Abstract
Sleep-dependent memory consolidation has been extensively studied. Neutral declarative memories and serial reaction time task (SRTT) performance can benefit from slow-wave activity, characterized by less than 1 Hz frequency cortical slow oscillations (SO). Emotional memories can benefit from theta activity, characterized by 4–8 Hz frequency cortical oscillations. Applying transcranial direct current stimulation (tDCS) during sleep entrains specific frequencies to alter sleep architecture. When applying cathodal tDCS (CtDCS), neural inhibition or excitation may depend on the waveform at the applied frequency. A double dissociation was predicted, with CtDCS at SO frequency improving neutral declarative memory and SRTT performance, and theta frequency CtDCS inhibiting negative emotional memory. Participants completed three CtDCS conditions (Theta: 5 Hz, SO: 0.75 Hz and control: sham) and completed an SRTT and word recognition task pre- and post-sleep, comprising emotional and neutral words to assess memory. In line with predictions, CtDCS improved neutral declarative memory when applied at SO frequency. When applied at theta frequency, no negative emotional word memory impairment was found but a positive association was found between post-stimulation theta power and emotional word recognition. SRTT performance was also not altered by either CtDCS frequency. Future studies should investigate overnight theta CtDCS and examine the effects of CtDCS during and after stimulation.
Highlights
Sleep facilitates memory consolidation [1,2,3,4], the mechanisms of this facilitation are debated [5,6,7]
As the theta waveform is not characterized by fully hyperpolarized down-states, instead acting as a ‘travelling wave’ [33], in principle, cathodal tDCS (CtDCS) can inhibit cortical excitability by further decreasing theta downstates, which may have a concomitant effect on emotional memory consolidation. transcranial direct current stimulation (tDCS) may, influence memory differently depending on more factors than the form of the stimulation
Alertness was measured pre-sleep and post-sleep in each CtDCS condition using the Sleepiness Scale (SSS) to ensure that task performance was not impacted by differences in alertness; these results are shown in figure 4
Summary
Sleep facilitates memory consolidation [1,2,3,4], the mechanisms of this facilitation are debated [5,6,7]. The synaptic homeostasis hypothesis suggests that global synaptic downscaling occurs due to slow-wave activity (SWA) mainly during slow-wave sleep (SWS), restoring balance after synaptic potentiation during wake [8]. This downscaling can eliminate weaker memory traces [5]. The active systems consolidation hypothesis [7] suggests that SWA orchestrates the reactivation and reintegration of newly encoded information through interplay between the hippocampus and neocortex [6] This dialogue involves slow-oscillatory (SO) neuronal activity which oscillates between hyperpolarizing ‘down-states’ and depolarizing ‘up-states’ [3]. As the theta waveform is not characterized by fully hyperpolarized down-states, instead acting as a ‘travelling wave’ [33], in principle, CtDCS can inhibit cortical excitability by further decreasing theta downstates, which may have a concomitant effect on emotional memory consolidation. tDCS may, influence memory differently depending on more factors than the form of the stimulation (cathodal or anodal)
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