Like a fisherman talking about the size of the one that got away, the brain exaggerates its memories.
This exaggeration is in the service of good, however. New research finds that when people exaggerate the differences between similar memories, they recall them better. The findings could help explain why memory works, and why it often declines with age.
The research involved asking people to match faces to objects, which often differed only slightly in color. When people mentally exaggerated the color differences between the objects, they were better at recalling which face went with which object. Brain imaging showed that this exaggeration was tied to activity in a region of the brain called the lateral parietal cortex.
"It's very fascinating to me to see that memory distortions can actually help us to tell these similar memories apart," said Yufei Zhao, the lead author of the study and a doctoral student in psychology at the University of Oregon.
Making memories
Zhao and her colleagues had previously conducted research on the hippocampus, a curved region deep in the brain that sits above the brainstem and is important for initially encoding memories. Brain imaging studies had shown some differences in how the hippocampus handled memories of two very similar events, but it wasn't clear whether there were any changes to the content of the memory itself.
In the new study, published in the Journal of Neuroscience on Feb. 22, Zhao and her co-authors focused on a part of the brain that doesn't encode memories but rather helps to recall them: the lateral parietal cortex, which sits beneath the top back of the skull.
"Parietal cortex is actually the place where the memory is housed when we retrieve our memory," Zhao told Live Science. "You will hold your memory in your parietal cortex, so investigating the parietal cortex can give us a very nice window to look at the details of our memory."
There were 29 participants in the study. On day one of the study, the participants were shown 24 different faces, each associated with a different everyday object, such as a beanbag, hat, balloon or umbrella. Unbeknownst to the participants, the researchers had chosen the objects so that they could later be paired up in a recall test. In half of the cases, these pairs were made up of two different objects — a balloon and a hat, perhaps — that were subtly different in color, just 24 degrees apart on a color wheel. In the other half of cases, the pairs were made up of the same objects — two beanbags — only different because their shades were also 24 degrees apart on the color wheel. One might be light green and the other darker green, for example.
Exaggerated differences
Two beanbags of slightly different color shades should be harder for the brain to remember than a balloon and a hat in those same shades, the researchers reasoned. Thus, if the brain distorts memories to remember them better, the participants should have exaggerated the gap between the colors of same-object pairs more than the gap between the colors of different-object pairs.
On day two of the study, the participants tested their recall. They were shown a picture of a face and the object associated with that face in grayscale. They then had to pick the color of the object on a color wheel. Sure enough, the participants exaggerated the gap in colors in the same-image condition but did not do so in the different-image condition.
This exaggeration was also associated with accuracy, the researchers found. The participants were better at remembering which face went with the correctly colored object when they exaggerated the color differences between the same-object pairs.
Then, the study authors tracked brain activity using functional magnetic resonance imaging (fMRI), which detects changes in oxygenation correlated with blood flow within the brain. Areas with more blood flow are more active. The researchers found differences in the patterns of activation in a wrinkle in the parietal cortex called the ventral intraparietal sulcus. These differences were focused in a region that encodes information about shape and color, and were more pronounced when the participants were recalling same-object pairs versus different-object pairs, meaning that the differences correlated with the exaggerations in the color gaps in people's memories.
"The neural pattern actually remembers them as less similar to each other, Zhao said. That dissimilarity is then correlated with better memory performance, she added.
Similar memories interfere with each other, becoming difficult to recall clearly (for example, it's easier to remember the one time you parked your car at Disneyland than one of the hundreds of times you parked at your office parking garage). The finding explains one way the brain reduces interference between similar memories, she said. Most likely, she said, this interference reduction starts in the hippocampus, where the brain may initially weigh the differences between two memories heavily in order to differentiate them. For example, if you went to the beach on two different days but one day was windy and the other was calm, the hippocampus might make special note of the weather difference when encoding the memory. Then, when you recall the memory, the parietal cortex may exaggerate the windiness of one day and the stillness of the other so that you retrieve the right day.
The participants in the study were all young, healthy adults with good memory recall, Zhao said — they were 98.9% accurate at recalling face-object matches when the objects were different and 93.2% accurate at remembering the matches when the objects were the same. The next step, she said, is to study older adults. Memory performance declines with age, Zhao said, and one reason might be that the brain becomes less skilled at reducing interference between memories. The researchers now want to find out if the brains of older adults fail to exaggerate the differences between their similar memories.
Originally published on Live Science.
