Non-invasive technologies that read or that stimulate the brain, such as EEG, TMS, or tDCS, frequently make assumptions about the anatomical structure of the brain. This is necessary as, for most participants, the structure of the brain is not visible. Two common examples of this sort of assumption would be the use of a standard brain model in computing the source of dipole components in EEG analysis (e.g., Marin, Guerin, Baillet, Garnero, & Meunier, 1998), or the projection of a tDCS-derived computed electric field onto a standard brain model (e.g., Bikson, Rahman, & Datta, 2012). However, to what extent can we rely on a standard brain image in understanding effects in an individual person? Are there particular regions of the brain where a standardized analysis is inadvisable? The use of transcranial magnetic stimulation (TMS) is a good example of a technology that depends strongly on understanding the structure of the brain. In TMS, a brief electric current through a coil of wire induces a rapidly varying