Modulation of network centrality and gray matter microstructure using multi‐session brain stimulation and memory training

Abstract

Neural mechanisms of behavioral improvement induced by repeated transcranial direct current stimulation (tDCS) combined with cognitive training are yet unclear. Previously, we reported behavioral effects of a 3‐day visuospatial memory training with concurrent anodal tDCS over the right temporoparietal cortex in older adults. To investigate intervention‐induced neural alterations we here used functional magnetic resonance imaging (fMRI) and diffusion tensor imaging (DTI) datasets available from 35 participants of this previous study, acquired before and after the intervention. To delineate changes in whole‐brain functional network architecture, we employed eigenvector centrality mapping. Gray matter alterations were analyzed using DTI‐derived mean diffusivity (MD). Network centrality in the bilateral posterior temporooccipital cortex was reduced after anodal compared to sham stimulation. This focal effect is indicative of decreased functional connectivity of the brain region underneath the anodal electrode and its left‐hemispheric homolog with other “relevant” (i.e., highly connected) brain regions, thereby providing evidence for reorganizational processes within the brain's network architecture. Examining local MD changes in these clusters, an interaction between stimulation condition and training success indicated a decrease of MD in the right (stimulated) temporooccipital cluster in individuals who showed superior behavioral training benefits. Using a data‐driven whole‐brain network approach, we provide evidence for targeted neuromodulatory effects of a combined tDCS‐and‐training intervention. We show for the first time that gray matter alterations of microstructure (assessed by DTI‐derived MD) may be involved in tDCS‐enhanced cognitive training. Increased knowledge on how combined interventions modulate neural networks in older adults, will help the development of specific therapeutic interventions against age‐associated cognitive decline.

Microstructural and functional plasticity following repeated brain stimulation during cognitive training in older adults

“…Simultaneous tDCS-fMRI application in proof-of-principle studies revealed changes in local activity and functional connectivity (temporally coherent activity between brain regions) that predicted behavioral performance gains 33 – 35 . Functional connectivity modulations in response to repeated training sessions may even reflect network level reorganizations, promoting longer-lasting neural plasticity 4 , 36 – 38 .…”

“…In the current paper, we used DTI acquired before and after the intervention for individual fiber tractography and quantification of white matter microstructure. Further, DTI allowed us to examine whether microstructural properties in the stimulation target would change due to the intervention as suggested previously 38 . The investigation of microstructural plasticity markers was complemented by resting-state functional magnetic resonance imaging (rs-fMRI) to analyze functional synchrony modifications within the targeted (frontoparietal) network.…”

Microstructural and functional plasticity following repeated brain stimulation during cognitive training in non-demented older adults

“…Simultaneous tDCS-fMRI application in proof-of-principle studies revealed changes in local activity and functional connectivity (temporally coherent activity between brain regions) that predicted behavioral performance gains 16,17,18 . Functional connectivity modulations in response to repeated training sessions may even re ect network level reorganizations, promoting longer-lasting neural plasticity 19,20,21,22 .…”

“…For individual ber tractography and quanti cation of white matter microstructure, we used DTI acquired before and after the intervention. Further, DTI allowed us to examine whether microstructural properties in the stimulation target would change due to the intervention as suggested previously 22 . The investigation of microstructural plasticity markers was complemented by resting-state functional magnetic resonance imaging (rs-fMRI) to analyze functional synchrony modi cations within the targeted (fronto-parietal) network.…”

Lasting Enhancements in Neural Efficiency by Multi-session Transcranial Direct Current Stimulation in Working Memory Training

“…More importantly, few studies have examined the aftereffects of tDCS on these neurophysiological measures immediately after the end of WMT, typically at the end [36,39] or one day after the last training session [4,[32][33][34]. A recent study utilizing resting-state functional magnetic resonance imaging (fMRI) found improved brain network processing efficiency with 3-day of tDCS+WMT one day after training [40]. However, no significant effects of tDCS on brain activity were found in a 3day WMT+tDCS study that quantified long-term neurobehavioral effects using ERPs at one month [38].…”