Transcranial magnetic and electrical stimulation compared: does TES activate intracortical neuronal circuits?
OBJECTIVE
To determine whether, and under which conditions, transcranial electrical stimulation (TES) and transcranial magnetic stimulation (TMS) can activate similar neuronal structures of the human motor cortex, as indicated by electromyographic recordings.


METHODS
Focal TMS was performed on three subjects inducing a postero-anterior directed current (p-a), TES with postero-anteriorly (p-a) and latero-medially (l-m) oriented electrodes. We analyzed the onset latencies and amplitudes (single-pulse) and intracortical inhibition and excitation (paired-pulse).


RESULTS
TMS p-a and TES p-a produced muscle responses with the same onset latency, while TES l-m led to 1.4-1.9 ms shorter latencies. Paired-pulse TMS p-a and TES p-a induced inhibition at short inter-stimulus intervals (ISI) (maximum: 2-3 ms) and facilitation at longer ISIs (maximum: 10 ms). No inhibition but a strong facilitation was obtained from paired-pulse TES l-m (ISIs 1-5 ms).


CONCLUSIONS
Our findings support the hypothesis, that current direction is the most relevant factor in determining the mode of activation for both TMS and TES: TMS p-a and TES p-a are likely to activate the corticospinal neurons indirectly. In contrast, TES l-m may preferentially activate the corticospinal fibres directly, distant of the neuronal body.


CONCLUSIONS
TES is a suitable tool to induce intracortical inhibition and excitation.
Brocke
J
J
Department of Neurology, Berlin NeuroImaging Center, Charité, Germany.
Irlbacher
K
K
Hauptmann
B
B
Voss
M
M
Brandt
S A
SA
D000328:Adult; D004558:Electric Stimulation; D004576:Electromyography; D006801:Humans; D008297:Male; D009044:Motor Cortex / Q000502:physiology*; D009433:Neural Inhibition / Q000502:physiology; D009434:Neural Pathways / Q000502:physiology; D011930:Reaction Time / Q000502:physiology; D050781:Transcranial Magnetic Stimulation