Bioimpedance spectroscopy method for investigating changes to intracranial dose during transcranial direct current stimulation

Tissue resistance changes upon application of DC current. We posit that in a similar fashion, that scalp and skull resistances during trancranial direct current stimulation (tDCS) are variable, resulting in changes to intracranial dose. Transcranial magnetic stimulation (TMS), electoencephelogram (EEG), functional magnetic resonance imaging (fMRI), proton magnetic resonance spectroscopy (¹H MRS) and functional near infrared spectroscopy (fNIRS) are technologies used to measure individual neural response to tDCS. These technologies are complex and may not be directly correlated to intracranial dose. We therefore present a bioimpedance spectroscopy method of measuring changes to the intracranial dose in vivo. Scalp resistance changes are measured during tDCS. Current flow through the scalp is calculated as the ratio of voltage measured on the scalp and scalp resistance. Variation of intracranial current is indirectly calculated from changes in the current shunted through the scalp. We thus demonstrate a novel methodology of on-line monitoring of scalp resistance and current as an objective feedback of estimated individual tDCS dose.