Chemical stimulation will be performed in distal esophagus by infusing an acidic solution (HCl 0.1N) in the esophagus. This is an adaptation of the Bernstein test, used in routine clinical practice since the early sixties. Infusion rate is controlled by a peristaltic infusion pump with a flow rate of 2ml/min. The stimulation will last for a maximum of 30 minutes or will be terminated when subjects reach pain tolerance threshold (PTT).