Your memory has more in common with a vintage, hand-cranked video camera than you may think.
Research by psychologists at the University of Toronto finds that like a video camera, your brain captures approximately seven snapshots per second that get stitched together when we recall the past. More specifically, memory oscillates at a theta rhythm with a frequency of 3 to 10 Hz.
In other words, whether you remember a specific moment depends on if it lands within one of these snapshots or falls outside of its frame.
"We’ve discovered the smallest timescale that our memories are divided into—the smallest chunks of our memories,” says Thomas Biba, a PhD candidate in the department of psychology and the first author of the paper published in Nature Human Behaviour.
“We know that when we're watching a movie, it’s an illusion. It's just 24 frames taken per second that contain enough information for us to fill in the gaps,” says Katherine Duncan, an associate professor of psychology in the Faculty of Arts & Science.
“The brain might be using those same tricks when it's storing memories,” says Duncan. “Instead of needing 24 frames per second, maybe you just need seven to capture the dynamics of our experience as humans because our minds are so good at smoothing over those rough edges as we’re recalling them.”
Researchers in the United States have already replicated preliminary results that Biba and Duncan presented over the last few years. “It speaks to the excitement that the field has had for these findings,” says Duncan.
In shedding light on how memories are formed from millisecond to millisecond, Biba and Duncan believe their study can unlock new ways to understanding neurodegenerative diseases like Alzheimer’s or Parkinson’s, and memory difficulties associated with mood disorders and post-traumatic stress disorder.
A Memory Time Trial
Scientists have long been stumped by the challenge of zooming into how memories form at fractions of a second.
“You have all these processes in research that take a number of seconds,” says Biba. “You present the image, then the participant has to see the image, think of a response, and press a button.”
For this study, Biba and Duncan developed a new approach that manipulates memory formation on a tightly timed course. They first presented participants with an attention-grabbing cue—either a blue screen or an orange screen—to reset brain rhythms. Participants were then asked to make a judgement about an image of a common object (like an apple) based on the colour they were shown.
Researchers precisely manipulated the delay between this cue and to-be-remembered images across trials. This let them test how well people formed memories at 30 different points per second. They then tested how well participants formed memories at each point with a recall task.
After running hundreds of trials and averaging their results, they found that people's ability to form memories rhythmically fluctuates, on average, seven times per second. Or more precisely, around 3 to 10 times per second.
“So somewhere in that range, we expect people to be cycling between being really good or really bad at forming memories,” Duncan says.
The researchers believe that the speed of this rhythm is shaped by the brain’s biology. Previous research has found that activity within the hippocampus—the seahorse-shaped region of the brain at the center of memory formation—rhythmically fluctuates at the same frequency.
Biba suggests this fluctuation may be happening because the hippocampus is shifting between what psychologists call “encoding” and “retrieval” states several times a second. In other words, the hippocampus may constantly cycle between taking new snapshots and recalling those taken previously.
The Fuel of Memory Rhythms
Researchers found that the strength of memory rhythms is influenced by indirect markers of acetylcholine, a vital neurotransmitter in the brain. Environmental factors that indicate something new or important is happening, like a car horn (or in this experiment, a coloured flash on a screen), can trigger its release.
These rhythms in memory aren't so apparent when people are in bad attentional states, when their minds are wandering, or when they're disengaging from the task.
“These rhythms in memory aren't so apparent when people are in bad attentional states, when their minds are wandering, or when they're disengaging from the task,” says Duncan.
They also measured memory rhythms in people who regularly consume nicotine, a drug known to desensitize acetylcholine receptors. These participants had weaker memory rhythms compared to people who do not regularly consume nicotine. Effectively, it's harder for their brains to open windows for strong memory creation.
“Acetylcholine is incredibly important for health and memory,” Duncan explains. “Many of the diseases that can cause problems with our memory system—like Alzheimer’s disease or Parkinson’s disease—are linked to this critical neurochemical.”
Funding Acknowledgement
This research was supported by the following agencies and grants: Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant, Canada Foundation for Innovation JELF and Ontario Research Fund, Canada Research Chair, and New Frontiers Research Fund.