The middle mouse button only changes the camera view. To turn your avatar around, you still need to use the Turn Right/Left icons. AFAIK, there're no 180degrees avatar turn icon in the game. Maybe, you can add it to the Wishlist.
If you have aim-mode enabled you can turn your avatar using the mouse. This is an extremely fast way to spin around once mastered. You can also start turning and/or running before the white screen disappears. Play with it a while and you'll soon get the hang of it