Premovement activity in the corticospinal tract is amplified by the placebo effect: an active inference account

The aim of this study is to investigate whether expectancy, induced through a placebo procedure, favors the activation of the corticospinal tract before movement initiation. By adopting the premovement facilitation paradigm, we applied transcranial magnetic stimulation over the left or right primary motor cortex at rest and 100 ms or 50 ms before movement onset while healthy volunteers performed a reaction time (RT) motor task consisting of abductions of the right or left thumb after a go signal. Participants in the placebo group received an inert electrical device applied on the right forearm along with information on its speed-enhancing properties. A control group received the same device with overt information about its inert nature, while another control group underwent no intervention. Along with RT, we measured the amplitude of the motor evoked potential (MEP) before and after the procedure. Compared to the control groups, the placebo group had faster RT and greater MEP amplitude before movement initiation. This study demonstrates that the placebo effect can boost the activity of the corticospinal tract before movement onset, and this modulation positively impacts motor performance. These results give experimental support to the active inference account.