Physiological sensing, stimulation, and protocol. The electrocardiogram and respiratory effort signals were measured at the locations shown on the left. Transcutaneous cervical vagus nerve stimulation (tcVNS) or sham stimulation was administered targeting the right carotid artery. The diagram on the right illustrates the protocol’s chronology. Protocol conditions included neutral videos, stimulation, and opioid cue audio and videos. The protocol lasted approximately two hours. Interstimulus intervals, illustrated using empty space between conditions, lasted approximately five minutes.