Behavioral and hemodynamic effects of prefrontal anodal stimulation in healthy older adults: A simultaneous tDCS-fNIRS study

Several studies have evaluated the effect of anodal transcranial direct current stimulation (tDCS) over the prefrontal cortex (PFC) for the enhancement of working memory (WM) performance. However, results are mixed, and the functional consequences of prefrontal tDCS duringWM tasks are still unknown, especially regarding potential benefits for cognitive aging. The present study addresses this question by testing healthy older adults with a multimodal approach. Twenty-one participants (mean age = 69.7 years; SD = 5.05) took part in the study attending three sessions. In the first two, participants performed aWMtask both before, during and after the delivery of 1.5 mA anodal tDCS/sham over the left PFC. The anode was placed between F3 and F7 (International 10-20 system), with the cathode over the contralateral shoulder. Anodal tDCS lasted 26 minutes, while the sham only 30 seconds, both with a fade-in and fade-out of 30 seconds. The order of the two sessions was counterbalanced across participants. During the stimulation, participants also received performance contingent feedback (high or low monetary incentives) for fast and correct responses at the WM task. In both sessions, hemodynamic activity of the bilateral frontal, motor and parietal areas was recorded with functional near-infrared spectroscopy (fNIRS). The third session consisted with a standard neuropsychological assessment. Results show a significant impact of tDCS on both WM performance and hemodynamic activity. Specifically, faster responses at the WM task were observed both during and after anodal tDCS, while no differences were found during and after the sham. This effect was however significant only taking into account invididual visuo-spatial WM capacity, with greater benefit found for low WM capacity participants. Moreover, increased hemodynamic activity was found in the bilateral PFC during and after the anodal tDCS, when compared with the sham stimulation, while no effects were detected in both motor and parietal areas.