Relationship between pain perception and motor cortex excitability. a: Relationship and pathway of pain perception and motor cortex excitability. b: Transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) can increase motor cortical excitability and pain threshold (= > decrease in pain perception). c: Motor tasks and motor observation increase pain threshold and motor excitability. d: Mental imagery decreases pain threshold and motor cortical excitability. Pictures modified from Flor ; Fregni et al. ; and Volz et al. .