Exploring anterior thalamus functional connectivity with cortical regions in prospective memory with ultra-high-field fMRI

Abstract

Introduction Prospective memory (PM), or memory for future intentions, engages particular cortical regions. Lesion studies also implicate the thalamus, with PM deterioration following thalamic stroke. Neuroimaging, anatomical, and lesion studies suggest the anterior nuclei of the thalamus (ANT) in particular are involved in episodic memory processing, with electrophysiological studies providing evidence for an active role in selection of neural assemblies underlying particular memory traces. Here we hypothesized that the ANT are also engaged in realizing prospectively-encoded intentions. Methods Participants (N = 14) performed an n-back working memory task as the ongoing task (OGT) with two cognitive loads, each with and without a PM component, during 7-Tesla functional magnetic resonance imaging. Seed-to-voxel whole brain functional connectivity analyses were performed to establish whether including a PM component in an OGT results in greater connectivity between ANT and cortical regions known to be engaged in PM. Repeated measures ANOVAs were applied to behavioral and connectivity measures, with the factors Task Type (with PM or not) and N-Back (2-back or 3-back). Results Response accuracy was greater and reaction times faster without the PM component, and accuracy was higher in the 2- than 3-back condition. We observed a main effect of Task Type on connectivity with an ANT seed, with greater connectivity between the ANT and dorsolateral prefrontal cortex (DLPFC) and superior temporal gyrus (STG) when a PM component was included. Post hoc testing based on a significant interaction showed greater ANT-DLPFC connectivity when PM was included with the low and ANT-STG connectivity with the high cognitive load OGT. There was no main effect of N-Back at these locations. Connectivity based on a dorsomedial nucleus of the thalamus seed did not differ according to Task Type at these locations. Conclusions Enhanced connectivity between the ANT and the DLPFC, a brain region with an established role in strategic monitoring for PM cues, arose with a low cognitive load OGT that enabled monitoring. This pattern was absent on directly increasing the cognitive load of the OGT without PM, suggesting specificity for PM. Greater ANT-STG connectivity on PM inclusion in the higher cognitive load OGT condition fits with reports of STG activation on PM through spontaneous retrieval. Differing connectivity based on a dorsomedial nucleus of the thalamus seed suggests ANT specificity. The findings fit with a coordinating role for the ANT in prospective remembering.