Individual response to transcranial direct current stimulation as a function of working memory capacity and electrode montage.

Introduction: Attempts to improve cognitive abilities via transcranial direct current stimulation (tDCS) have led to ambiguous results, likely due to the method's susceptibility to methodological and inter-individual factors. Conventional tDCS, i.e., using an active electrode over brain areas associated with the targeted cognitive function and a supposedly passive reference, neglects stimulation effects on entire neural networks. Methods: We investigated the advantage of frontoparietal network stimulation (right prefrontal anode, left posterior parietal cathode) against conventional and sham tDCS in modulating working memory (WM) capacity dependent transfer effects of a single-session distractor inhibition (DIIN) training. Since previous results did not clarify whether electrode montage drives this individual transfer, we here compared conventional to frontoparietal and sham tDCS and reanalyzed data of 124 young, healthy participants in a more robust way using linear mixed effect modeling. Results: The interaction of electrode montage and WM capacity resulted in systematic differences in transfer effects. While higher performance gains were observed with increasing WM capacity in the frontoparietal stimulation group, low WM capacity individuals benefited more in the sham condition. The conventional stimulation group showed subtle performance gains independent of WM capacity. Discussion: Our results confirm our previous findings of WM capacity dependent transfer effects on WM by a single-session DIIN training combined with tDCS and additionally highlight the pivotal role of the specific electrode montage. WM capacity dependent differences in frontoparietal network recruitment, especially regarding the parietal involvement, are assumed to underlie this observation.

Age-related cognitive effects of the COVID-19 pandemic restrictions and associated mental health changes in Germans.

Restrictive means to reduce the spread of the COVID-19 pandemic have not only imposed broad challenges on mental health but might also affect cognitive health. Here we asked how restriction-related changes influence cognitive performance and how age, perceived loneliness, depressiveness and affectedness by restrictions contribute to these effects. 51 Germans completed three assessments of an online based study during the first lockdown in Germany (April 2020), a month later, and during the beginning of the second lockdown (November 2020). Participants completed nine online cognitive tasks of the MyBrainTraining and online questionnaires about their perceived strain and impact on lifestyle factors by the situation (affectedness), perceived loneliness, depressiveness as well as subjective cognitive performance. The results suggested a possible negative impact of depressiveness and affectedness on objective cognitive performance within the course of the lockdown. The younger the participants, the more pronounced these effects were. Loneliness and depressiveness moreover contributed to a worse evaluation of subjective cognition. In addition, especially younger individuals reported increased distress. As important educational and social input has partly been scarce during this pandemic and mental health problems have increased, future research should also assess cognitive long-term consequences.

Making the rich richer: Frontoparietal tDCS enhances transfer effects of a single-session distractor inhibition training on working memory in high capacity individuals but reduces them in low capacity individuals.

Working memory (WM) performance depends on the ability to extract relevant while inhibiting irrelevant information from entering the WM storage. This distractor inhibition ability can be trained and is known to induce transfer effects on WM performance. Here we asked whether transfer on WM can be boosted by transcranial direct current stimulation (tDCS) during a single-session distractor inhibition training. As WM performance is ascribed to the frontoparietal network, in which prefrontal areas are associated with inhibiting distractors and posterior parietal areas with storing information, we placed the anode over the prefrontal and the cathode over the posterior parietal cortex during a single-session distractor inhibition training. This network-oriented stimulation protocol should enhance inhibition processes by shifting the neural activity from posterior to prefrontal regions. WM improved after a single-session distractor inhibition training under verum stimulation but only in subjects with a high WM capacity. In subjects with a low WM capacity, verum tDCS reduced the transfer effects on WM. We assume tDCS to strengthen the frontostriatal pathway in individuals with a high WM capacity leading to efficient inhibition of distractors. In contrast, the cathodal stimulation of the posterior parietal cortex might have hindered usual compensational mechanism in low capacity subjects, i.e. maintaining also irrelevant information in memory. Our results thus stress the need to adjust tDCS protocols to well-founded knowledge about neural networks and individual cognitive differences.

No modulation of pupil size and event-related pupil response by transcutaneous auricular vagus nerve stimulation (taVNS).

Transcutaneous auricular vagus nerve stimulation (taVNS) bears therapeutic potential for a wide range of medical conditions. However, previous studies have found substantial interindividual variability in responsiveness to taVNS, and no reliable predictive biomarker for stimulation success has been developed so far. In this study, we investigate pupil size and event-related pupil response as candidate biomarkers. Both measures have a direct physiological link to the activity of the locus coeruleus (LC), a brainstem structure and the main source of norepinephrine in the brain. LC activation is considered one of the key mechanisms of action of taVNS, therefore, we expected a clear increase of the pupillary measures under taVNS compared to sham (placebo) stimulation, such that it could serve as a prospective predictor for individual clinical and physiological taVNS effects in future studies. We studied resting pupil size and pupillary responses to target stimuli in an auditory oddball task in 33 healthy young volunteers. We observed stronger pupil responses to target than to standard stimuli. However, and contrary to our hypothesis, neither pupil size nor the event-related pupil response nor behavioral performance were modulated by taVNS. We discuss potential explanations for this negative finding and its implications for future clinical investigation and development of taVNS.