Efficacy of slow oscillatory‐transcranial direct current stimulation on EEG and memory – contribution of an inter‐individual factor

Abstract Despite many reports on beneficial effects of anodal slow oscillatory‐transcranial direct current stimulation (so‐ tDCS ) during non‐rapid eye movement ( NREM ) sleep on memory consolidation, frequent negative outcomes have also been observed. Our working hypothesis is that so‐ tDCS efficacy is strongly dependent upon the susceptibility of the underlying network. One component determining susceptibility of the network is hypothesized to be reflected in learning or ‘task‐induced’ plastic changes. Another component is hypothesized to represent inter‐individual confounds. Twenty‐five (15 female) healthy students participated in two learning conditions with and without so‐ tDCS during early nocturnal NREM sleep and in one control condition without learning tasks. So‐ tDCS was applied in five 5‐min blocks. EEG was assessed during two time windows: an acute period with five 1‐min epochs after each stimulation block and a 150‐min post‐stimulation time period. Inter‐individual differences were assessed by a memory quotient ( MQ ) and subjects classified into high‐ vs. low‐scoring groups. Although so‐ tDCS was efficient in enhancing fast spindle parameters in the 150‐min time period in all subjects, so‐ tDCS failed to modulate memory consolidation. In contrast, in subjects with a high MQ , memory retention on a figural paired‐associate task was significantly increased after so‐ tDCS . Task‐induced slow spindle density was modulated in the opposite direction in subjects with high vs. low MQ being increased in the high‐ MQ group only. Effects of so‐ tDCS on EEG were limited to fast spindle modulations in both time windows. These results reveal that inter‐individual confound can impact so‐ tDCS efficacy, suggesting potential use of such factors as biomarkers.