The impact of tACS stimulation timing on the modulation of working memory

Previous studies have demonstrated that tACS can expand cognitive resources, thereby improving working memory (WM) performance, while some studies failed to observe a definitive advantage of tACS in boosting WM. Regarding this ongoing debate, this study sought to explore the effect of tACS on working memory by involving the factor of psychological inertia. Psychological inertia refers to the unconscious, automatic behavior
of an individual. The conflict between psychological inertia and rational decision-making could lead to a loss of optimal choice.
In Experiment 1, 36 participants completed a color recall reporting task. Active or sham tACS was applied before or after practice to manipulate the amount of memory resources at the initial stage. The results confirmed our hypothesis that participants who received active tACS after practice (post-practice) adopted similar memory strategies to those receiving sham stimulation due to a psychological inertia. They exhibited a trade-off between quantity and quality, i.e., post-practice tACS could boost memory accuracy with comparable quantity; while pre-practice stimulation improved memory quantity with comparable accuracy.
To investigate whether the regulatory effects of the first task would carry over to the second task, 56 participants completed both color and orientation recall reporting tasks in Experiment 2, where tACS stimulation was applied after the practice of the first task. The results revealed that memory accuracy was improved while the quantity was unchanged in the first task, which is consistent with the results from the group receiving post-ptactice tACS in Experiment 1. However, the memory quantity was enhanced and the accuracy was kept unchanged in the second task, consisting with the results of pre-practice tACS in Experiment 1. This indicates that, psychological inertia can be disrupted in new situations, leading to new strategies.
The regulatory effects of tACS on WM are modulated by the timing of stimulation. That is, pre-practice tACS influences the memory quantity, and post-practice tACS affects memory accuracy. Furthermore, the regulatory effects that were established during the first task may be disrupted when encountering a new task. These findings clarify the optimal temporal window for tACS intervention, providing a precise reference for clinical cognitive rehabilitation, such as early intervention in Alzheimer's disease.