Memory issues and confusion are reported in one in nine adults aged 45 and older, and 50 percent experience limitations in daily activities such as cooking, cleaning, and taking medications, according to the Centers for Disease Control and Prevention (CDC).
Research published in Nature Neuroscience on Aug. 22 reveals a new potential way to enhance the memory of the elderly without relying on medications.
Cognitive neuroscientist Robert Reinhart and other Boston University researchers showed that repetitive electrical stimulation over four days in the brains of adults aged 65 and older improved memory for at least one month after intervention.
Interestingly, different results were obtained by stimulating parts of the brain with lower or higher frequencies of electrical currents.
Lasting memory benefits
Previous studies hinted at the presence of a “capacity-limited working memory (WM) store” to briefly maintain a store of information and an “unlimited long-term memory (LTM) store” for sustained maintenance of information. Although the dorsolateral prefrontal cortex and the inferior parietal lobule were thought to be involved in these mechanisms, previous research with electrical stimulation did not produce definitive results.
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Modulation of synchronous low-frequency activity in the parietal cortex improved working memory on days three and four and one month after intervention, while high-frequency activity in the prefrontal cortex improved long-term memory on days two through four and one month after intervention.
Larger improvements and memory over the first four days were predictive of the degree of improvement one month after intervention.
Additionally, adults with lower baseline cognitive function enjoyed larger and longer-lasting boosts to their memory.
“Our findings demonstrate that the plasticity of the aging brain can be selectively and sustainably exploited using repetitive and highly focalized neuromodulation grounded in spatiospectral parameters of memory-specific cortical circuitry,” the authors stated.
Repetitive brain stimulation
The authors recruited 150 participants between the ages of 65 and 88 from the greater Boston metropolitan area and used a brain stimulation technique called transcranial alternating current stimulation (tACS), in which electrical currents are transmitted through electrodes placed on the scalp. In this randomized, double-blinded study, subjects received 20 minutes of tACs daily for four consecutive days.
In the “free recall task,” participants were asked to immediately recall a list of 20 words after it was presented. Every day, five runs of the free recall task were performed while undergoing neuromodulation for approximately 20 minutes each day.
High-frequency electrical stimulation to the dorsolateral prefrontal cortex improved recall of words at the beginning of the lists, which is associated with long-term memory. In contrast, low-frequency neuromodulation of the inferior parietal lobe improved recall of items later in the lists, which is associated with working memory.
Additional experiments were conducted as a control and a test for replication of the primary findings. Switching the neuromodulation frequency between the brain regions did not confer any benefits, supporting the idea that “it is the combination of anatomical location and rhythmic frequency that determines the appropriate substrate for memory improvement.”
Furthermore, a sham protocol “in which the electrical currents were applied only briefly at the beginning and end of the task to mimic the sensation of brain stimulation, did not boost memory.”
While the Boston University study offers promising results about a non-pharmacological method of boosting memory in the elderly population, the generalizability of the findings is limited due to the selected population of only elderly adults in the greater Boston metropolitan area with data only up to one month after intervention.
Follow-up studies at six months, one year, and longer would provide valuable information about the safety and potential side effects of the technique, as well as the duration of benefits. In addition, while a free recall task of 20 words is helpful for quantifying improvements in memory, activities of daily living such as cooking, cleaning, managing medications, and paying bills likely require different facets of memory and the brain.
The team is already looking at expanding the tACs study to include people with Alzheimer’s disease because the greatest benefits may be experienced by those with “poor cognitive function to begin with.” Alzheimer’s disease affects an estimated 6.5 million Americans aged 65 and older, of which 4 million are women and 2.5 million are men.
Despite the limitations of the study, Simon Hanslmayr, a cognitive neuroscientist at the University of Glasgow in the UK, stated, “I was both impressed and surprised by this paper.”
The authors were able to demonstrate a protocol linked to “consistent and quite strong improvements in memory.” He points out factors that may have contributed to the study’s success, including how several previous studies only administered a single session of tACS and in a much younger population.