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The experience of pain influences behaviour and motor function. Indeed, pain-induced reduction of the cortico-motor pathway excitability has been repeatedly demonstrated using motor-evoked potentials (MEP). However, it is unclear how pain specifically impacts the functional state of the motor cortex. Therefore, we explored pain-induced changes at cortical level using TMS-evoked potentials (TEPs) - obtained by combining transcranial magnetic stimulation (TMS) and electroencephalography (EEG) - as a direct measure of motor cortical excitability. In 20 subjects, we recorded TEPs and MEPs evoked by TMS over the primary motor cortex and assessed their modulation by tonic pain elicited by topical application of capsaicin on the contralateral hand. The effect was evaluated in comparison to vehicle using a within-subject design. Both measures were acquired at baseline and every 15 minutes during capsaicin/vehicle application (75 minutes). Capsaicin elicited long-lasting pain accompanied by a suppression of MEP amplitudes, whereas vehicle led to a progressive MEP increase. At cortical level, capsaicin caused an increase of the early TEP component P30, likely reflecting pain-induced changes in motor cortical excitability, while certain later components were reduced, possibly reflecting several parallel processes. The earliest TEP component N15, previously associated with cortico-spinal motor output, increased over time in both conditions, possibly due to cortical disinhibition by repeated stimulation. This is the first study using TEPs to characterize the functional changes at the cortical level during experimentally induced tonic pain. It shows a dissociation between pain-induced changes in TEPs and MEPs, suggesting pain-motor interactions at both cortical and spinal levels.
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Sulcova, D., Courtin, A., Filbrich, L., Lenoir, C., & Mouraux, A. (2022). Changes in motor excitability during capsaicin-induced pain: TMS-EMG+EEG study. Pain Mechanisms and Therapeutics Conference, Verona (Italy). https://hdl.handle.net/2078.5/102677