Can Transcranial Electrical Stimulation Localize Brain Function?

Abstract

Transcranial electrical stimulation (TES) uses constant (TDCS) or alternating currents (TACS) to modulate brain activity. Most TES studies apply low-intensity currents through scalp electrodes (≤2 mA) using bipolar electrode arrangements, producing weak electrical fields in the brain (<1 V/m). Low-intensity TES has been employed in humans to induce changes in task performance during or after stimulation. In analogy to focal transcranial magnetic stimulation, TES-induced behavioral effects have often been taken as evidence for a causal involvement of the brain region underlying one of the two stimulation electrodes, often referred to as the active electrode. Here, we critically review the utility of bipolar low-intensity TES to localize human brain function. We summarize physiological substrates that constitute peripheral targets for TES and may mediate subliminal or overtly perceived peripheral stimulation during TES. We argue that peripheral co-stimulation may contribute to the behavioral effects of TES and should be controlled for by “sham” TES. We discuss biophysical properties of TES, which need to be considered, if one wishes to make realistic assumptions about which brain regions were preferentially targeted by TES. Using results from electric field calculations, we evaluate the validity of different strategies that have been used for selective spatial targeting. Finally, we comment on the challenge of adjusting the dose of TES considering dose–response relationships between the weak tissue currents and the physiological effects in targeted cortical areas. These considerations call for caution when attributing behavioral effects during or after low-intensity TES studies to a specific brain region and may facilitate the selection of best practices for future TES studies.

Transcranial alternating current stimulation (tACS): from basic mechanisms towards first applications in psychiatry

“…Similarly, cranial electrotherapy stimulation, a close alternating current stimulation tool applied on the forehead and mastoids, induces electrical brain changes via direct stimulation of cranial nerves [ 223 , 224 ]. Although this hypothesis of the retina and peripheral somatosensory stimulation cannot be fully excluded, evidence supports the direct causality of tACS to entrain brain oscillations [ 20 , 222 , 225 ]. Consistent with this conclusion, tACS revealed frequency-, phase- and montage- and state-specific effects [ 67 , 198 , 222 , 226 ].…”

“…Although this hypothesis of the retina and peripheral somatosensory stimulation cannot be fully excluded, evidence supports the direct causality of tACS to entrain brain oscillations [ 20 , 222 , 225 ]. Consistent with this conclusion, tACS revealed frequency-, phase- and montage- and state-specific effects [ 67 , 198 , 222 , 226 ]. Further research could try to estimate the contribution of these indirect mechanisms to the whole tACS using modelling techniques, as well as active control groups [ 222 ].…”

“…A second major drawback of tACS is the indirect mechanisms of action rather than direct entrainment of endogenous brain oscillations. Stimulation of peripheral nerves and the retina could account for the entrainment of brain oscillations [213,[218][219][220][221][222]. Similarly, cranial electrotherapy stimulation, a close alternating current stimulation tool applied on the forehead and mastoids, induces electrical brain changes via direct stimulation of cranial nerves [223,224].…”

Precision non-implantable neuromodulation therapies: a perspective for the depressed brain

“…The neurobiological effects of NIBS can also be assessed during or after application sessions, [115][116][117] in what have been termed ''online'' and ''offline'' approaches, respectively. 115 The ''online'' approach allows use of imaging techniques to quantify local neural network properties and application of NIBS so as to interfere with ongoing neuronal processing, visualizing how NIBS modulates the level or timing of neuronal activity with imaging and electrophysiology.…”

“…The neurobiological effects of NIBS can also be assessed during or after application sessions, 115 - 117 in what have been termed “online” and “offline” approaches, respectively. 115 …”