They said neurons in a memory center of the brain called the hippocampus make a mental map of new places and experiences, then store them away for future use.
But when two experiences begin to seem very much alike, these mental maps overlap and start to blur.
"Deja vu occurs when this ability is challenged," said Susumu Tonegawa, a professor of biology and neuroscience at the Massachusetts Institute of Technology in Cambridge, whose work appears in the journal Science.
It is really just a malfunction in the brain's ability to sort through new information, something called episodic memory.
"This is very important for an intelligent animal like human beings so you know what's going on around you and you can recall it later," said Tonegawa in a telephone interview.
He and colleagues studied mice that were genetically altered to lack a gene in a specific part of the hippocampus called the dentate gyrus, which they found to be critical in forming the ability to sort through similar experiences.
Mice who lacked this ability were moved from one cage to a second, similar cage and then back to the first cage. In one cage, they got a mild electrical shock to the foot. In the other, they did not.
The mutant mice associated both cages with danger and began to freeze when placed in either cage -- they could not determine in which cage they got shocked.
Healthy mice quickly learned the difference and only froze in the dangerous cage.
When the researchers tested the animals' brain activity, the mutant mice reacted similarly in both cages, but the brain activity of the healthy mice was different in each.
Tonegawa said the type of memory that allows people to quickly distinguish different faces and places fades with age.
"Since we know the molecular and cellular pathway based on our results, there is a possibility to use those molecular targets to develop a drug to improve this connection," he said.
That is especially the case for neurodegenerative diseases like Alzheimer's.
He said the study settles 35 years of debate over how the brain can distinguish between similar places and experiences.
"One big question about the memory is now taken care of," he said.--------------------------------------------------------