Slow-theta power decreases during item-place encoding predict spatial accuracy of subsequent context recall.

Human hippocampal theta oscillations play a key role in accurate spatial coding. Associative encoding involves similar hippocampal networks but, paradoxically, is also characterized by theta power decreases. Here, we investigated how theta activity relates to associative encoding of place contexts resulting in accurate navigation. Using MEG, we found that slow-theta (2-5Hz) power negatively correlated with subsequent spatial accuracy for virtual contextual locations in posterior hippocampus and other cortical structures involved in spatial cognition. A rare opportunity to simultaneously record MEG and intracranial EEG in an epilepsy patient provided crucial insights: during power decreases, slow-theta in right anterior hippocampus and left inferior frontal gyrus phase-led the left temporal cortex and predicted spatial accuracy. Our findings indicate that decreased slow-theta activity reflects local and long-range neural mechanisms that encode accurate spatial contexts, and strengthens the view that local suppression of low-frequency activity is essential for more efficient processing of detailed information.

Induction of a depression-like negativity bias by cathodal transcranial direct current stimulation.

Cognitive control (CC) over emotional distraction is of particular importance for adaptive human behaviour and is associated with activity in the left dorsolateral prefrontal cortex (dlPFC). Deficient CC, e.g., presenting as negativity bias, has been suggested to underlie many of the core symptoms of major depression (MD) and is associated with impairments of dlPFC function. Correspondingly, enhancement of dlPFC activity with anodal transcranial direct current stimulation (tDCS) can ameliorate these impairments in patients with MD. Here, we tested the hypothesis that a reduction of dlPFC activity by cathodal tDCS induces CC deficits, thus triggering a depression-like negativity bias in healthy subjects. Twenty-eight individuals participated in a double-blinded, balanced randomized crossover trial of cathodal (1 mA, 20 min) and sham tDCS applied to the left dlPFC. To assess CC we conducted a delayed response working memory (DWM) task and an arithmetic inhibition task (AIT) with pictures of varying valent content (negative, neutral, positive) during and immediately after stimulation. Cathodal tDCS led to impaired CC specifically over negative material as assessed by reduced response accuracy in the DWM and prolonged response latency in the AIT. Hence, the current study supports the notion that left dlPFC is critically involved in CC over negative material. Together with previously reported beneficial anodal effects, it indicates that the hypoactivation of left dlPFC causes deficits in CC over negative material, which is a possible aetiological mechanism of depression.