One of the main signs of cognitive aging is the fading ability to retrieve details of one’s experiences. This phenomenon has now been tied to the so-called anterior shift: When a memory is retrieved, the region of the brain that is most active is farther forward than when the memory was encoded.
Researchers from The University of Texas at Dallas’ Center for Vital Longevity (CVL) published a study March 2 in The Journal of Neuroscience that shows the greater the size of the shift — the distance in the brain between the peak of the activity elicited during encoding versus retrieval — the foggier the recall.
While age is a factor in the size of the anterior shift, it is not the only one, the researchers found.
CVL director Dr. Michael Rugg, holder of the Distinguished Chair in Behavioral and Brain Sciences and professor in the School of Behavioral and Brain Sciences (BBS), is the senior author of the article, which examines several aspects of the anterior shift with a new degree of detail.
“There was prior evidence for an anterior shift when remembering visual scenes after a brief interval, but no one had studied the shift when remembering something that occurred 10 to 15 minutes ago,” Rugg said. “We also are the first research group to report the shift when recalling faces as well as scenes. And our study is the first to show that the shift is, on average, larger in older people than younger people — although it varies significantly within both age groups.”
Cognition and neuroscience doctoral student Sabina Srokova, lead author of the paper, said the results provide new context for understanding cortical reinstatement theory — the idea that there is overlap between brain regions activated during the encoding of a memory and its later retrieval.
“We’ve known for decades that the neurons that respond when we experience an event are later reactivated when we remember the event,” she said. “In recent years, though, a shift has been documented. Instead of the exact same neuronal populations being reactivated, we see this anterior shift. It’s not necessarily only a reactivation of the same neurons.”
Rugg explained that “reinstatement theory is part of neuroscience dogma and has been repeatedly demonstrated both in people and in animals. The anterior shift, however, shows there’s more to it than that.
“We interpret the anterior shift as reflecting a bias in favor of semantic, abstract information at the expense of perceptual detail. When someone is unable to remember detailed information about an event, they’re forced to rely on a more generic, ‘gist-like’ representation.”
In the researchers’ experiment, 48 healthy participants — 24 young adults with a mean age of 22 and 24 older adults with a mean age of 70 — underwent functional MRI (fMRI) as they viewed word-image pairs that each appeared for two seconds. The image was either a face or a scene.
“We … are the first research group to report the shift when recalling faces as well as scenes. And our study is the first to show that the shift is, on average, larger in older people than younger people.”
Dr. Michael Rugg, holder of the Distinguished Chair in Behavioral and Brain Sciences and professor in the School of Behavioral and Brain Sciences
After 10 to 15 minutes, participants viewed a series of words and were asked to indicate if they remembered seeing each word in the first phase of the experiment. If they had, they were then asked if the word had been paired with a face or a scene, or if they did not know.
The fMRI scans allowed the researchers to pinpoint which brain regions were active during each phase of the experiment. Rugg credited Srokova with developing an innovative method to quantify the anterior shift at the level of individual people.
“We didn’t average the data over participants,” he said. “Sabina figured out an analysis method to quantify the shift at the participant level so that we were able to calculate correlations with memory performance.”
Results showed that those who had the best memory for the word-image pairs had the smallest anterior shift.
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“We looked at younger and older people, and we know there’s a difference — older people show a bigger shift than younger people do,” Rugg said. “Nevertheless, it’s important to note that there is overlap in the shift between high-performing older adults and poorly performing younger ones. Whether you’re aged 20 or 70, the bigger your anterior shift, the worse your memory performance.”
Srokova said the experiments supported existing ideas about the nature of the memories that older adults retrieve.
“The fact that there’s an age difference in the anterior shift suggests that older adults generally retrieve memories that are of lower fidelity and more abstract than the memories retrieved by younger people,” she said. “That is consistent with behavioral evidence indicating that older adults are prone to retrieve memories lacking in detail.”
Next, the researchers will extend their studies to the auditory modality. They will also investigate whether changing the testing conditions will affect outcomes.
“The auditory system is set up rather like the visual system. Representations become increasingly abstract along the posterior to anterior axis of the temporal cortex,” Rugg said. “So, we would predict an anterior shift in auditorily selective cortex similar to what we found for visually selective cortex.”
Srokova said: “We also want to know if we can modulate the size of the anterior shift. The shift might have been especially prominent in our study because we didn’t require participants to retrieve fine details about the scene and face images. We predict that if the memory test requires retrieval of more detailed information, the shift will be smaller.”
Dr. Paul Hill, a research scientist formerly at UT Dallas and now at the University of Arizona, contributed to the study.
The research was supported by the National Science Foundation (1633873) and the National Institute on Aging (2RF1AG039103-06A1), a component of the National Institutes of Health.