Abstract
Electric brain stimulations such as transcranial direct current stimulation (tDCS), transcranial random noise stimulation (tRNS), and transcranial alternating current stimulation (tACS) electrophysiologically modulate brain activity and as a result sometimes modulate behavioral performances. These stimulations can be viewed from an engineering standpoint as involving an artificial electric source (DC, noise, or AC) attached to an impedance branch of a distributed parameter circuit. The distributed parameter circuit is an approximation of the brain and includes electric sources (neurons) and impedances (volume conductors). Such a brain model is linear, as is often the case with the electroencephalogram (EEG) forward model. Thus, the above-mentioned current stimulations change the current distribution in the brain depending on the locations of the electric sources in the brain. Now, if the attached artificial electric source were to be replaced with a resistor, or even a negative resistor, the resistor would also change the current distribution in the brain. In light of the superposition theorem, which holds for any linear electric circuit, attaching an electric source is different from attaching a resistor; the resistor affects each active electric source in the brain so as to increase (or decrease in some cases of a negative resistor) the current flowing out from each source. From an electrophysiological standpoint, the attached resistor can only control the extracellular impedance and never causes forced stimulation; we call this technique transcranial extracellular impedance control (tEIC). We conducted a behavioral experiment to evaluate tEIC and found evidence that it had real-time enhancement and depression effects on EEGs and a real-time facilitation effect on reaction times. Thus, tEIC could be another technique to modulate behavioral performance.
Highlights
Electroencephalogram (EEG) recording is a real-time observation of electrophysiological brain activities
The large/small relations indicated that Type I (Type II) had higher sensitivity to nearby EEG generators and lower sensitivity to distant EEG generators in comparison with the Sham condition
The EEG magnitude of Type I (Type II) for any channel was almost always smaller than that of the Sham condition, because the Type I (Type II) resistor attached to an EEG channel and the reference decreased the impedance between any EEG channel and the reference
Summary
Electroencephalogram (EEG) recording is a real-time observation of electrophysiological brain activities. EEG generators in the neocortex are the sums of excitatory and inhibitory postsynaptic potentials of apical dendrites of pyramidal neurons [1,2,3,4]. These postsynaptic potentials generate intracellular and extracellular currents (these two currents must electrically continue, i.e., they must be equal at the cell boundary) and a portion of the extracellular currents cause voltage drops on the scalp that are recorded as EEGs. While the dynamics of EEG generators and their networks are nonlinear, the relationship between the generators and volume conduction is linear [4]. Since the typical impedance between two EEG electrodes with an appropriate scalp pretreatment is several kV, the amplifiers used for EEG measurements usually provide a high impedance, say 1 MV or higher, and do not generate any additional current paths
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