The assessment of somatosensory cortex plasticity during sleep deprivation by paired associative stimulation

Many animal studies suggest that during sleep deprivation (SD) synaptic strength should progressively increase, leading to the saturation of the ability to induce long-term potentiation (LTP). Nevertheless, direct evidences about the effects of sustained wakefulness on cortical plasticity in humans are still lacking. The aim of the present study was to assess changes in the ability to induce LTP-like mechanism in humans during a period of SD by means of a paired associative stimulation (PAS) protocol, which combines median nerve stimulation with transcranial magnetic stimulation (TMS) applied over the contralateral somatosensory cortex. During a 41-h SD protocol, 16 healthy subjects, defined as responders to the PAS protocol after a pre-selection session, were involved in 4 experimental sessions (11.00 a.m. and 11.00 p.m. of first and second day) with: a) pre-PAS somatosensory evoked potentials (SEPs) recordings; b) PAS protocol; c) post-PAS SEPs recordings. The effect of PAS on SEPs early components (N20-P25 complex) was assessed. During the first experimental session (without SD) no significant PAS effects on SEPs components amplitude have been found, and large intra- and inter-individual variability have been observed. A lack of significant changes has been observed also in the subsequent sessions. Our results index a low intra- and inter-individual reliability of the PAS protocol, suggesting particular caution when longitudinally evaluating the effect of this technique on cortical plasticity.