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?