Abstract
Introduction: This study investigated cognitive-load-dependent effects
of high-definition transcranial direct current stimulation (HD-tDCS) on
the executive vigilance (EV) decrement and its modulation by aperiodic
electroencephalography (EEG) markers. Given the relevance of vigilance for
daily functioning and its susceptibility to decline over time, we examined
whether HD-tDCS could counteract this decrement under varying cognitive
demands.
Methods: In a between-participant design (N = 180), anodal HD-tDCS was
applied over the right posterior parietal cortex (rPPC) during performance
of a single-, dual-, or triple-task, with on-task EEG recorded pre- and
post-stimulation. Power spectra were parametrized to extract aperiodic
(non-oscillatory) components; the aperiodic exponent and offset across two
frequency ranges (1–35 and 30–45 Hz).
Results: HD-tDCS induced a reduction in the aperiodic exponent within the
30–45 Hz range (i.e., flattening of the spectral slope), consistent with increased
cortical excitation. This change was associated with a mitigated EV decrement
under high task demand and an exacerbated decrement under low demand,
suggesting a mechanistic link between changes in excitation/inhibition balance
and behavioral outcomes. However, these effects reached significance only
under a directional hypothesis and seemed to be obscured by a push-pull
relationship with the aperiodic offset, indicating a more complex interaction
between local excitability and broadband spectral dynamics. Baseline aperiodic
markers did not significantly moderate the stimulation effect but predicted
overall task performance, independent of stimulation.
Discussion: These findings suggest a mechanistic understanding of how
endogenous neural activity, specifically, aperiodic EEG features, modulates brain
stimulation outcomes. By demonstrating that HD-tDCS effects vary as a function
of cognitive load and spectral dynamics, the study underscores the need for
future research, centered on