 Participants will be in this study for about 25 months. An initial screening period that may last up to 6 weeks will include a medical history review, various examinations, and the use of a wrist-mounted device called an Actiwatch to record pain severity and mood over seven consecutive days. Eligible participants will then undergo surgical implantation of the VNS system. This will be followed by a 2-week recovery period, then a 2-week period of gradually increasing the electrical output of the VNS system, and finally a 12-week VNS treatment period during which the electrical output level will remain constant, unless a participant develops intolerable side effects. Study visits will occur weekly for the first month after implantation and then every 2 weeks during the treatment period. During most study visits, participants will undergo a clinical assessment, including evaluation of any side effects, and they will complete written assessments on pain, mood, fatigue, and quality of life and sleep. The VNS system will also be checked for safety and programmed appropriately. During the Week 16 visit, some of the screening evaluations will be repeated. Also at Week 16, which will mark the end of the treatment period, participants will have the option to continue receiving treatment. All participants will have follow-up evaluations at Months 6, 9, 12, and 24. Participants who wish to retain the VNS implantation beyond the length of the study will be referred to a qualified neurologist for follow-up care. For the other participants, the VNS system will be turned off and surgically explanted. 