Pain and touch differentially modulate corticospinal excitability, independent of afferent inhibition

Pain can profoundly impact motor functioning for self-preservation but is also associated with motor and somatosensory disturbances. Despite considerable research, it remains unclear whether pain and touch modulate motor processes independently or interacting. Across two transcranial magnetic stimulation (TMS) experiments, we tested the effects of tactile and nociceptive inputs on corticospinal and sensorimotor processes using an afferent inhibition (AI) paradigm. In Experiment 1, a single electrocutaneous stimulus was delivered to the left index finger before single pulse-TMS over the right first dorsal interosseous (FDI) motor hotspot at one of five delays (15, 25, 35, 45, 60, or 160 ms). In Experiment 2, the same paradigm examined the effects of pain by administering moderate tonic heat pain to the forearm. In both experiments, significant reductions in TMS-induced motor output occurred at 25, 35, and 160 ms, with facilitation at 60 ms. This was unaffected by tactile afferent duration (Experiment 1) or tonic heat pain (Experiment 2). However, overall, corticospinal excitability was significantly reduced in painful compared to painless conditions. In summary, our findings show that tonic pain has a direct (inhibitory) effect on motor processes; however, in this context, tactile sensorimotor interactions remain unaltered.