P097 Direct measurement of electric fields in human and monkeys during transcranial electric stimulation

Abstract

Introduction Transcranial electric stimulation (TES) is an emerging technique to non-invasively modulate brain function. However, the spatiotemporal distribution of electric fields during TES remains poorly understood. Objectives In this study we perform direct intracranial measurements of the electric field generated by transcranial alternating current (tACS) in epilepsy patients and cebus monkeys and evaluate the capacity of finite element method (FEM) models to predict the spatial distribution of measured electric fields. Methods Two presurgical epilepsy patients, with ca. 100 intracranially implanted electrodes participated in a single TES session. Two sponge electrodes (25 cm2) were attached over the left and right temporal cortex and a current of 1 mA with a frequency of 1 Hz was applied for 2 min. In two cebus monkeys three electrodes, with a total of 32 contacts were permanently implanted with posterior-anterior orientation. In multiple sessions intracranial EEG was recorded during TES. We varied the frequency of stimulation from 1–150 Hz and computed amplitude and phase relationships of recorded voltages. We constructed FEM models with increasing anatomical complexity for one epilepsy patient and compared the measured and simulated electric fields. Results Voltage magnitude slightly decreased with stimulation frequency up to 10% and small phase differences between electrode contacts up to a few degrees were observed. Electric field strengths were strongest in superficial brain regions with maximum values of 0.5 mV/mm ( Download : Download high-res image (1MB) Download : Download full-size image Fig. 1). Comparison of measured and simulated potentials and electric fields showed very high correlation values for the potentials (r = 0.95) and correlations of r = 0.7 for the electric fields. Evaluating the predictive value of increasingly complex FEM models highlighted the importance of accurate skull modeling especially in the vicinity of skull defects ( Download : Download high-res image (746KB) Download : Download full-size image Fig. 2). Conclusion We conducted a comprehensive evaluation of intracranial electric field during TES in both human patients and monkeys. Our results indicate that TES currents spread in a linear ohmic manner and capacitive effects are small indicating that the quasi-static approximation is well justified in the low frequency range. The spatial variation of the electric fields can be captured using realistic FEM models.