Exploration of cortical effect of PAS

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