Electric vs magnetic trans-cranial stimulation of the brain in healthy humans: a comparative study of central motor tracts 'conductivity' and 'excitability'

Motor evoked potentials (MEPs) were elicited in the thenar muscles of 11 healthy volunteers via individual electric unifocal and magnetic trans-cranial stimuli (TCS). The effects of TCS strength, of the muscular state (relaxed, contracted) as well as of the amplitude-latency characteristics and the duration of the motor tracts central conduction times (CCTs) to hand muscles, were evaluated and compared between the two types of brain excitation. MEPs with the shortest latency (18.91 +/- 1.31 ms) were recorded in the voluntarily contracted muscle during electric TCS, whilst those with maximal latency (23.3 +/- 1.63 ms) were found after magnetic TCS with an intensity at threshold for eliciting an MEP of about 0.1 mV in the relaxed muscle. Mean CCTs for electric and magnetic TCS calculated in the contracted target muscles, were respectively 5.07 +/- 0.51 and 6.34 +/- 0.46 ms. MEPs with larger amplitudes and durations were observed during magnetic TCS, being maximal when suprathreshold stimuli were delivered. A restricted range of liminar values of magnetic TCS was obtained by defining the threshold for raising motor responses in complete muscle relaxation, indicating that magnetic pulses might represent a useful probe for testing the 'excitability' of the motor tracts.