"Recreational drugs like cocaine don’t just usurp the brain’s reward circuit; they have powerful effects on learning and memory. Many brain imaging studies, using positron emission tomography, show that cues like viewing drug paraphernalia are enough by themselves to activate memory circuits and unleash drug craving. Where you are and what you are doing when you use a drug like cocaine is inextricably linked with the high. And these associations are stored not just in your conscious memory, but also in memory circuits outside your awareness. This kind of pathologic learning lies at the heart of compulsive drug use. Long after someone has apparently kicked the habit, long after withdrawal symptoms subside, the individual is vulnerable to these deeply encoded unconscious associations that can set off a craving, seemingly out of the blue."