A unique enhancement of associative learning produced by methylenedioxyamphetamine

The effects of methylenedioxyamphetamine (MDA) on classical conditioning of the rabbitʼs nictitating membrane response were assessed in four experiments. Experiment 1 established a dose-effect curve for MDA doses of 0, 1, 3, and 10 μmol/kg. Both the 3 and 10/μmol/kg doses significantly enhanced the rate of learning. Experiment 2 established that the 10 μmol/kg dose of MDA had no effect on non-associative determinants of responding to the conditioned stimulus (CS). However, this dose of MDA apparently sensitized the response to the unconditioned stimulus (US). Experiments 3 and 4 assessed the effects of the 10 μmol/kg dose of MDA on sensory processing of the CS and US, respectively. MDA had no effect on the intensity thresholds for eliciting responses to either of these stimuli. Thus, the enhanced rate of acquisition observed in experiment 1 cannot be attributed to an increase in nonassociative responding to the CS or to enhanced sensory processing of the CS or US. MDA apparently enhanced the rate of acquisition in experiment 1 by facilitating the association between the CS and US. These findings are unique in that no other hallucinogen we have examined has enhanced acquisition without also affecting either non-associative responding or sensory processing of the CS and US.