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J Neurophysiol (June 7, 2006). doi:10.1152/jn.00360.2006
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Submitted on April 6, 2006
Accepted on May 22, 2006

Origin of Facilitation of Motor Evoked Potentials after Paired Magnetic Stimulation: Direct Recording of Epidural Activity in Conscious Humans

Vincenzo Di Lazzaro1*, Fabio Pilato1, Antonio Oliviero1, Michele Dileone1, Eleonoara Saturno1, Paolo Mazzone, Angelo Insola, Paolo Profice1, Federico Ranieri1, Fioravante Capone, Pietro A Tonali, and John C. Rothwell2

1 Institute of Neurology, Universita Cattolica, Rome, Italy
2 Sobell Department Neurophysiology, Institute of Neurology, London, United Kingdom

* To whom correspondence should be addressed. E-mail: vdilazzaro{at}rm.unicatt.it.

A magnetic transcranial conditioning stimulus given over the motor cortex at intensities below active threshold for obtaining Motor Evoked Potentials (MEPs) facilitates EMG responses evoked at rest in hand muscles by a suprathreshold magnetic stimulus given 10 to 25ms later. This is known as intracortical facilitation (ICF). We recorded descending volleys produced by single and paired magnetic motor cortex stimulation through high cervical epidural electrodes implanted for pain relief in six conscious patients. At interstimulus intervals (ISIs) of 10 and 15ms, although MEP was facilitated there was no change in the amplitude or number of descending volleys. An additional I wave sometimes was observed at 25ms ISI. In one subject we also evaluated the effects of reversing the direction of the induced current in the brain. At 10ms ISI, the facilitation of the MEPs disappeared and was replaced by slight suppression, at 2ms ISI there was a pronounced facilitation of epidural volleys. Subsequent experiments on healthy subjects showed that a conditioning stimulus capable of producing ICF of MEPs had no effect on the EMG response evoked by transmastoidal electrical stimulation of corticospinal tract. We conclude that ICF occurs because either (1) the conditioning stimulus has a (so far undetected) effect on spinal cord excitability that increases its response to the same amplitude test volley or (2) that it can alter the composition (but not the amplitude) of the descending volleys set up by the test stimulus such that a larger proportion of the activity is destined for the target muscle.




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