Theta’s functional role in encoding, nocturnal reactivation and retrieval for the formationof declarative memory traces

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The aim of my studies is to investigate the functional relevance of oscillatory theta activity during wakefulness and sleep in different stages of the memory process. We plan to test the influence of transcranial alternating current stimulation in the range of the theta frequency band (theta-tACS, 6 Hz) on the memory performance after sleep in a Dutch-German wordpair paradigm to explore a possible functional role of theta activity before encoding, nocturnal reactivation and retrieval. We hypothesize that theta-tACS conducted before encoding (study 1) as well as retrieval (study 2) of acoustically and visually presented items will modify the ongoing brain state. Previous findings revealed that higher theta before encoding is essential for an improved delayed retention. Therefore, we assume for study 1 that the increased theta power resulting from the simulation will lead to a better retention performance the next morning. Given that the brain state before retrieval is as important as during encoding and that theta is seen as important in the attempt of retrieving information, for study 2 we also expect a higher retention performance resulting from such a stimulation. In general, we do not expect an improvement in memory performance after applying sham-tACS. Given the modality preference for auditory presentation we expect a generally better result for acoustically presented items compared to visually presented items. Additionally, with regard to the context dependency we also expect a better memory performance for items which are cued during recall in the same modality as they are presented during the study phase. In study 3 we apply theta-tACS shortly before the nocturnal reactivation of previously encoded items (auditory and visually presented) which is induced by us. We assume that auditory item reactivation during SWS leads generally to an improved retention performance after sleep compared to items which are not encoded previously. Additionally, we expect a better retention performance of acoustically presented items compared to visually presented items due to the context similarity during only auditory nocturnal reactivation. The additional theta-tACS (applied before reactivation of a part of the previously learned items) should lead to an increase in power in the respective frequency band and thus, result in better retention performance the next morning compared to sham-tACS as well as completely new words. Such a result could be an indication for the role theta is playing in improving the memory strength of reactivated items. In general, the results could provide insights about a potential functional role of the theta frequency band for formation processes of declarative memories.