The brain is a complex organ with complex processing. But it's slow. Early AIs were written to explore the highest level of brain functions — things like planning, logic, natural language, creativity, problem solving, mathematics. Like the higher levels of the brain, solving these problems is slow. In game AI, planning and pathfinding are examples of the higher level processing, and these are often slow.
In the body, when fast reactions are needed, we have reflexes: a fast but dumb reaction that occurs before the brain can fully process and respond to a signal. The classic example is putting a hand on a hot potato – you will pull your hand away before the brain even receives the pain signal. Many reflexes are handled by the spinal cord, before the signals reach the brain.
The body has both:
- A global slow, smart system: processing in the brain
- A local fast, dumb system: reflexes closer to where the input occurred
In game AI we can often do the same. An example is in games where pathfinding is slow, and has to be split up among several frames. We need to give instant feedback to the player when the unit is clicked. If pathfinding is slow, then we need something else in its place. In some games, you'll see animation and sound for the unit getting ready to move. But the reflex-inspired approach would be to have a secondary pathfinding system that's fast and dumb, and have the unit start following that path right away. The most straightforward approach would be to start moving in a straight line; another approach would be to see if any nearby units are moving to the same general area, and start following them. The slow, smart system can fix the path later. In some cases you don't need the smart system at all because the dumb path is fine. That way, you can get both instantaneous action and good paths.
In planning tasks we also have places where reflexes can be useful. For example, suppose three different squads are attacked simultaneously. Their reflex action should be to fight back. In the background a slower processing task can analyze these three positions, the size of the attacking forces, and the information gathered from scouting. It can guess what the enemy is going to do next, and order all three squads to fall back to a safer position. Fire first, then fall back, instead of waiting for the analysis before performing any action.
Lots of games already use these techniques. They're part of a general pattern: use both slow/smart and fast/dumb event handlers, so that you can react quickly and then go back and react smartly in cases where you need it.