Finzi began musing on the facial feedback hypothesis, which holds that physical expression is an integral component of emotion. You can feel sad or angry without moving your face, but, the theory holds, those emotions will not be as strong or persist for as long if your face is not moving in the expected manner. Put another way, relative to emotion, the face and its muscles are a privileged area of the body. Whatever you do with your face transmits feelings back to your brain.
In 2003, Finzi treated several subjects suffering from moderate to severe depression with Botox, paralyzing the muscles in their brows that create expressions of sadness, anger, and fear. The results were astonishing.