Fig. 1.

Dose-effect curves for cocaine self-administration before (filled circles), during (open circles), and after (open squares) oral MPD treatment in four monkeys. Each point on the “baseline” and “postmethylphenidate” curves is the mean of the last 3 days of availability of a cocaine dose or saline; data for the “during methylphenidate” curve are from single-session tests of that cocaine dose. When a cocaine dose or saline was made available on more than one occasion, error bars indicate the standard error of the mean. Ordinates: number of cocaine injections earned; abscissae: available cocaine dose.