Automated measurement of motor activity in human subjects: effects of repeated testing and d-amphetamine

Repeated exposure to a test setting decreases, and amphetamine increases, motor activity in animals. To evaluate whether these effects also occur in human subjects, we recorded motor activity levels from 12 subjects during a double-blind oral drug discrimination (placebo vs. 75 mg tripelennamine) study. Before each 4-h session, activity monitors were attached to the subject's wrist and ankle. During each session, subjects rated their drug effects hourly (task periods), and could freely choose among leisure activities during intertask intervals (recreational periods). Habituation was evaluated by comparing activity response during initial (training phase) vs. later (discrimination phase) placebo sessions. During later sessions the two training drugs, as well as diazepam (2.5, 5 mg P.O.) and d-amphetamine (5, 10 mg P.O.) were administered. Consistent with animal studies, repeated exposure to the test environment significantly decreased, and d-amphetamine significantly and selectively increased, wrist motor activity. These data indicate that human motor activity is sensitive to environmental factors (task, time), drug class, and d-amphetamine dose. Activity measures may, therefore, be useful in evaluating environment/psychostimulant interactions in humans.