Electric Fields Induced in the Brain by Transcranial Electric Stimulation: A Review of In Vivo Recordings

Abstract

Transcranial electrical stimulation (tES) techniques, such as direct current stimulation (tDCS) and transcranial alternating current stimulation (tACS), cause neurophysiological and behavioral modifications as responses to the electric field are induced in the brain. Estimations of such electric fields are based mainly on computational studies, and in vivo measurements have been used to expand the current knowledge. Here, we review the current tDCS- and tACS-induced electric fields estimations as they are recorded in humans and non-human primates using intracerebral electrodes. Direct currents and alternating currents were applied with heterogeneous protocols, and the recording procedures were characterized by a tentative methodology. However, for the clinical stimulation protocols, an injected current seems to reach the brain, even at deep structures. The stimulation parameters (e.g., intensity, frequency and phase), the electrodes' positions and personal anatomy determine whether the intensities might be high enough to affect both neuronal and non-neuronal cell activity, also deep brain structures.

Keywords: electric fields; intracranial recordings; neuromodulation; transcranial alternating current stimulation; transcranial direct current stimulation; transcranial electric stimulation.

Figure 1

Coronal section of the brain showing the deep brain structures that were targeted for recording. Ruhnau et al., 2018 [36]; Chhatbar et al., 2018 [34]; Esmaeilpour et al., 2017 [37].

Figure 2

Sagittal section (left) and cerebral cortex (right) of the right hemisphere showing regions that were targeted for recordings. Datta et al., 2016 [14]; Opitz et al., 2016 [12]; Huang et al., 2017 [10]; Lafon et al., 2017 [32]; Krause et al., 2019 [29]; Louviot et al., 2022 [38]; Esmaeilpour et al., 2017 [37]; Salimpour et al., 2017 [26]; Johnson et al., 2019 [30].

Figure 3

Sagittal section (left) and cerebral cortex (right) of the left hemisphere showing regions that were targeted for recording. Opitz et al., 2016 [12]; Kar et al., 2017 [28]; Krause et al., 2019 [29]; Datta et al., 2016 [14]; Huang et al., 2017 [10]; Louviot et al., 2022 [38]; Lafon et al., 2017 [32]; Alekseichuk et al., 2019 [39].

Figure 4

Representation (“+” is anode; “−“ is cathode) of the presumed in vivo effects of transcranial electrical stimulation on non-neural components of the brain at intensities applied in human protocols.