Want to know if a smile is sincere? The secret is in the eyes. As Guillaume Benjamin Amand Duchenne discovered through his pioneering studies in electrophysiology, smiles that are the result of true happiness occur spontaneously and involve the muscles around the eyes as well as those of the mouth. This type of smile is known today as a “Duchenne smile.” When someone puts on a perfunctory smile, like those found in class photos or on the faces of store greeters, the muscles surrounding the eyes play no part. Because the muscle movement in a Duchenne smile is involuntary, it is very difficult to fake.
Demystifying the smile was just one of Duchenne’s many achievements. A Frenchman descended from a long line of seafarers and fishermen, Duchenne broke family tradition to study medicine in Paris. Following graduation, he began a medical practice in his hometown, Boulogne-sur-Mer. Duchenne developed a keen interest in nerve and muscular disorders. In the mid 1830s, he applied electrical stimulation to a patient suffering from neuralgia. Other scientists had performed similar experiments, but little was gained by the efforts, and the patients often experienced tissue damage. Intrigued by the possibility of using electrification as a form of therapy, Duchenne designed and built his own machine for electrical stimulation. The portable device was the first of its kind to include surface electrodes that enabled the user to localize the stimuli.
By experimenting with his machine, Duchenne realized that he could trigger the movements of specific muscles with electric stimulation by varying the placement of the electrodes. Initially he used this information to develop treatments for individuals with muscular dystrophy and other neuromuscular disorders. Eventually he began using electrical stimulation in diagnostic studies. In 1842, he moved to Paris, began a new practice and searched local hospitals for interesting cases. He accumulated a tremendous amount of information on neuromuscular disorders and was the first to describe several of them in detail.
Around the middle of the 19th century, Duchenne turned his attention toward understanding the muscles of the face and their role in expressing emotions. Using his machine, he applied electric current to the faces of subjects, causing them to contort. He maintained the electric stimulation long enough to take pictures of the expressions that resulted; Duchenne was one of the earliest scientists to use photography for medical research.
Finding people willing to have electricity applied to their faces, especially for the longer periods necessary to take photographs in those days, was not an easy task. Duchenne’s work, therefore, concentrated on a single subject whose affliction with palsy made him almost completely impervious to the pain normally caused by prolonged electrical stimulation. In The Mechanism Of Human Facial Expression, published in 1862, Duchenne described this subject as “an old toothless man, with a thin face, whose features, without being absolutely ugly, approached ordinary triviality, and whose facial expression was in perfect agreement with his inoffensive character and his restricted intelligence.” Using techniques he perfected over time, Duchenne found that he could “... stimulate his individual muscles with as much precision and accuracy as if I were working with a still irritable cadaver.”
Duchenne’s long-running study of facial muscles, as well as his work with neuromuscular disorders, gained him professional prestige, at least in countries outside France. The rulers of England and Spain invited him to visit their courts, a number of foreign universities offered him honorary degrees, and several foreign academies of science included him among their members. Duchenne was never, however, offered membership into the French Academy of Science.
Today Duchenne is often remembered as a pioneer in electrophysiology and the father of electrotherapeutics. His collection of photographs depicting his tests of facial muscles served as a great resource for future studies of emotions and expressions. Charles Darwin referred to and included copies of several of them in The Expression of the Emotions in Man and Animals (published in 1872), which argued that facial expressions are universal among humans and animals.