Functional magnetic resonance imaging (fMRI) brain scans are not very good at distinguishing between true and false memories, Stanford psychologists and law experts have found in experiments.

“We were able to differentiate between rich recollection, strong familiarity or a weak memory,” said Anthony Wagner, an associate professor of psychology who conducted the experiments with postdoctoral fellow Jesse Rissman and law Professor Hank Greely. “But there was very weak evidence that what the subject actually experienced was etched in the brain.”

In the first of two experiments, the researchers had 16 subjects study photographs of 200 faces. During an fMRI scan, the subjects were shown each of those 200 faces again, plus another 200 faces they had never seen. Every time someone looked at a face, they were asked to rate how well they recognized that image.

Using multivoxel pattern analysis, the researchers could decode the subject’s sense of recognition with “remarkable accuracy,” Rissman said.

But the researchers found that memories could play tricks on people’s brains. Sometimes a picture of a face would spark a memory even if the subject had never before seen that face, and in these cases the brain activity pattern closely mimicked that of a real memory.

A second experiment was designed to gauge people’s memories when they weren’t being asked to remember anything. In contrast to the first experiment, when the subjects weren’t prompted to say how well they remembered a face they were shown, their brain activity failed to indicate whether or not they had a memory related to the image.

“We are by no means at the level you’d want for a technique that might be used in a courtroom to probe a defendant’s memory and uncover the truth about his or her past experiences,” Rissman said. “Brain imaging analysis will definitely develop, but I’m doubtful that the technology will ever be capable of providing a 100 percent reliable determination of whether somebody actually had a particular experience.”

More info: Stanford University news and Detecting individual memories through the neural decoding of memory states and past experience