This is a drill adapted by 55 Defense from writings done by Marcus Wynne. The focus of this drill is understanding target discrimination backward and forward.
What is backward target discrimination? Shooting everything EXCEPT what is called out. Many of us have done discrimination drills in which we shoot the target(s) called by a range partner. This builds the brain to identify targets from an audible stimulus. If you can, add a system in which you can use a visual stimulus.
Now, our goal will be to shoot everything BUT what was called. Have a partner intermittently say EXCEPT. This will be your indicator to shoot all bit the targets that are called. This creates a situation in which you still have to do the same brain work (choose and identify the targets) and then consciously filter out those targets and shoot the others. Though it seems counter intuitive, it will make your target identification skills faster.
This is a great addition on to a normal range drill and does require a partner. It can be done with a deck of cards as well, if you need to, by using even and odd numbers, as well as using red for shoot what is shown and black for shoot the opposite. This is just a suggestion, you can use whatever setup of the deck you like.
Shoot straight and have fun!