The esophagus was studied clinically to elucidate the subjective experience called heartburn. Apparatus was developed for recording acidity within the esophagus simultaneously with intraesophageal and intragastric pressure and with signals representing the patient's sensations. The evidence showed that heartburn is distinct from the sensation that accompanies distension of the esophagus. Heartburn occurred when acidity was high in the esophagus; and it disappeared when the acidity decreased.
Tuttle SG, Bettarello A, Grossman MI. Gastroesophageal Regurgitation. JAMA. 1961;176(6):498-500. doi:10.1001/jama.1961.03040190020006