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.

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Electroencephalography (EEG) has been widely used in the neurophysiological investigation of major depressive disorder (MDD) during past decades. An approach that has attracted particular interest over the past 20 years is current source density (CSD) that assesses current source in extracellular spaces, which are the local generators of the field potentials caused by the activation of neurones. Our aim was to review the current literature regarding resting state CSD analysis in MDD patients. To date, the most prominent aspects in such studies comprise the identification of clinical endophenotypes on the basis of resting state CSD, and the investigation of CSD with respect to treatment outcome prediction. Increased alpha band resting state CSD in frontal regions is typical for MDD, while increased theta band activity in the rostral anterior cingulate gyrus (rACC) has been found to be a good predictor of better antidepressant response. However, differences in the methods used in different studies could be responsible for some contradictions in reported findings. Further research is needed for better distinction of depressive patients from patients with other psychiatric disorders, as well as from healthy subjects.

Sleep spindles were one of the first rhythms associated with learning and memory consolidation. Current research shows spindles can reflect features of trait and time-varying properties of neuroplasticity. A new study has now used feedback-controlled spindle frequency stimulation to show that sleep spindles modulate endogenous brain electric activity and behavior.