Cross-hemispheric Alternating Current Stimulation During a Nap Disrupts Slow Wave Activity and Associated Memory Consolidation

Background: Slow Wave Activity (SWA), the low frequency (<4 Hz) oscillations that characterize Slow Wave Sleep (SWS) are thought to relate causally to declarative memory consolidation during nocturnal sleep. Evidence is conflicting relating SWA to memory consolidation during nap however. Objective/hypothesis: We applied transcranial alternating current stimulation (tACS) - which, with a cross-hemispheric electrode montage (F3 and F4 - International 10:20 EEG system), is able to disrupt brain oscillations-to determine if disruption of low frequency oscillation generation during afternoon nap is causally related to disruption in declarative memory consolidation. Methods: Eight human subjects each participated in stimulation and sham nap sessions. A verbal paired associate learning (PAL) task measured memory changes. During each nap period, five 5-min stimulation (0.75 Hz cross-hemispheric frontal tACS) or sham intervals were applied with 1-min post-stimulation intervals (PSI's). Spectral EEG power for Slow (0.7-0.8 Hz), Delta (1.0-4.0 Hz), Theta (4.0-8.0 Hz), Alpha (8.0-12.0 Hz), and Spindle-range (12.0-14.0) frequencies was analyzed during the 1-min preceding the onset of stimulation and the 1-min PSI's. Results: As hypothesized, power reduction due to stimulation positively correlated with reduction in word-pair recall post-nap specifically for Slow (P < 0.0022) and Delta (P < 0.037) frequency bands. Conclusions: These results provide preliminary evidence suggesting a causal and specific role of SWA in declarative memory consolidation during nap. (C) 2015 Elsevier Inc.