In 1965 at MIT, Melzack and Wall developed the gate control theory of pain which states that pain is "gated" or modulated by past experience. Gate control theory led to the valuable discovery of endorphins and enkephalins, the body's natural opiates. He is also noted for work on stress-induced analgesia, phantom-limb pain and the theory of neuromatrix. He proposes that we are born with a genetically determined neural network that generates the perception of the body, the sense of self, and can also generate chronic pain, even when no limbs are present.