Inter-individual differences in baseline dynamic functional connectivity are linked to cognitive aftereffects of tDCS

Abstract

Transcranial direct current stimulation (tDCS) has the potential to modulate cognitive training in healthy aging; however, results from various studies have been inconsistent. We hypothesized that inter-individual differences in baseline brain state may contribute to the varied results. We aimed to explore whether baseline resting-state dynamic functional connectivity (rs-dFC) and/or conventional resting-state static functional connectivity (rs-sFC) may be related to the magnitude of cognitive aftereffects of tDCS. To achieve this aim, we used data from our double-blind randomized sham-controlled cross-over tDCS trial in 25 healthy seniors in which bifrontal tDCS combined with cognitive training had induced significant behavioral aftereffects. We performed a backward regression analysis including rs-sFC/rs-dFC measures to explain the variability in the magnitude of tDCS-induced improvements in visual object-matching task (VOMT) accuracy. Rs-dFC analysis revealed four rs-dFC states. The occurrence rate of a rs-dFC state 4, characterized by a high correlation between the left fronto-parietal control network and the language network, was significantly associated with tDCS-induced VOMT accuracy changes. The rs-sFC measure was not significantly associated with the cognitive outcome. We show that flexibility of the brain state representing readiness for top-down control of object identification implicated in the studied task is linked to the tDCS-enhanced task accuracy.

Figure 1

Experimental design and methods. (a) The crossover design involved two sessions with real 2 mA stimulation/ sham tDCS with a concurrent working memory task. Prior to and after the stimulation, participants performed a visual object matching task (VOMT) and underwent resting-state fMRI. (b) Offline VOMT—subjects respond whether the two consecutive objects are the same or different by pressing a YES/NO button in two difficulty levels (conventional view of objects—lower difficulty level; unconventional view of objects—higher difficulty level). (c) Online WMT—subjects view a block of faces and scenes (2 + 2, randomized order) preceded by a specific command on how to react to a probe that follows each block. Subjects respond whether the probe is consistent/ inconsistent with the prior instruction by pressing a YES/NO button. Freely available face photographs from Chicago and Glasgow face databases were used as a face stimuli in the task.

Figure 2

Ten ICA components utilized for the rs-dFC analysis.

Figure 3

Four identified rs-dFC states (1–4 from the upper left to lower right). Each matrix depicts mutual correlations between each component identified using the ICA. Dark blue suggests a high negative correlation, and dark red suggests a high positive correlation. Note: DMN default-mode network, SMN sensorimotor network, DAN dorsal attentional network.