Improving working memory in older healthy adults: A simultaneous fNIRS-tDCS study

Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique, in which a low intensity electric current is applied to the scalp through at least two electrodes, the anode and the cathode. In recent years, tDCS has been increasingly used in both research and clinical practice, especially with the aim to possibly enhance cognitive functions. In particular, the use of tDCS to boost working memory (WM) performance, e.g. in older adults, holds great promise. However, controversial results have been presented so far, and new evidence is needed to understand the neural underpinnings of tDCS effects on WM. In a recent study, Jones and colleagues1 tested the effect of tDCS on WM performance of healthy young adults, and employed fNIRS to evaluate the hemodynamic responses to a WM task performed before and after tDCS administration. Results of this study showed that anodal tDCS over the left prefrontal cortex (PFC), when combined with the presence of reward motivation, can actually give rise to both a significant WM performance improvement as well as a significant increase in the oxy-hemoglobin (HbO) hemodynamic response. Based on these results, the present study was aimed to evaluate the impact of a novel experimental design, in which brain activity in healthy older adults was monitored with fNIRS, while simultaneously stimulating with anodal tDCS (over the left PFC) during WM task performance combined with a reward motivation manipulation. Methods: Twenty-two participants (70.2 ± 5.15 y. o.) took part in the study and attended 3 experimental sessions. In the first two, WM capacity was tested with a novel WM task, divided in three experimental blocks. In the first and last block, participants performed a memory-probe task where only performance feedbacks were delivered, while in the middle block every fast and correct response was rewarded by either high (20 cent) or low (2 cent) reward, in a random way. In this middle block, 1.5 mA tDCS was delivered by a battery driven constant-current stimulator (BrainSTIM, EMS-CE certified), through two electrodes. The anode was placed over the left PFC, while the cathode was placed on the contralateral shoulder. Anodal and sham sessions were counterbalanced across participants. Importantly, hemodynamic activity was recorded for the whole session, i.e. not only before and after tDCS administration, but also during the application of the electrical current over the left PFC. A multi-channel fNIRS system was employed, equipped with 32 sources and 8 detectors, forming 36 standard channels (3 cm) and 2 short-separation channels (0.8 cm), located over the bilateral frontal, supplementary motor and parietal areas. In the third session, participants were administered a battery of standard neuropsychological tests to assess their cognitive functions and reward motivation. Results/Discussion: In line with the results of Jones and colleague1, we found a significant impact of tDCS on both WM performance and hemodynamic activity. Specifically, we found that HbO hemodynamic responses appeared to be sensitive to the combination of reward motivation and anodal tDCS (i.e., during the simulation period), especially in the PFC. Finally, we found significant correlations between the behavioural performance at the WM task and both the hemodynamic activity and the standard measures of WM and reward motivation. 1Jones et al., Neuroimage 105, 2015