Alpha-Oscillatory Current Application Impacts Prospective Remembering Through Strategic Monitoring

Abstract

Prospective memory (PM) is the ability to remember to execute future intentions. PM requires engagement of attentional networks, in which oscillatory activity in the alpha frequency range has been implicated. The left dorsolateral prefrontal cortex (DLPFC) and inferior parietal cortex are assumed to be engaged during PM tasks. We hypothesized that the selective application of transcranial alternating current stimulation (tACS) at alpha frequency to these areas can modulate PM-associated event-related potentials. Participants were assigned to alpha-tACS, theta-tACS, or Sham stimulation. They performed a working memory task (OGT), with a PM component, pre-, during, and post-stimulation. EEG was recorded post-stimulation. Accuracy and reaction times (RTs) were computed. Following EEG source reconstruction of mean amplitude, source activity was contrasted between conditions in which performance was modulated by tACS using cluster-based permutation tests. RTs were slower on introducing the PM task, consistent with strategic monitoring. PM accuracy improved in the alpha-tACS group only. During PM trials, source activity in the posterior cingulate cortex (PCC) was lower following alpha-tACS than after Sham stimulation. Source activity in the DLPFC following alpha-tACS was lower during PM than in OGT trials following alpha-tACS. Performance modulation through alpha-tACS, and the lower DLPFC activity in PM than in OGT trials provide evidence of a role for alpha oscillations during strategic monitoring for a PM cue. Lower PCC activity in the alpha-tACS than Sham group is consistent with facilitation of disengagement of the default mode network, supporting re-direction of attention from the OGT to the PM task and task-switching.

Keywords: EEG; delayed intention; future intention; prospective memory; source activation; strategic monitoring; tACS.

FIGURE 1

Timeline of the experiment. The Baseline consisted of one training run and one run of only the 2‐back task. Directly after, the three main blocks (pre‐, during, and post‐stimulation) followed, with four runs each. Colored rectangles represent the four possible letter colors, one of which was assigned as the prospective memory cue (surrounded here in a rectangular border) for each new run.

FIGURE 2

(A) 2‐back working memory task with embedded prospective memory (PM) task. In this example, the prospective memory cue was the color blue. Responding to the prospective memory cue was prioritized over the ongoing task. The next letters H (yellow) and K (yellow) were both nontargets, followed by H (red), a target of the ongoing task. (B) Simulated electric field magnitude (V/m²) for the transcranial alternating current stimulation montage.

FIGURE 3

(A) Mean accuracy and (B) reaction time for the ongoing task trials during Baseline and Pre‐stimulation; Circles = individual mean values; * = p < 0.001.

FIGURE 4

(A) Mean accuracy and (B) reaction time in the prospective memory trials Pre‐, During, and Post‐stimulation for each stimulation group (Sham, theta‐tACS, alpha‐tACS); Circles = individual mean values; * = significant difference at threshold p < 0.017 (after Bonferroni correction).

FIGURE 5

(A) Mean accuracy and (B) reaction time in the ongoing task (OGT) trials pre‐, during, and post‐stimulation for each stimulation group (Sham, theta‐tACS, alpha‐tACS); Circles = individual mean values; * = significant difference at threshold p < 0.017 (after Bonferroni correction).

FIGURE 6

Grand‐average event‐related potentials following sham stimulation for a parietal region of interest (‘P3’, ‘Pz’, ‘P4’). The greater peak amplitude during prospective memory (PM) than ongoing task (OGT) trials in the time window between 600 to 800 ms is consistent with previously reported prospective positivity; Red dashed lines = significant difference at threshold p < 0.05.

FIGURE 7

(A) Source reconstruction of mean amplitude differences between prospective memory (PM) and ongoing task (OGT) trials after alpha‐tACS at the border between the left anterior prefrontal cortex and dorsolateral prefrontal cortex. (B) Source reconstruction of mean amplitude differences between PM trials after alpha‐tACS and after sham stimulation in the left dorsal posterior cingulate cortex; The vertex cluster value indicates the temporal extent of the cluster at that vertex, measured in samples.

FIGURE 8

Mean alpha (left) and theta (right) power peak values after sham stimulation for prospective memory (PM) and ongoing task (OGT) trials at the time/location of ERP components reported in prospective memory. (A) For the time/location of prospective positivity, averaged over a time window from 600 to 800 ms and electrode locations ‘P3’, ‘Pz’, ‘P4’. (B) for the time/location of the N300, averaged over a time window from 200 to 400 ms and electrode locations ‘PO10’, ‘PO9’, ‘POz’; Circles = individual mean values; * = significant difference at threshold p < 0.05.

FIGURE 9

Mean alpha (left) and theta (right) power peak values following sham and alpha‐tACS for ongoing task trials at the time/location of frontal positivity, averaged over a time window from 250 to 400 ms and electrode locations ‘Fz’, ‘AF3’, ‘AF4’, ‘AF7’, ‘AF8’; Circles = individual mean values; * = significant difference at threshold p < 0.05.