Neurostimulation improves word memory

Neurostimulation improves word memory

A subject undergoing brain stimulation. ©Robert Reinhart

Brain stimulation with mild electrical stimuli can affect brain activity and even memory. In a new study, researchers have therefore investigated the effect of targeted stimulation of two brain areas with specific alternating current frequencies on word memory. The test persons were stimulated by means of an electrode cap both when memorizing a word list and when remembering it. It was shown that 20 minutes of stimulation given on four consecutive days improved both working memory and long-term memory of the subjects for the words - albeit only minimally. This effect lasted for a month.

The areas and cells of our brain communicate not only via neurotransmitters, but above all via electrical signals. These can be derived as brain waves, for example in the electroencephalogram (EEG). Such EEG recordings show that different types of brain waves are closely linked to specific functions and states of consciousness. In particular, brain waves in the theta range of four to eight hertz and gamma waves with frequencies from around 30 hertz upwards seem to be closely linked to short- and long-term memory, as previous studies suggest. Short-term memory is a type of working memory: it retains information for a maximum of two seconds and has a very limited capacity for words. For example, it ensures that when we read, we remember the beginning of the sentence to the end of the sentence. The long-term memory, on the other hand, is largely unlimited and takes over relevant information from the main memory for permanent "storage". Scientists have been investigating for a long time whether targeted stimulation of the brain with electrical or magnetic fields can promote and improve these parts of memory - with contradictory results so far.

Appropriate and inappropriate stimulation

That's why Shrey Grover and his colleagues from Boston University have now carried out several experiments in which they specifically stimulated two areas of the brain that are closely linked to short- and long-term memory with weak alternating currents at different frequencies. "We tested the hypothesis that modulation of theta rhythms in the inferior parietal lobe enhances auditory-verbal short-term memory, while modulation of gamma waves in the prefrontal cortex enhances auditory-verbal function of long-term memory," explains the team. For this purpose, a total of 150 healthy test persons between the ages of 65 and 88 were fitted with an electrode cap for 20 minutes once a day, via which the parietal lobe and the frontal lobe were stimulated with a weak alternating current.

A group of subjects received the appropriate stimulation with a theta frequency of 4 Hertz in the parietal lobe and gamma waves of 60 Hertz in the prefrontal cortex. In another group, both frequencies were switched and a third group received only a sham treatment. During the brain stimulation, all test subjects heard a list of 20 words to remember in five rounds. Immediately after listening to a word list, they were then asked to recall and name as many words from the list as they could in a two-minute period. This experiment was repeated on four consecutive days. Before the start of the experiments and after one month, a final test was carried out, in which the word memory was tested in addition to the basic cognitive performance.

Verbal short- and long-term memory strengthened

The evaluation showed that the test persons whose parietal lobes and forehead were stimulated with the appropriate frequency during the task performed significantly better than the test persons from the other two groups. On average, they remembered slightly more words both on the treatment days and a month later. "This suggests that these stimulation-induced improvements depend specifically on frequency and area," explain Grover and his colleagues. The positive effect was only evident when the stimulation was carried out with the correct frequency in the correct brain area. Two separate effects were also found: The stimulation of the frontal lobe with the gamma alternating current improved above all the memory of words from the beginning of the 20-part list - from an average of 40 percent to 60 percent probability of memory. "This suggests that neurostimulation of the prefrontal cortex in the gamma wave range primarily promotes long-term memory," explains the team.

The second effect occurred specifically through stimulation of the parietal lobe with alternating currents in the low-frequency theta rhythm. They helped the subjects remember the last four words on the list better. The hit rate was around 70 percent, in the control groups it was only around 50 percent. This indicates improved short-term memory. "Our results thus suggest that a targeted modulation of the functionally specific brain wave rhythms can improve the memory function of older people," write Grover and his colleagues. Neurostimulation apparently affects two different cognitive functions – working memory and long-term memory. Seniors with initially poor cognitive performance benefited particularly from the stimulation. In addition, the positive effects of the four-day treatment lasted for around a month.

No therapy for dementia yet

However, the scientists also emphasize that their experiments did not improve general memory, but only very narrowly defined aspects of memory. Overall, the effect of the stimulation was also relatively small and affected, for example, only the words at the beginning and end of the word lists, but not the total number of words remembered. The team initially sees their experiments primarily as a tool for basic research in order to further decode how human memory works. The extent to which such treatments can also help people with dementia and other neurodegenerative diseases still needs to be researched further. Johannes Levin from the German Center for Neurodegenerative Diseases (DZNE) in Munich takes a similar view: "We must not forget that the brains of dementia patients are pathologically different from those of healthy people. I am therefore always critical when such or comparable studies raise hopes that a treatment option for cognitive decline in old age or for people with dementia may have been found," says the neuroscientist who was not involved in the study.

Source: Shrey Grover (Boston University, Boston) et al., Nature Neuroscience, doi: 10.1038/s41593-022-01132-3

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