During each stimulation, subjects will be instructed to record perception of stimuli using pain scoring scale. This allows the subject to scale perception and pain on a scale from 0 to 10. First perception, pain perception threshold (PPT) and pain tolerance threshold (PTT) will be recorded. All types of esophageal stimulations will be immediately terminated when the pain tolerance threshold is reached. At the time when the pain tolerance threshold is reached (PTT), the subjects will be asked to draw the referred pain area, to identify where the pain is located.