There is another wrinkle to the dopamine-as-craving story. Give a monkey a single squirt of juice and its brain shows a spike of dopamine, but repeat the process several times and eventually the dopamine neurons settle down. But now give the monkey two squirts when it expected one, and dopamine perks up once again. Give it three squirts and dopamine perks up even more. Yet if these three squirts are now repeated, dopamine once again settles back down.
