Age-related declines in working memory were reversed temporarily with non-invasive transcranial alternating current stimulation (tACS), a small study of older adults demonstrated.
Stimulating temporal and prefrontal brain areas simultaneously at a specific rhythm improved older adults’ working memory and the effect outlasted a 50-minute period after stimulation stopped, according to Robert Reinhart, PhD, and John Nguyen, BS, both of Boston University.
Working memory deficits in older adults appear to emerge from weak cross-frequency coupling between gamma and theta rhythms and compromised theta phase synchronization between prefrontal and temporal areas, they reported in Nature Neuroscience.
This research supports theories of neurocognitive aging that suggest cortical disconnection underlies age-related cognitive decline and shows negative, age-related changes are not unchangeable, Reinhart said in a press briefing: “We can bring back the more superior working memory function that you had when you were much younger.”
“Working memory is a fundamental building block of human cognition,” Reinhart noted. “It allows us to hold information in our minds over a period of seconds … [it’s] where we think, where we problem-solve, where we reason, plan, perform mathematical calculations, and make decisions.”
Throughout the adult lifespan, working memory exhibits a strong linear decline, and the mechanism behind this decline is unknown.
Working memory is linked to specific neural interactions within and between brain regions and is thought to involve two patterns of neural oscillation — gamma and theta rhythms. A specific form of cross-frequency coupling known as theta-gamma phase-amplitude coupling — in which the amplitude of gamma rhythms is coupled to the phase of theta rhythms — has been observed in the temporal cortex and is thought to reflect the local processing and storage of memory contents.
In their study, the researchers used high-definition tACS (HD-tACS, a form of stimulation that offers more precise targeting of cortical structures) to modulate wave interactions associated with working memory and electroencephalography to monitor how these interactions changed.
They evaluated 42 younger adults (ages 20 to 29) and 42 older adults (ages 60 to 76) in a working memory task, with and without HD-tACS. When performing the task, younger adults displayed increased interactions between theta and gamma rhythms in the left temporal cortex and increased synchronization of theta rhythms in the frontotemporal regions.
Without brain stimulation, older adults were slower and less accurate at the task than younger adults: “They struggled to maintain robust representations of task-relevant information,” Reinhart observed. When they received stimulation, however, older adults exhibited a mean accuracy level statistically indistinguishable from that of younger adults at baseline.
The effect occurred relatively quickly — within about 12 minutes of HD-tACS delivery — and continued past a 50-minute follow-up period. Improvements in task accuracy correlated with increased theta-gamma coupling in the temporal cortex and enhanced theta phase synchronization across the frontotemporal cortex.
Working memory deficits and functional connectivity problems are central to many brain disorders, including schizophrenia, autism, and Alzheimer’s disease, Reinhart noted. “Our hope is that this work will help lay the basic science groundwork for an entirely new avenue of research where we develop non-invasive new neuroscience tools to help treat people who are suffering from brain disorders,” he said.
The authors declared no conflicts of interest.
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