Spatiotemporal structure of intracranial electric fields induced by transcranial electric stimulation in humans and nonhuman primates

Abstract

Transcranial electric stimulation (TES) is an emerging technique, developed to non-invasively modulate brain function. However, the spatiotemporal distribution of the intracranial electric fields induced by TES remains poorly understood. In particular, it is unclear how much current actually reaches the brain, and how it distributes across the brain. Lack of this basic information precludes a firm mechanistic understanding of TES effects. In this study we directly measure the spatial and temporal characteristics of the electric field generated by TES using stereotactic EEG (s-EEG) electrode arrays implanted in cebus monkeys and surgical epilepsy patients. We found a small frequency dependent decrease (10%) in magnitudes of TES induced potentials and negligible phase shifts over space. Electric field strengths were strongest in superficial brain regions with maximum values of about 0.5 mV/mm. Our results provide crucial information of the underlying biophysics in TES applications in humans and the optimization and design of TES stimulation protocols. In addition, our findings have broad implications concerning electric field propagation in non-invasive recording techniques such as EEG/MEG.

Limitations of ex vivo measurements for in vivo neuroscience

“…Details of implantation and experimental procedures can be found in ref. 10. One female Cebus monkey (11 y, 2.9 kg) was implanted with a MRI-compatible headpost (Cilux) and four linear array depth electrodes (Adtech), inserted through a small craniotomy over the left occipital cortex; this was subsequently sealed with nonconducting bone cement.…”

Measurements and models of electric fields in the in vivo human brain during transcranial electric stimulation

“…A recent study has reported the electric field produced by TES as measured by stereotactic EEG electrodes in two epilepsy surgical patients (Opitz et al, 2016). While the study does not leverage or evaluate accuracy of computational models, it does estimate maximal electric field to be 0.5 V/m with 1 mA TES.…”

“…Bangera et al (2010) performed in vivo intracranial recordings from human, but the data were used to better predict EEG surface recordings, not for calibrating models of electric field with transcranial current stimulation. The most recent studies (Opitz et al, 2016;Koessler et al, 2016) involve similar in vivo intracranial recordings on human subjects and monkeys, but they do not provide a comparison and validation of current-flow models.…”

“…We confirmed this in our models, which show only minimal differences in field estimates when modeling the craniotomy as highly conducting CSF (in practice it is largely fluid-filled) versus entirely resistive (air filled). However, when electrodes are directly placed over the skull opening, as in Opitz et al (2016), one expects larger electric field intensities than normal as current flows directly into the skull. We thus feel that our estimate is closer to what one may expect in normal skull anatomy.…”

“…We thus feel that our estimate is closer to what one may expect in normal skull anatomy. Opitz et al (2016) also measured voltages for different stimulation frequencies and found a 10% drop in voltage recording between 1-100 Hz. In our equipment we find a 25% drop which we attribute to a non-uniform gain of the equipment (in saline we measured a gain difference of 25% between 1 Hz and 100 Hz).…”