Working memory is a type of short term memory that reflects our ability to remember a small piece of information (such as a telephone number) for a few seconds. There is experimental evidence that working memory can be enhanced by electrical stimulation of specific brain areas, such as the parietal and frontal cortices. The main purpose of this study was to systematically evaluate whether electrical stimulation of the above mentioned areas with low and high frequency stimulation (in the theta and gamma bands) would improve participants’ performance in neuropsychological tests assessing working memory. The results of the study suggest that bilateral parietal and right-fronto parietal theta stimulation affected the oscillatory pattern of the brain and improved performance on a subset of working memory tests. On the contrary, the gamma stimulation protocol failed to modulate brain oscillation and performance on the working memory tests. Additional studies, employing more participants are necessary to draw conclusions about the efficiency of electrical stimulation on working memory enhancement.
In the present study several parameters were manipulated in order to systematically assess the effect of tACS in working memory. Manipulated variables included the frequency of stimulation (theta and gamma frequency band), the type of working memory task (n-back and change detection) and the placement of tACS electrodes (bilateral frontal, bilateral parietal, left fronto-parietal, and right-fronto parietal). The researchers focus on both electrophysiological data, acquired by EEG, and neuropsychological data, acquired by the behavioral tests. EEG data was recorded before and after stimulation, and during performance on the working memory tasks
- Duration: 15 min
- mA: The amplitude was increased stepwise by 250 μA (duration per step = 30 s) starting with 1250 μA until a maximum of 2000 μA was reached. After each increase in amplitude, the participants were asked to report the presence of a skin sensation. For the remaining experiment, stimulation intensity was kept 250 μA below the lower threshold for skin sensations
- Hz: “In an attempt to match the stimulation frequency to the endogenous oscillatory state, individual theta and gamma stimulation frequencies were determined based on pre-stimulation resting EEG data”
- Anode and cathode position varied between the groups : “There were 4 groups: bilateral parietal stimulation (P3 & P4), left frontoparietal stimulation (F3 & P3), right frontoparietal Stimulation (F4 & P4) and bilateral frontal stimulation (F3 & F4)”
In respect to electrophysiological data the present study showed that theta but not gamma tACS significantly affects resting EEG data. Additionally, this effect was location-specific, as bilateral parietal and right frontoparietal theta tACS affected P3 amplitude and latency, whereas this was not observed after bilateral frontal and left frontoparietal theta tACS. Bilateral parietal stimulation in the theta frequency band affected both resting EEG data (frequency-dependent modulation of EEG oscillations) and task-based EEG data (decreased P3 latency, correlations between P3 amplitude and n-back accuracy).
In respect to behavioral data acquired by working memory tests, the results of the study show that gamma tACS failed to improve performance on all aspects of both n-back and change detection working memory tests, while theta tACS improved performance on the n-back tests when the electrodes were placed over bilateral parietal areas or right fronto-parietal areas.
- “One of the goals of this study was to provide causal evidence for the roles of theta and gamma-band oscillations in frontal and parietal areas in working memory. The other goal was related to the fact that there is a lack of consensus concerning the optimal parameters of tACS for reliable physiological and behavioral changes.”
- “The effects were only observed when the electrodes were placed over bilateral parietal areas and over right fronto-parietal areas, thereby partially supporting the hypothesis that stimulation involving at least one target electrode placed over posterior parietal areas would elicit the greatest behavioral effects.”
- This quote highlights the importance of posterior parietal cortices in tACS-mediated working memory enhancement. Now we know where to place electrodes to improve working memory!
- “There is no evidence to suggest that gamma tACS affected working memory performance. This may stem from the lack of significant effects of gamma tACS on resting EEG amplitude in comparison to pre-stimulation EEG data or in relation to sham stimulation.”
- This implies that lack of success of gamma stimulation to improve performance in working memory tests may be due to insufficient modulation of the brain oscillatory patterns, under the specific experimental settings. Other research has shown Gamma tACS can improve focus, however it may not be the correct setting to improve working memory.
Pahor, A., & Jaušovec, N. (2018). The effects of theta and gamma tACS on working memory and electrophysiology. Frontiers in human neuroscience, 11, 651.