Age-dependent effects of brain stimulation on network centrality

Functional magnetic resonance imaging (fMRI) studies have suggested that advanced age
may mediate the effects of transcranial direct current stimulation (tDCS) on brain function.
However, studies directly comparing neural tDCS effects between young and older adults are
scarce and limited to task-related imaging paradigms. Resting-state (rs-) fMRI, that is
independent of age-related differences in performance, is well suited to investigate age
associated differential neural tDCS effects. Three “online” tDCS conditions (anodal, cathodal,
sham) were compared in a cross-over, within-subject design, in 30 young and 30 older
adults. Active stimulation targeted the left sensorimotor network (active electrode over left
sensorimotor cortex with right supraorbital reference electrode). A graph-based rs-fMRI data
analysis approach (eigenvector centrality mapping) and complementary seed-based
analyses characterized neural tDCS effects. An interaction between anodal tDCS and age
group was observed. Specifically, centrality in bilateral paracentral and posterior regions
(precuneus, superior parietal cortex) was increased in young, but decreased in older adults.
Seed-based analyses revealed that these opposing patterns of tDCS-induced centrality
modulation were explained from differential effects of tDCS on functional coupling of the
stimulated left paracentral lobule. Cathodal tDCS did not show significant effects. Our study
provides first evidence for differential tDCS effects on neural network organization in young
and older adults. Anodal stimulation mainly affected coupling of sensorimotor with
ventromedial prefrontal areas in young and decoupling with posteromedial areas in older
adults.