Abstract
Transcranial electric stimulation aims to stimulate the brain by applying weak electrical currents at the scalp. However, the magnitude and spatial distribution of electric fields in the human brain are unknown. We measured electric potentials intracranially in ten epilepsy patients and estimated electric fields across the entire brain by leveraging calibrated current-flow models. When stimulating at 2 mA, cortical electric fields reach 0.8 V/m, the lower limit of effectiveness in animal studies. When individual whole-head anatomy is considered, the predicted electric field magnitudes correlate with the recorded values in cortical (r = 0.86) and depth (r = 0.88) electrodes. Accurate models require adjustment of tissue conductivity values reported in the literature, but accuracy is not improved when incorporating white matter anisotropy or different skull compartments. This is the first study to validate and calibrate current-flow models with in vivo intracranial recordings in humans, providing a solid foundation to target stimulation and interpret clinical trials.
Highlights
Transcranial electric stimulation (TES) delivers weak electric currents to the scalp with the goal of modulating endogenous brain activity (Ruffini et al, 2013)
A modest drop in magnitude was observed with increasing frequency (25% at 100 Hz; Figure 3B), which we ascribe to a non-uniform gain across frequencies for the measurement equipment and electrodes as confirmed with recording of voltages in saline
Since the magnetic resonance imaging (MRI) of most patients is truncated at the base of the skull, we extended the field of view (FOV) using a standard head that captures the average anatomy of the lower head
Summary
Transcranial electric stimulation (TES) delivers weak electric currents to the scalp with the goal of modulating endogenous brain activity (Ruffini et al, 2013). More recently weak alternating currents (tACS) have been used in an effort to entrain or modulate brain activity (Herrmann et al, 2013; Reato et al, 2013; Ali et al, 2013; Alekseichuk et al, 2016; Lustenberger et al, 2016). Because of their simplicity, flexibility and safety profile, these techniques have been investigated in over 70 neuropsychiatric conditions, including major depression (Bikson et al, 2008), epilepsy (Fregni et al, 2006d; Auvichayapat et al, 2013), tinnitus (Frank et al, 2012), Parkinson’s disease (Fregni et al, 2006b), pain control (Fregni et al, 2006a, 2006c, 2007), and stroke rehabilitation (Schlaug et al, 2008; Baker et al, 2010) among others. In healthy subjects tDCS may benefit declarative memory (Marshall et al, 2004), working memory (Fregni et al, 2005), motor learning (Reis and Fritsch, 2011), verbal fluency (Pereira et al, 2013), and planning ability (Dockery et al, 2009)
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