During each of six later sessions ( within-subject , double-blind , randomized crossover design ) , volunteers could respond on a 12-trial choice progressive ratio task to earn units ( 1 or 2 mg ) of the sampled hydromorphone dose ( drug A or B ) vs money ( $ 2 ) following different oral yohimbine pretreatment doses ( 0 , 16.2 , and 32.4 mg ) .
