High-definition tDCS of the motor cortex affects prefrontal and primary motor activity differently in young and older...

The ability to suppress irrelevant or distracting inputs that interfere with goal-driven behavior is known to decline with increasing age. Although noninvasive stimulation of the motor cortices has been shown to modulate age-related changes in motor activity, the findings remain preliminary, and it is unknown whether this effect extends beyond the motor cortex. In this study, 125 healthy adults, categorized into young (20-35 years) and older groups (55-72 years) underwent three visits (i.e., anodal, cathodal, and sham). During each visit, they received 20 min of high-definition transcranial direct current stimulation (HD-tDCS) applied to their left primary motor cortex (M1) and completed a flanker task during high-density magnetoencephalography (MEG). Statistically significant oscillatory responses were imaged and analyzed using voxel-wise, whole-brain, and point of stimulation approaches. Our results showed increased gamma flanker interference effects within the contralateral M1 in older relative to younger adults following anodal stimulation, and after anodal compared to cathodal HD-tDCS in older adults. We also found polarity-based differences in beta and gamma M1-prefrontal connectivity as a function of age group. Critically, these data indicate distinct spectrally- and polarity-dependent effects of M1 HD-tDCS on the local and network-level neurophysiological responses serving motor performance in young versus older healthy adults.