Sensory stimulation enhances the capacity of human visual working memory, study finds

To complete tasks that require storing relevant visual details for short periods of time, such as solving a puzzle, reading or comparing different objects, humans leverage their so-called visual working memory. This is a cognitive system that allows people to store important visual information temporarily, typically for a few seconds or minutes.

Some neuroscientists found that the capacity of the human visual working memory could be increased by electrically stimulating specific regions of the brain. In a paper published in Communications Psychology, a research team at Universitat Pompeu Fabra in Barcelona set out to determine whether sensory stimulation can also affect visual working memory capacity.

“This study had two main goals: to explore the functional role of brain oscillations in working memory and to test whether sensory stimulation could replace transcranial electrical stimulation (tACS),” Indre Pileckyte, first author of the paper, told Medical Xpress.

“For the theoretical goal, we examined whether brain oscillations merely reflect synchronized neuronal activity (i.e., essentially are a by-product of an active brain), or do they actively shape cognitive processes (i.e., how we perceive and act in the world).”

Recent studies have linked the human brain’s oscillatory activity, particularly its phase and power, to perception and attention. Pileckyte and her colleague Salvador Soto Faraco set out to investigate whether this activity also influences higher cognitive functions, such as working memory.

“Methodologically, we aimed to replicate findings from tACS studies using sensory stimulation (i.e., rhythmic visual flickers and auditory beeps),” said Pileckyte.

“If effective, sensory stimulation could provide a versatile, accessible way to modulate brain oscillations across various settings beyond the lab. For instance, it could be integrated into everyday environments, like offices or cars, to enhance our working memory performance.”

The hypothesis tested by the researchers was based on a computational model, known as the theta-gamma model. This model predicts that the capacity of the human working memory depends on the frequency (i.e., speed) of theta brain oscillations.

The recent study by Pileckyte and Soto Faraco builds on this model, combining it with the findings of past studies that relied on transcranial electrical stimulation. The hypothesis it tested was that speeding up or slowing down theta oscillations would either decrease or increase the study participants’ working memory capacity.

“We used a technique called neural entrainment to manipulate the speed of brain oscillations,” explained Pileckyte. “It is based on the brain’s natural tendency to synchronize its internal rhythms with external rhythmic stimuli. These external stimuli can take various forms, such as electrical currents, magnetic pulses, or, as in our study, rhythmic visual flickers and auditory beeps.”

To test their hypothesis, Pileckyte and Soto Faraco combined sensory entrainment strategies with a straightforward visual memory task. They used sensory entrainment to manipulate the oscillations in the brains of 209 participants in total and tested their working memory by having them complete a simple task.

The participants were shown four to seven squares of different colors for a very brief time. After a short delay, a new set of squares appeared, which was either identical to the first one or one of the squares was of a different color. The participants were then asked to indicate whether any color had changed between the two sets.

“We found that sensory stimulation, both at faster and slower theta frequencies, significantly improved participants’ working memory capacity,” said Pileckyte. “This result contradicted our initial prediction, as we had expected faster stimulation to decrease capacity. Therefore, our findings suggest that sensory stimulation operates differently from electrical stimulation.”

Interestingly, Pileckyte and Soto Faraco observed that improvements in visual working memory capacity were most pronounced in participants who exhibited a smaller working memory at control conditions (i.e., when presented with static images or pure tone instead of visual flickers and beeps). This suggests that sensory stimulation interventions could be most beneficial for people who struggle to memorize information for shorter periods of time.

The findings gathered as part of this recent study could pave the way for further research exploring the potential of sensory stimulation interventions for boosting people’s working memory. In addition, psychologists could try to determine whether the improvements observed by Pileckyte and Soto Faraco also extend to specific clinical populations known to present working memory deficits, such as patients diagnosed with schizophrenia.

“As is often the case in research, we ended up raising more questions than we answered,” added Pileckyte. “In future studies, we would like to better understand how sensory entrainment operates in the brain—how it propagates from the sensory cortices to higher brain areas, and how it simultaneously influences multiple cognitive functions, such as perception and attention, alongside working memory.”

In their next studies, Pileckyte and Soto Faraco also plan to disentangle the extent to which the improvements in working memory that they observed were driven by modification of brain oscillations, as opposed to the alerting effects of sensory stimuli.

In addition, they hope to better understand the reasons why sensory stimulation appears to be particularly beneficial for participants with a lower baseline memory capacity.

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