A Local Signature of LTP-Like Plasticity Induced by Repetitive Paired Associative Stimulation

Repetitive paired associative stimulation (rPAS) repeatedly pairs electrical nerve stimulation (ENS) with transcranial magnetic stimulation (TMS) of the contralateral motor hand area (M1) at 5 Hz frequency. So far, there are only few studies concerning the effects of PAS on the modulation of EEG power. Hence, aim of the present study was to investigate rPAS long term after-effects on cortical excitability looking at EEG power spectra. In four experimental sessions, separated by 2 weeks interval, 12 awake subjects received rPAS of the right median nerve and left M1 at a fixed interval (ISI) of 25 ms (real condition), 5 Hz-TMS on left M1, 5 Hz-ENS, of the right median nerve, and rPAS with changing ISI (sham condition). We measured peak-to-peak MEP amplitude, evoked from the target muscle (right abductor pollicis brevis muscle) at rest and the absolute power (POW) in four frequency bands: α (8-12 Hz), β (13-30), θ (4-7) and δ (1-3), under rest conditions. All these parameters were evaluated in three detection blocks: baseline, immediately after and after 30' from the end of the conditioning protocol. Real rPAS induced a long-lasting homotopic cortical excitability modulation, as indexed by MEP amplitude increase, that was paralleled by a long-lasting reduction of α/β-POW and by a widespread θ-δ-POW modulation. rPAS applied over the sensory-motor cortex induced an LTP-like plasticity, as indexed by a robust reduction in the α/β POW positively correlated with the MEP amplitude increase. rPAS25ms may be a useful tool for motor neurorehabilitation promoting a sensory-motor coupling within β oscillations.