Pixar’s Inside Out portrays memories as glowing individual spheres that we replay in our minds like a movie on a projector screen. But in real life, neuroscientists have found that memories are not compartmentalized into perfect little bubbles; they are represented over a largely distributed set of brain regions. And the same brain regions at play when we remember something reactivate when we try to recall the memory. In a new study, researchers found that the memories we recall most vividly have the greatest patterns of brain reactivation.
“Recollection is what happens when we suddenly feel like we are reliving a moment from our past inside our head, says Marie St-Laurent of the Rotman Research Institute at Baycrest. “It is the subjective experience of being transported back in the moment.”
She and colleagues wanted to understand how the subjective experience of recalling a memory matches up with the physical experience of reactivating memory regions in the brain. “Our goal was to provide evidence for something that is fairly intuitive, but for which there is not much evidence in the current scientific literature.” St-Laurent says.
As published online in the Journal of Cognitive Neuroscience last month, St-Laurent’s team designed a study that asked participants to watch various video clips while they measured brain activity in the fMRI. The participants saw each film clip 21 times and had to recall them several times. “By asking people to access the same memories several times, we could track how each particular memory varied from time to time,” St-Laurent says. “Sometimes the memory felt more vivid, and reactivation was also more precise.”
The 11 video clips were diverse to evoke patterns of activity across the brain that distinguished them from one another. They included scenes of an avalanche, a roller coaster, a dance routine, the Coliseum, someone diving, someone crying, someone cutting a piece of roast beef, a dog riding a skateboard, a super cute owl, and President Obama making a joke about his birth video.
The researchers used “multivoxel pattern analysis” on the fMRI data to identify the different neural activity patterns distinct to each video. St-Laurent described that the technique is like partitioning the brain into little cubes a few millimeters wide, and then measuring whether activity in each little cube increases or decreases in relation to a baseline. “Every 1.5 seconds, we can look at the overall pattern of activation that is distributed across all your brain cubes, it’s like a 3-dimensional tapestry,” St-Laurent says.
So, for example, when someone first watches the film clip of President Obama telling a joke, researchers might observe activation in brain regions that “perform face recognition, speech perception and, if the joke is funny, pleasure, or if you voted Republican, maybe annoyance,” St-Laurent says. “So we can assess which portions of your brain are active and which are less active when you hear Mr. Obama’s joke.”
By then asking participants to think back and imagine Obama telling his joke again, without showing the clip, the researchers could measure activity across all “brain cubes” and compare that pattern to the pattern measured when they first watched the clip. “We observe a lot of similarities between the perception pattern and the memory patterns, which means that a lot of brain regions recreate their state of activation when you remember something,” she says.
In addition to observing a strong correspondence between reactivation and recollection — the subjective quality of the memory — the researchers found a surprising pattern of brain activity. Many previous brain imaging studies have shown that a part of the cortex called the angular gyrus is active when people remember, and some research has suggested that it supports the representation of the memory. In the new study’s results, however, the more active the angular gyrus was, the less specifically participants reactivated their memories.
St-Laurent says that they are still trying to make sense of this puzzling result, but that one possibility is that because participants retrieved the same memories over and over, after several trials, they could access their memory without having to dig too much for it. “Participants directed their efforts toward visualizing the memory rather than toward ‘finding’ it,” she says. “So it’s possible that the angular gyrus is engaged when a memory is accessed following an effortful search, but that it is less active when participants need to maintain their focus on a memory that is easily within reach.”
The study is an important contribution to the study of memory, especially with our population aging, and memory deficits of growing concern for older adults. “Our results show that the patterns we can pick up in the brain reflect something that is meaningful: They are a good index of what memory feels to you,” St-Laurent says. “So it gives us confidence that the new tools scientists are developing measure something that has clinical relevance.”
The research team is now working on a new paper that reports results from a patient with amnesia who was tested on the same fMRI task. I” won’t spoil the results, but like I said, techniques like the ones in our study have enormous potential as clinical tools to assess memory deficits, and our group is hoping to explore some of this potential in the near future by testing more individuals with different kinds of memory disorders,” St-Laurent says.
-Lisa M.P. Munoz
The paper, “Distributed Patterns of Reactivation Predict Vividness of Recollection” by Marie St-Laurent, Hervé Abdi, and Bradley R. Buchsbaum, were published online in the Journal of Cognitive Neuroscience on June 23, 2015.