P098 Cortical layer-specific distribution of TES electric fields

Abstract

Introduction Cellular targets of transcranial electric stimulation (TES) are not well understood. Due to a number of factors including size, packing, myelination and orientation of predominant cell types and related variations in conductivity across layers, some cortical layers may be more susceptible to stimulation than others. Current biophysical models of TES do not account for laminar specific differences in the electric field distribution. Objectives In this study we systematically mapped the electric field distribution during TES across cortical layers as a function of electrode montage, stimulation frequency and cortical depth. Methods Using laminar multielectrode recordings (100 μm spacing, 24 contacts, implanted in V1) in an anesthetized cebus monkey we recorded TES induced potentials for two montages: 1. Anterior-Posterior current direction, electrodes placed over V1 and forehead. 2. Left–Right current direction, electrodes placed over bilateral temples. Stimulation intensity was 100 μA applied through round (3.14 cm2) Ag/AgCl electrodes. Measurements were performed using alternating currents with frequencies from 1–150 Hz for different depths (from dura through GM and WM). Electric fields were computed as the first spatial derivative of recorded potentials. Results We found a marked influence of the laminar structure on the electric field distribution ( Download : Download high-res image (624KB) Download : Download full-size image Fig. 1A) with locally increased field strengths (Layer IV/V). This effect was dependent on current direction (Fig. 1B). The amplitude and phase of recorded potentials varied in a frequency and spatially dependent manner ( Download : Download high-res image (444KB) Download : Download full-size image Fig. 2). Conclusion Our results highlight differential effects of electric field propagation across cortical layers. Locally enhanced electric fields are likely due to currents passing across conductivity mismatches between layers. This effect is dependent on current orientation. Magnitude and phase of recorded potentials varied across the depth of cortex demonstrating changes in local electric properties of brain tissue. Larger phase shifts in deeper WM regions could be related to stronger capacitive influences from myelinated axons. In summary our results show previously unreported laminar specific effects of TES electric fields making a first step in the identification of cell specific targets for TES.