Amphetamine and barbiturate effects on two tasks performed singly and in combination

A study was made of the influence of two drugs on performance of two tasks. The tasks were performed both separately and simultaneously. The drug conditions used were amphetamine (20 mg phentermine HCL), barbiturate (600 mg hexobarbital sodium), a placebo and a control condition. The tasks were (a) randomization, i.e. the production of a random sequence, and (b) pursuit tracking. The drug conditions were arranged in a 4 × 4 Latin Square with three subjects assigned to each order. One session consisted of 33 trials of 120 sec, each followed by a 40 sec rest period. On randomization both the barbiturate and the secondary task resulted in a shift towards more repetition. Given most subjects' preference for too few repetitions this resulted in a more random response sequence. Amphetamine had the opposite effect. Barbiturate and a secondary task both gave higher integrated error scores on the pursuit tracking. Amphetamine brought about a lower score. Neither with randomization, nor with the tracking there was an interaction between drug treatment and single vs. dual task. On the basis of this additivity it was concluded that the drugs and the dual task affect different mechanisms. An attempt was made to give a more precise specification of these mechanisms.