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Does brain stimulation's effect depend on what the brain is doing?
Open access · cc by · source: Europe PMC
Brief electrical stimulation of the right frontal cortex changed activity and connectivity in distant brain networks, but the direction of change depended on whether people were doing a task or resting and on the current's polarity.
Study at a glance
- Design
- Human experiment — Within-subject factorial design crossing brain state (choice reaction task vs rest) with brief anodal, cathodal or sham tDCS to the right inferior frontal gyrus, delivered inside the fMRI scanner.
- N
- N=26 · 26 healthy volunteers, each completing three tDCS-fMRI runs; heart rate was monitored in 23 of them.
- Population
- Healthy adult volunteers (13 women, 13 men, mean age 38) with no neurological or psychiatric history, all new to tDCS
- Outcome
- BOLD activity and psychophysiological-interaction functional connectivity of salience and default mode network nodes; choice reaction time and accuracy
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Key findings
Stimulation did not change task performance, but both polarities produced widespread activity changes far from the electrode, generally exaggerating the normal pattern of each state: during the task they boosted task-activated regions such as the dorsal anterior cingulate, while at rest anodal stimulation increased default mode network activity. Polarity effects flipped with state: during the task cathodal stimulation produced more activation of task regions, whereas anodal produced more deactivation of default mode regions. Cathodal stimulation during the task also reduced coupling between the stimulated frontal region and the ventromedial prefrontal cortex while increasing connectivity within the default mode network.
Methodology
Healthy adults lay in an fMRI scanner doing blocks of a simple left/right arrow choice task alternating with rest blocks. During each block they received anodal, cathodal or sham transcranial direct current stimulation (tDCS) over the right inferior frontal gyrus, a hub of the salience network, lasting only seconds per block. The researchers compared brain activity and connectivity across all six state-by-stimulation combinations.
Limitations
Only one target site was stimulated, so the authors cannot say whether these effects are specific to the right inferior frontal gyrus. The design cannot measure after-effects, and carry-over between short blocks cannot be fully excluded even though block order was pseudorandomised. Because behaviour did not change, the study does not show that these network changes matter for cognition. BOLD changes cannot reveal which neuron populations were affected, so the proposed cellular explanations remain speculative.
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