Brain stimulation revives working memory in older adults
Over the last century, there has been an astounding rise in the prevalence of declining brain health and function. Along with this, the occurrence of dementia in older adults has also staggeringly increased. As more people around the world get older, these numbers are expected to snowball in the coming decades. We must understand how the brain typically ages and develop methods to maintain or improve brain function in older adults to prevent an avalanche of dementia.
Now, researchers have developed a short, noninvasive brain stimulation procedure for putting a freeze on dementia. The series of soft shocks improved short-term memory important for reasoning and decision-making — working memory — in adults over 60 years old. Following 25 minutes of harmless stimulation to specific brain regions, older adults could perform working-memory tasks as well as younger adults in their 20s. These results lay the groundwork for future non-drug-based interventions targeting features of declining brain function.
The mental work space
Working memory is central to how we experience life. It acts as a mental work space to keep things in mind, think, and make decisions. Working memory allows you to drive a car, order from a menu, and assemble flat-pack furniture out of a box. It’s how you are stringing together the information in this very article — word by word, from start to finish — into a story.
Working memory is where your short-term and long-term memory come together so that you can carry out a task, achieve a goal, or solve a problem. For example, to calculate a tip, we use our knowledge of math, stored in long-term memory, to work on numbers temporarily held in short-term memory. In terms of memory storage, working memory plays a key role in what information from what we experience moves into long-term memory.
Also, attention, or concentration, is controlled by working memory. This helps us take in and hold onto important information while filtering out distractions. Working memory helps us keep our tasks in mind long enough to finish them. Without working memory, it would become extremely difficult to remember simple tasks at hand or what you just read.
Deficits in working memory occur during normal brain aging. Complications with working memory are also key to the speedy deterioration of brain function linked to dementias including Alzheimer’s disease. The decline in age-related memory and brain function is thought to be linked to changes in how brain areas communicate. This is likely due to the loss of connections between brain areas that normally work in an orchestrated manner.
Coupled and *NSYNC
Currently, neuroscientists think that the basis for working memory is communication within and between networks in regions of the cortex — the outer layer of the brain critical for consciousness. Specifically, there are two mechanisms that could explain how brain areas communicate: coupling and synchronization.
Coupling occurs when different types of brain activity patterns within the brain link up. During working memory, coupling happens between two different types of brain rhythms within a cortical region, called the temporal cortex. Importantly, this is thought to be key to performing tasks that require holding onto relevant information through time.
Synchronization occurs when the same type of brain activity patterns from different brain regions begin to coordinate. During working memory, it is thought that there is synchronization of a brain rhythm between two separate cortical regions: the prefrontal and temporal cortices. This is thought to be important for performing tasks that require combining information from the use of our senses.
To summarize, it is thought that two mechanisms — coupling of two different rhythms within the temporal cortex, and synchronization of one type of rhythm between prefrontal and temporal areas — are the basis for proper working-memory maintenance in young, healthy brains. For this reason, it is likely that older adults with deficits in working memory have issues with coupling or synchronization.
Little shock, lots of awe
Brain stimulation involves modulating the brain’s activity with brief, safe, and innocuous electric shocks. The neat thing about brain stimulation methods is that the electrical stimulation produced can be customized to the brain it is influencing.
In some cases, electrodes are implanted in the brain to directly deliver electricity. Alternatively, the electricity can be given indirectly and noninvasively through electrodes placed on the scalp or by using magnetic fields applied to the head.
Brain stimulation, when paired with brain imaging, can track and affect how brain regions communicate. This combo has shown great promise in identifying and modulating rhythms of specific brain networks that lead to lasting changes in behavior.
One such brain stimulation tool is called transcranial alternating current stimulation (tACS). Growing evidence shows that tACS can influence behavior by targeting specific regions of the cortex. So, it may be possible to influence working memory by targeting cortical regions with tACS.
To test this, researchers used a brain imaging tool — electroencephalogram (EEG) — and a form of tACS that provides more precise targeting of cortical structures.
The researchers first compared recorded brain activity with EEG during a working-memory task in young adults in their 20s and older adults over 60. They found that working-memory performance is impaired in older adults. Rhythms within the temporal cortex were uncoupled in older adults during working-memory maintenance. Also, synchronization between temporal and prefrontal cortices was compromised in older adults.
Next, the researchers examined the impact of brain stimulation on neural coupling and synchronization as well as behavior. The application of tACS improved working memory in older adults in a stable and lasting manner. After 25 minutes of stimulation, older adults showed a significant increase in task performance. The age-related impairment in working-memory accuracy was eliminated by tACS — they were indistinguishable from participants in their 20s.
After stimulation, the researchers saw an increase in the temporal cortical coupling that mirrored the improvement in working memory in older adults. Also, tACS corrected the loss of synchronization between temporal and prefrontal cortical regions in older adults.
Finally, the investigators were able to use brain stimulation in younger adults to either increase or decrease working-memory performance. When they used tACS designed to desynchronize interactions between temporal and prefrontal cortices, working-memory deficits were rapidly induced. They even used tACS to synchronize these interactions in the brains of young people with poor working memory and saw improvements.
Ohm, watt’s next?
This shows that brain stimulation can rapidly boost working-memory function in older adults, with potentially long-term effects. However, this research didn’t examine how long the positive effects on memory lasted. To understand how long the effects of brain stimulation methods last on working memory, more research is needed.
Also, future experiments will be critical for understanding how these changes affect high-level brain functions that rely on working memory, such as decision-making and understanding language and math. This would lay the foundation for making future non-drug-based treatments aimed at reducing deficits in brain function during normal aging as well as disease.