Extracting the individual theta frequency from associative memory EEG for personalized brain stimulation

Abstract

Non-invasive brain stimulation (NIBS) has gained increased interest in research and therapy of associative memory (AM) and its impairments. However, the one-size-fits-all approach yields inconsistent findings, thus putting forward the need for the development of personalized frequency modulated NIBS protocols to increase the focality and the effectiveness of the interventions. Here we present the new method for extracting the individual theta-band frequency (ITF) to be used as an input parameter for personalized frequency-modulated NIBS approaches – transcranial altering current stimulation (tACS) and transcranial oscillatory current stimulation (otDCS). In a sample of 42 healthy volunteers, we extracted the frequencies with the highest event-related spectral perturbation from 19 overlapping time windows and six centroparietal electrodes from the EEG signal recorded during successful encoding in an AM task. The ITF was defined as modal frequency (4-8Hz in 0.5 Hz steps) in time x electrode matrix. The method showed 93% success rate, good reliability, and the full range of variability of the extracted ITFs. We present and discuss individual differences in theta-band activity during AM encoding, together with ITF-extraction challenges. Finally, we provide a rationale behind the adopted analytic approach and critically evaluate it in comparison to the alternative methods that have been reported in the literature.