A scientist performs an experiment called passive avoidance, where a mouse is placed on the lighted side of a two chamber apparatus with a small entrance connecting the two chambers.$$ One side is heavily lighted, the other side is dark. $$If the mouse ventures into the dark side of the chamber it receives a mild electric shock every $2$ seconds, until it leaves the dark side.$$ The experiment is repeated until the mice avoid the dark side completely.$$ This is an example of: