Thermal stimulation will be performed by re-circulating a saline solution (NaCl 0.09%), heated by a water bath, through the polyurethane balloon mounted on the probe. Stimulation temperature will be steadily increased by increasing the flow rate from the water bath to the balloon. Flow rate will be controlled by a computer operated pump. The volume in the balloon will be kept constant at 5mL to avoid mechanical stimulation of the esophagus. A temperature sensor present in the balloon will continuously monitor the stimulation temperature, which will be displayed on a computer display throughout the study. Maximum stimulation temperature is set at 62°C. Thermal stimulation will be terminated when the subject reaches PTT.