Inter-subject Variability in Electric Fields of Motor Cortical tDCS

•TDCS electric fields are modeled in anatomically-accurate models of 24 subjects.•We propose a surface-based inter-subject registration method for variability analysis of electric fields.•The extent and main reasons of inter-subject variation are quantitatively characterized.•Cerebrospinal fluid thickness explains one-half of the inter-subject variability in the electric fields.