Effects of posture on electric fields of non-invasive brain stimulation.

The brain moves when the orientation of the head changes. This inter-postural motion has been shown to affect the distribution of cerebrospinal fluid (CSF). As CSF layer thickness affects the distribution of electric fields (EF) in non-invasive brain stimulation methods such as transcranial direct current (TDCS) and magnetic (TMS) stimulation, possible differences in body position between sessions could affect the stimulation efficacy. Additionally, inter-postural differences might distort the modeling results of TDCS and TMS, as the models are usually built based on magnetic resonance images (MRI) obtained while the subject is in the supine position, whereas the actual stimulation is given while the subject is in an upright position. Here, we studied the effects of changing the position of the subject between supine, prone, and left lateral on the conformation of the brain. This study aimed to determine whether small inter-postural changes in the shape of the brain can affect TDCS and TMS EFs as hypothesized. We obtained MRI from five subjects in each position and used them to build anatomically realistic models for use in finite element simulations of the EFs. Position was found to affect EFs, with them being approximately 10% stronger and more diffuse while subjects were in the prone and left lateral than in the supine positions for TDCS. In TMS, a similar trend was observed, but the effect was smaller, approximately 2%, than that observed for TDCS. Thus, the effect of posture should be considered in the design of TDCS and TMS experiments.