Figure 7: Étude du méchanisme et de l'expression du muscle frontal chez un vieillard: à droit, attention; à gauche, repos

Duchenne embarked on the first scientific, systematic exploration of the physiology of human facial expression and hoped, through photographs of his experiments, to teach artists how to portray those emotions. The experiments were performed by applying electrical stimulation to a single muscle on one side of the face, so that the viewer could clearly see the impact of the muscular contraction.