New evidence suggests that researchers can tell which memory of a past event a person is recalling from the pattern of their brain activity alone.
The results, reported online on March 11th in Current Biology, a Cell Press publication, follow an earlier discovery by the same University College London team that they could tell where a person was standing within a virtual reality room in precisely the same way (http://www.eurekalert.org/pub_releases/2009-03/cp-cts030509.php). The researchers say the new results move this line of research along because our episodic memories—those recollections of the everyday events that make up the autobiography of our lives—are expected to be more complex, and thus more difficult to crack, than your basic spatial memory would be.
"We've been able to look at brain activity for a specific episodic memory—to look at actual memory traces," said senior author of the study Eleanor Maguire. "We found that our memories are definitely represented in the hippocampus. Now that we've seen where they are, we have an opportunity to understand how memories are stored and how they may change through time."
In order to pull this off, Maguire and her colleagues Martin Chadwick, Demis Hassabis, and Nikolaus Weiskopf showed ten people each of three very short films before brain scanning. Each movie featured a different actress and a fairly similar everyday scenario. For instance, Maguire explained, in one of the films a woman rifles through her purse to find an envelope that she then drops in a mailbox. In a second film, another actress finishes her cup of coffee and drops the empty cup in a trash can.
The researchers scanned the participants' brains using functional magnetic resonance imaging (fMRI) while the participants were asked to recall each of the films. The researchers then ran the imaging data through a computer algorithm designed to identify patterns in the brain activity associated with memories for each of the films. Finally, they showed that those patterns could be identified in independent fMRI data to accurately predict which film a given person was thinking about when he or she was scanned.
The results imply that the neuronal traces of episodic memories are stable, and thus predictable, even over many re-activations, the researchers report. Although the patterns in individual brains do vary from one another, they also showed remarkable similarities in the parts of the hippocampus that were active, Maguire added.
"Now that we have shown it is possible to directly access information about individual episodic memories in the human hippocampus in vivo and noninvasively, this offers new opportunities to examine important properties of episodic memory, to explore possible functional topographies, and to examine neural computations within hippocampal subfields," the researchers conclude.
The researchers include Martin J. Chadwick, Demis Hassabis, Nikolaus Weiskopf, and Eleanor A. Maguire, of University College London, London, UK.