Personalized Frequency Modulated Transcranial Electrical Stimulation for Associative Memory Enhancement

Abstract

Associative memory (AM) is the ability to remember the relationship between previously unrelated items. AM is significantly affected by normal aging and neurodegenerative conditions, thus there is a growing interest in applying non-invasive brain stimulation (NIBS) techniques for AM enhancement. A growing body of studies identifies posterior parietal cortex (PPC) as the most promising cortical target for both transcranial magnetic stimulation (TMS) and transcranial electrical stimulation (tES) to modulate a cortico-hippocampal network that underlines AM. In that sense, theta frequency oscillatory tES protocols, targeted towards the hallmark oscillatory activity within the cortico-hippocampal network, are increasingly coming to prominence. To increase precision and effectiveness, the need for EEG guided individualization of the tES protocols is proposed. Here, we present the study protocol in which two types of personalized oscillatory tES-transcranial alternating current stimulation (tACS) and oscillatory transcranial direct current stimulation (otDCS), both frequency-modulated to the individual theta-band frequency (ITF), are compared to the non-oscillatory transcranial direct current stimulation (tDCS) and to the sham stimulation. The study has cross-over design with four tES conditions (tACS, otDCS, tDCS, sham), and the comprehensive set of neurophysiological (resting state EEG and AM-evoked EEG) and behavioral outcomes, including AM tasks (short-term associative memory, face-word, face-object, object-location), as well as measures of other cognitive functions (cognitive control, verbal fluency, and working memory).

Keywords: EEG; associative memory; cognitive assessment; individual theta-band frequency; personalized brain stimulation; transcranial alternating current stimulation (tACS); transcranial direct current stimulation (tDCS); transcranial oscillatory current stimulation (otDCS).

Figure 1

The overview of the study design—recruitment and the selection of participants; first experimental session (S0) in which ITF is determined based on EEG data recorded during the AM task; tES-EEG experiment consisting of four experimental sessions (S1–S4) for each participant (counterbalanced cross-over design). Each of S1–S4 adopts the same experimental procedure: (1) questionnaires, (2) resting state EEG (rsEEG), (3) one of tES protocols (sham/tDCS/tACS/otDCS), during which participants performed Short-term AM task first and the Simon task second, (4) rsEEG, (5) Face–word AM task with EEG recording, (6) Animal–location AM task with EEG recording, (7) verbal fluency task, (8) Face–object AM task with EEG recording, (9) n-back task, and (10) Backward span task.

Figure 2

(a) The tES stimulation protocols–tACS (30 s gradual ramp up to 1 mA, 19 min of 0 ± 1 mA at ITF, 30 s gradual ramp down), otDCS (30 s gradual ramp up to 1.5 mA, 19 min of 1.5 ± 0.5 mA at ITF, 30 s gradual ramp down), tDCS (30 s gradual ramp up to 1.5 mA, 19 min of 1.5 mA, 30 s gradual ramp down), and sham (30 s gradual ramp up to 1.5 mA, 30 s gradual ramp down to 0 mA, 19 min 0 mA, 30 s gradual ramp up to 1.5 mA, 30 s gradual ramp down); (b) the electrode montage—the positions of EEG recording electrodes (green) and the position of tES electrode over left PPC (orange); the return electrode is positioned on the right cheek (orange).

Figure 3

Schemes of the AM tasks employed in the study (a) the short-term AM task; (b) the Face–word AM task; (c) the Animal–location task; (d) the Face–object AM task.