Slow Theta tACS of the Right Parietal Cortex Enhances Contralateral Visual Working Memory Capacity

Bender, Monika; Romei, Vincenzo; Sauseng, Paul · 2019 · Crossref

DOI: 10.1007/s10548-019-00702-2

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Summary

This study investigates whether focal transcranial alternating current stimulation (tACS) of the right parietal cortex can enhance visual working memory (WM) capacity, specifically addressing whether previous findings were driven by co-stimulation of prefrontal or subcortical structures. The research is grounded in the theoretical framework that WM capacity is limited by the nesting of fast gamma oscillations within slower theta waves; slowing theta frequency theoretically allows more gamma cycles to be integrated, thereby increasing capacity. While prior studies using broad electrode montages demonstrated that slowing parietal theta frequency improved WM, it remained unclear if these effects required stimulation beyond the parietal lobe. The authors hypothesized that focal tACS at a slow theta frequency (4 Hz) would increase WM capacity, while faster theta stimulation (7 Hz) would decrease it, even without prefrontal or subcortical involvement. The study comprised two experiments involving healthy young participants performing a visuo-spatial WM delayed match-to-sample task. Participants retained items presented in a cued visual hemifield and compared them to a probe. In Experiment 1, 18 participants received focal tACS over the right parietal lobe (electrode P4) with return electrodes at Oz, Cz, and T8 to ensure high focality. Stimulation intensities were 1000 µA peak-to-peak at 4 Hz, 7 Hz, or sham. Experiment 2 involved 14 participants using the same protocol but with increased stimulation intensity (2000 µA) to achieve electric field strengths comparable to previous literature, albeit with slightly reduced focality. Working memory capacity was calculated using the parameter *k*, and data were sham-corrected for analysis. Experiment 1 yielded no significant effects on WM capacity. However, Experiment 2 revealed a significant interaction between stimulation condition and visual hemifield. Specifically, 4 Hz tACS significantly increased WM capacity for stimuli presented in the left visual hemifield (contralateral to the right parietal stimulation) compared to sham and 7 Hz conditions. This enhancement was driven exclusively by trials with the highest memory load (six items). No significant decrease in capacity was observed with 7 Hz tACS, though a trend toward reduced hit rates was noted. The authors suggest that modulating theta toward slower frequencies may be more effective than shifting toward alpha-range frequencies, and that effects are most pronounced during cognitively demanding tasks. The findings confirm that focal stimulation of the right parietal cortex is sufficient to enhance contralateral visual working memory capacity, ruling out the necessity of prefrontal or subcortical co-stimulation for this effect. The results support the theta-gamma coupling theory of working memory, demonstrating that slowing parietal theta oscillations can expand memory capacity. However, the study indicates that relatively strong stimulation intensity is required to achieve these behavioral effects, and the benefits are specific to high-load conditions and the visual field opposite the stimulation site.

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• working memory