Determining the Individual Theta Frequency for Associative Memory Targeted Personalized Transcranial Brain Stimulation

Abstract

Non-invasive brain stimulation (NIBS) methods have 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 electroencephalography (EEG)-guided personalized frequency-modulated NIBS protocols to increase the focality and the effectiveness of the interventions. Still, extraction of individual frequency, especially in the theta band, turned out to be a challenging task. Here we present an approach to extracting the individual theta-band frequency (ITF) from EEG signals recorded during the AM task. The method showed a 93% success rate, good reliability, and the full range of variability of the extracted ITFs. This paper provides a rationale behind the adopted approach and critically evaluates it in comparison to the alternative methods that have been reported in the literature. Finally, we discuss how it could be used as an input parameter for personalized frequency-modulated NIBS approaches-transcranial alternating current stimulation (tACS) and transcranial oscillatory current stimulation (otDCS) directed at AM neuromodulation.

Keywords: EEG; associative memory; brain stimulation; individual theta frequency (ITF); personalization.

Figure 1

The AM task (Encoding and Recognition block) was performed while EEG was recorded; the EEG signals from the Encoding block were used to extract individual theta frequencies (ITF) for subsequently correctly recognized pairs.

Figure 2

(a) The group-level time-frequency plot of the EEG signal during the successfully encoded pairs. The ERSP values, averaged across participants and electrodes, were plotted within the window of −800 and 1250 ms (x-axis), with stimulus onset marker (black line) and 1–12 Hz (y-axis) split into alpha, theta, and delta band. (b) The examples of individual time-frequency plots (three participants); the same x-axes and y-axes as well as color coding as in (a) were used. (c) The example of participant-level time x electrode matrix with Hz with highest ERSP value in each cell, marked according to the alpha (green), theta (orange), and delta (blue) band, with ITF of 4.5 Hz, i.e., modal value marked bold.

Figure 3

(a) The AM task performance: the overall success rate in % and the % of correctly identified targets, i.e., successfully encoded pairs. Individual participants overlay the box-plot marking median and interquartile range (b) The individual differences in theta activity during the AM encoding: examples of time-frequency plots of the EEG signal during the successfully encoded pairs for six participants; the ERSP values, averaged across 9 centro-parietal electrodes, are plotted within the window of −800 and 1250 ms (x-axis) and 4–8 Hz (y-axis), with the extracted ITF for each participant marked; (c) The individual theta frequency (ITF) distribution: the number of participants and their extracted ITF values in Hz (x-axis).