Recording motor-evoked potentials (MEPs) in response to transcranial direct current stimulation (tDCS)?

Question

The amplitudes of motor-evoked potentials (MEPs) are often recorded as a measure of (change in) cortical excitability, after administering some form of non-invasive brain stimulation (NIBS). In a typical scenario, magnetic stimulation (TMS) is used to deliver pulses to the motor cortex, and MEPs are recorded from e.g. the index finger.

It has never been clear to me why MEPs can not also be recorded in response to other forms of NIBS that also modulate cortical excitability, most notably electrical stimulation (tDCS). This paper seems to state this axiomatically, without really giving a reason:

Compared with tDCS, TMS provides a better spatial resolution; when used over the motor cortex, it also allows the quantification of motor-evoked potentials (MEPs), a measure of the excitability of the motor system, which tDCS does not.

Any thoughts as to why it is that MEPs are not also prompted by tDCS?

Answer 1

Compared to TMS which is usually brief and high-intensity, tDCS stimulation protocols are fairly weak and long-duration. This protocol doesn't elicit MEPs by itself, because you need a stronger stimulus.

In some studies (see references below), tDCS has been shown to modulate MEPs, but it is probed by using TMS to elicit the MEP, and comparing the evoked MEPs with and without tDCS.

The mechanistic idea is that tDCS is modulating neuronal thresholds, but you still need some sort of a "kick" to actually evoke a MEP.

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Nitsche, M. A., & Paulus, W. (2000). Excitability changes induced in the human motor cortex by weak transcranial direct current stimulation. The Journal of physiology, 527(3), 633-639.

Nitsche, M. A., & Paulus, W. (2001). Sustained excitability elevations induced by transcranial DC motor cortex stimulation in humans. Neurology, 57(10), 1899-1901.

Nitsche, M. A., Fricke, K., Henschke, U., Schlitterlau, A., Liebetanz, D., Lang, N., ... & Paulus, W. (2003). Pharmacological modulation of cortical excitability shifts induced by transcranial direct current stimulation in humans. The Journal of physiology, 553(1), 293-301.