Excitability changes induced in the human motor cortex by weak transcranial direct current stimulation.

TLDR: Transcranial electrical stimulation using weak current may be a promising tool to modulate cerebral excitability in a non‐invasive, painless, reversible, selective and focal way.

Abstract: The approach taken in this study to produce localised changes of cerebral excitability in the intact human was modulation of neuronal excitability by weak electric currents applied transcranially. So far, this technique has mainly been used in animal research, primarily through modulation of the resting membrane potential (Terzuolo & Bullock, 1956; Creutzfeld et al. 1962; Eccles et al. 1962; Bindman et al. 1964; Purpura & McMurtry, 1965; Artola et al. 1990; Malenka & Nicoll, 1999). In general, cerebral excitability was diminished by cathodal stimulation, which hyperpolarises neurones. Anodal stimulation caused neuronal depolarisation, leading to an increase in excitability (Bindman et al. 1962; Purpura & McMurtry, 1965), as was shown by spontaneous neuronal discharges and the amplitudes of evoked potentials (Landau et al. 1964; Purpura & McMurtry, 1965; Gorman, 1966). However, in single cortical layers opposite effects were seen (Purpura & McMurtry, 1965), underlining the fact that the effects of DC stimulation depend on the interaction of electric flow direction and neuronal geometry. Enduring effects of 5 h and longer have been described if the stimulation itself lasts sufficiently long, about 10–30 min. These prolonged effects are not simply due to prolonged membrane potential shifts or recurrent excitation, because intermittent complete cancellation of electrical brain activity by hypothermia does not abolish them (Gartside, 1968a,b). Long-term potentiation (LTP) and long-term depression (LTD) have been proposed as the likely candidates for this phenomenon (Hattori et al. 1990; Moriwaki, 1991; Islam et al. 1995; Malenka & Nicoll, 1999). The concept described here was an attempt to induce neuronal excitability changes in man by application of weak DC stimulation through the intact skull. It has already been demonstrated within invasive presurgical epilepsy diagnostics that intracranial currents of sufficient strength can be achieved in humans by stimulation with surface electrodes at intensities of up to 1.5 mA (Dymond et al. 1975). A suitable candidate for evaluating cortical excitability changes is transcranial magnetic stimulation (TMS), because it allows the quantification of motor-cortical neurone responses in a painless and non-invasive manner. The amplitude of the resulting motor-evoked potential (MEP) represents the excitability of the motor system. In the following, we confirm the principal possibility of altering cortical excitability by applying weak DC. Furthermore we show that systematic DC stimulation with minimum stimulation duration and intensity is necessary for an effective application of weak current in humans. This is of particular importance for inducing effects which outlast the duration of stimulation.