Bruno de Matos Mansur

Strategic monitoring impaired ongoing task performance and increased central–parietal alpha/beta power. Alpha- and beta-band activity showed a context-dependent role. In the no-PM condition, greater oscillatory power—particularly late alpha—was associated with slower responses, indicating reduced cognitive flexibility, whereas in the with-PM condition, increased alpha/beta power was linked to higher accuracy and faster responses, supporting goal maintenance during prospective monitoring.

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.

Prospective memory, or memory for future intentions, engages particular cortical regions. Lesion studies also implicate the thalamus, with prospective memory deterioration following thalamic stroke. Neuroimaging, anatomical, and lesion studies suggest the ANT (anterior nuclei of the thalamus) in particular are involved in episodic memory, with electrophysiological studies suggesting an active role in selecting neural assemblies underlying particular memory traces. Here we hypothesized that the ANT are engaged in realizing prospectively-encoded intentions, detectable using ultra-high-field strength functional MRI. Using a within-subject design, participants (N = 14; age 20-35 years) performed an ongoing n-back working memory task with two cognitive loads, each with and without a prospective memory component, during 7-Tesla functional MRI. Seed-to-voxel whole brain functional connectivity analyses were performed to establish whether including a prospective memory component in an ongoing task results in greater connectivity between ANT and cortical regions engaged in prospective memory. Repeated measures ANOVAs were applied to behavioral and connectivity measures, with the factors Task Type (with prospective memory or not) and N-Back (2-back or 3-back). Response accuracy was greater and reaction times faster without the prospective memory component, and accuracy was higher in the 2-than 3-back condition. Task Type had a main effect on connectivity with an ANT seed, with greater ANT–DLPFC (dorsolateral prefrontal cortex) and ANT–STG (superior temporal gyrus) connectivity when including a prospective memory component. Post hoc testing based on a significant interaction showed greater ANT–DLPFC connectivity (p-FWE = 0.007) when prospective memory was included with the low and ANT–STG connectivity (p-FWE = 0.019) with the high cognitive load ongoing task. Direct comparison showed greater functional connectivity between these areas and the ANT than DMNT (dorsomedial nucleus of the thalamus) during prospective remembering. Enhanced ANT–DLPFC connectivity, a brain region with an established role in strategic monitoring for prospective memory cues, arose with a low cognitive load ongoing task that enabled monitoring. This connectivity was significantly less on direct comparison with increasing the cognitive load of the ongoing task without prospective memory, suggesting specificity for prospective memory. Greater ANT–STG connectivity on prospective memory inclusion in the higher cognitive load ongoing task fits with reported STG activation on prospective memory through spontaneous retrieval. Lower connectivity on direct comparison with a DMNT seed suggests ANT specificity. The findings fit with a coordinating role for the ANT in prospective remembering. Given the small sample, these findings should be considered preliminary, with replication required.