Abstract
The crucial role of a hippocampal-cortical replay that gradually strengthens hippocampus-independent cortico-cortical connections has been recently emphasized for the prefrontal cortex (PFC). In a prior analysis, we demonstrated that bi-frontal anodal tDCS significantly decreased total sleep time in healthy participants. To further elucidate the relevance of prefrontal activity levels for sleep-related memory consolidation, we explore the effects of bi-frontal tDCS on pre-post sleep memory consolidation. 18 healthy participants (13 females, age 53.1 ± 6.6 years) underwent three experimental nights with one of three counterbalanced tDCS protocols (anodal, cathodal, sham) immediately prior to polysomnography (11 pm to 7 am, interval of 3–7 d). tDCS was transferred via two pairs of electrodes (5 × 7 cm over FP1/FP2 and 10 cm x 10 cm over P3/P4) covered with electrode cream (1 mA over each target electrode, ramp up/down of 30 s, interstimulus interval 20 min). At 8 pm, 46 semantically related word-pairs were presented in a randomized order (Presentation® software) followed by repeatedly tapping a five-element sequence of numbers as fast and accurately as possible. At 8 am on the following morning, memory retrieval was assessed in a cued recall test for word-pairs (percentage of correctly retrieved referred to encoded words) and in a finger tapping test session (percentage of average speed and accuracy improvement). Bi-frontal tDCS did not result in significant changes in pre-post sleep memory consolidation in either of the examined memory systems. Specifically, repeated measures analyses of variance detected no significant condition effect for the percentage of correct word pairs ( F = 0.2, p = 0.820, pETA 2 = 0.012), finger tapping speed ( F = 1.9, p = 0.167, pETA 2 = 0.113) or accuracy ( F = 0.1, p = 0.827, pETA 2 = 0.010). In contrast to our finding of a decreased total sleep time in healthy participants following bi-frontal anodal tDCS, the stimulation protocol did not influence sleep-related memory consolidation. As mainly remote memory has been proposed to be dependent on the PFC, the test interval could have been too short to detect long-term memory effects. Besides, the results support that sleep modulating tDCS can be considered a safe and promising tool without severe adverse cognitive effects.
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