Brain excitability and electroencephalographic activation: non-invasive evaluation in healthy humans via transcranial magnetic stimulation

Excitability changes of the central motor tracts as a function of the electroencephalographic (EEG) characteristics has been investigated in 10 healthy volunteers. Transcranial magnetic stimulation (TCS) was administered to the right motor cortex with an intensity 5-10% above threshold for the elicitation of motor evoked potentials (MEPs) in the left forearm muscles. Simultaneously, the right median nerve was stimulated to provoke an H-reflex in the forearm flexors and EEG activity was recorded from the left hemiscalp. Subjects were completely relaxed and were asked at random either to keep the eyes closed while maintaining mental anactivity (A) or to open their eyes and perform mental arithmetics (B). Latencies and amplitudes of MEPs and H-reflexes were statistically matched with the spectral content of the EEG. In condition A, MEPs of 119 ± 61 μV, with up to 36% of missing responses and background EEG activity dominated by rhythms in the alpha range were found. In condition B, MEPs of 219 ± 66 μV (P <0.001), with