11.   The PET scans showed activation in the mesolimbic dopamine system as the addicts described feeling intense cravings for cocaine.

12.   Their dopamine systems could not tolerate this, he said, so they would switch to button B.

13.   Their dopamine system, operating moment to moment in predicting rewards, goes berserk when they are losing and they switch strategies too often.

14.   They had started out as occasional users but over time the drug hijacked their natural dopamine systems.

15.   When it comes to a general genetic susceptibility, the leading suspect is a defect in the dopamine system, experts say.

16.   When the dopamine system goes seriously awry, she said, people lose their excitement for life and can no longer move their limbs.

17.   Winning in gambling can also hijack the dopamine system, Berns said.

18.   The dopamine system is responsible for movement, may be involved in learning and rewards and has been implicated in addiction.