Context: Transcranial Magnetic stimulation (TMS) is a non-invasive stimulation technique of the brain,
which allows the investigation of the neuronal plasticity and cortical circuits in humans. This method is
based on the generation of an impulse by using an electromagnetic induction (1). Paired associative
stimulation (PAS) is a non-invasive technique used to induce and to study the plasticity in the human
motor cortex. Essentially, it is the repeated pairing of a peripheral stimulation with a cortical stimulation
(2). PAS induces an LTP-like effect with an inter-stimulus interval of 25ms. The latencies of afferent
signals from the median nerve stimulation at the wrist to reach the motor cortex (M1) after 20-25ms in
order to produce synchronous pre- and postsynaptic activation (3). This change of excitability in the
motor cortex induced by PAS is quantified by recording motor evoked potentials (MEP) before and after
the intervention. Another way to study alterations in the motor cortex with TMS is to record the long
interval intra-cortical inhibition (LICI) before and after PAS. It is very important to know that the PAS
response is characterized by a high inter/intra-subject variability (4). This variability could be partially
explained by the fact that every subject has a different level of excitability before the PAS. So, a reason
to investigate the effect of PAS on MEP and LICI is to understand if PAS could be a method to set every
subject at the same level of excitability before studying his plasticity in order to have more comparable
results.
Objectives: This study is part of a larger project that investigates potential interactions between different
techniques exploring focal dystonia in patients with the Complex Regional Pain Syndrome (CRPS).
This study explores and compares two protocols of PAS, the conventional low frequency PAS (0.2 Hz)
with a high frequency (5Hz) PAS.
Method: In our study, we will investigate the plasticity of the motor cortex and inhibition in 15 healthy
subjects. The PAS protocol is a 2 stimuli sequence separated by a specific interval (25ms). We will
deliver an electric stimulus on the median nerve followed by a TMS pulse at the level of the motor
cortex with an 8-shaped magnetic coil. we record the MEP from the abductor digiti minimi muscle
(ADM), Dorsal interossei of the hand (FDI). The paired-pulse TMS paradigm (ppTMS) resides in the
combination of a subthreshold conditioning stimulus with a suprathreshold test stimulus at different
intervals. LICI will be recorded by ppTMS using 100ms as interval with two suprathreshold stimulus.
Conclusions: Both 5Hz and 0.2Hz PAS protocols have comparable effects on motor cortex, about ¾ of
the subjects show lpong-term potentiation. There were no side effects, but some subjects found 5Hz
unpleasant.