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

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.