Rats will readily learn to run up an alley for a cocaine infusion. In one study, they have this opportunity once a day for fourteen consecutive days, but instead of going faster each day, as they would with other drugs such as heroin, they begin quickly and just before reaching the infusion source are apt to turn and run the other way. They usually go back and forth several times—“Yes! I want it” / “No! I don’t”—suggesting to researchers what every addict already appreciates: that cocaine addiction is a mix of positive and negative motivational states and that the negative consequences
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