The invention provides methods and devices to treat obesity by ablating tissue from an exterior surface of a stomach of a patient. For example, the invention may involve ablation of exterior stomach tissue to alter gastric myoelectric activity or muscle function from the exterior surface of the stomach to produce abnormal gastric peristalsis without damaging the mucosal lining of the stomach. The invention may further provide methods and devices to monitor the gastric myoelectric activity before and after the ablation procedure.