In a garage in Palo Alto, Calif., in the 1980s, some friends and I were the first humans to experience becoming avatars—that is, movable representations of ourselves in cyberspace. Amazingly, all these years later, almost no one else has been able to experience a hint of what will be one of the great cognitive adventures of this century.

It has been possible for some years for visitors to theme parks to try out virtual-reality "rides," but these don't capture the experience. Becoming an avatar in virtual...