanting electrodes in the brain to monitor patients' brain activities, and sometimes direct stimulation of the brain is necessary to see if/where seizures can be induced.
It was discovered that with direct electrical stimulation in the brain, deja vu may be artificially induced in some epileptic patients.
Specifically, deja vu is often observed when the neural activities in the rhinal cortices, hippocampus and amygdala are more synchronised.
This finding suggests that the neural basis of deja vu may exist in the co-operation between these brain areas.
Assistant Professor Hsieh Po-jang,
Neuroscience and Behavioural Disorders Programme,
Duke-NUS Graduate Medical School Singapore
Have a burning science question? E-mail us at STscience@sph.com.sg
This article was first published on September 4, 2015.
Get a copy of The Straits Times or go to straitstimes.com for more stories.