Quantifying the Impacts of Transcranial Electrical Stimulation on Cortical Activity in Human Visual Cortex

Abstract

Transcranial electrical stimulation (tES) has become a popular interventional method of stimulating human brains noninvasively. Despite reports of modulation of membrane potentials or BOLD responses by tES, it is far from conclusive whether and how tES affects neural activity. One prominent factor contributing to this inconclusion is that the baseline variability of noises intrinsic to measurements, which occur with diverse origins not just between but also within experimental sessions, have not been properly handled in previous studies. For example, the intrinsic variability of hemodynamic responses within and between scans causally confounds tES and thus complicates the attribution of observed effects in BOLD. To overcome this problem, we developed an experimental protocol that allows for statistically dissecting tES effects and other intrinsic noises in BOLD activity. By applying this protocol to human visual cortex, we demonstrate that tES induces substantial changes not only in the temporal dynamics of hemodynamic response function (HRF) but also in cortical population responses to dynamic stimuli, which cannot be reduced to the changes in HRF. Our findings imply that tES, when applied in protocols with statistical rigor and power, can manifest its impacts on BOLD signals in much more complicated and nuanced ways than previously reported.