Both theories have camps of staunch advocates, drawing support from particular amnesia cases that only their model can explain. But as neither theory perfectly fits together all pieces of the puzzle, the field has entered a stalemate.
Researchers from Johns Hopkins University have come up with a new theory that just might settle some of this controversy. Their explanation rests on the premise that memories are transformed each time we revisit them. According to this theory, a memory is first encoded by the coordinated activity of neurons in the hippocampus and cortex. The hippocampus acts as the brain’s director, telling the cortex which particular neurons to activate. Each time we recall that memory, a similar, but not identical set of neurons are activated. Neurons that are frequently activated become part of the permanent memory trace in the cortex, while the rarely activated ones are lost. Every reactivation re-encodes the memory, and depending on what cortical neurons are engaged, can strengthen, weaken or update particular memory features.
On the surface, this new model sounds a lot like the earlier ones. But it breaks the longstanding stalemate by proposing that what we do with a memory, rather than its age or type, determines where it’s stored in the brain. While the competing theories debated whether the hippocampus is only needed for recent memories or episodic memories, the new model suggests that what really matters is how often you revisit the memory. When a memory is recalled often, it will more rapidly become stored in the cortex, become less episodic and become independent of the hippocampus. But a memory that’s rarely revisited will remain dependent on the hippocampus. Older memories might be recalled more often, but the relationship isn’t perfect. This would explain why one amnesic’s memory impairment extends back forty years, while another’s extends only ten years.
The theory also nicely accounts for our subjective sense for how our memories change over time; namely, how the hippocampus and cortex collaborate to gradually fade or distort our memories. Say you’re reminded of that beach vacation every summer. With each memory reactivation, some features are reinforced while others disappear, explaining why the memory seems to get fuzzy over time. And the more details that are lost, the less “episodic” and the more “semantic” the memory becomes, accounting for the sense of personal detachment often associated with very old memories. Each time you think back to your Florida vacation, you re-encode fewer details, making the memory feel less vivid now than it did decades ago. Today, you might still be able to recall your striped blue bathing suit even though the smell of the ocean air is lost.
Each mental trip back to Florida is not only an opportunity to strengthen or weaken the memory, but also to incorporate fictional tidbits. You used to be sure that vacation was in Fort Lauderdale, but your sister always talks about the fun family trip to Miami. Every time you reminisce together, the memory of Fort Lauderdale is reactivated, but so is a competing representation of Miami. Next time you think of the vacation, the Fort Lauderdale and Miami representations conflict, causing uncertainty over where you actually went. Recall the beach in Miami enough times and voilà, a false memory is born!
Memories fade and transform as they age. This intriguing new theory suggests that these changes have to do less with the age or content of a memory, and more with what we do with that memory. Changing the past just might be easier than we thought. Chances are, you do it every time you remember.
Are you a scientist who specializes in neuroscience, cognitive science, or psychology? And have you read a recent peer-reviewed paper that you would like to write about? Please send suggestions to Mind Matters editor Gareth Cook, a Pulitzer prize-winning journalist and regular contributor to NewYorker.com. Gareth is also the series editor of Best American Infographics, and can be reached at garethideas AT gmail.com or Twitter @garethideas.