Classical conditioning during nitrous oxide treatment: Influence of varying the interstimulus interval

Classical conditioning of the rabbit nictitating membrane response (NMR) wa s accomplished by presenting a 75-ms tone conditioned stimulus (CS) at inte rvals of 0, 100, 200, 400, and 800 ms before the presentation of a 100-ms s hock unconditioned stimulus. Following every four paired trials (tone follo wed by shock), the occurrence of conditioned responses (CRs) was tested on every fifth trial in which only tone was presented (test trials). Three dos es of nitrous oxide in oxygen (0, 33, and 67%) were used during conditionin g. Nitrous oxide produced dose-dependent decrements of learning. Conditione d responding was related to the interstimulus interval (ISI) by a concave-d own function. The higher dose of nitrous oxide caused more decrements of le arning at several ISIs compared to the other two doses, changing the shape of the curve. Trace conditioning, which was examined in the present study, was more impaired under the influence of nitrous oxide than conditioning in a previous study, which used the standard delay paradigm. Thus, the drug i mpairs explicit memory more than implicit memory. (C) 1999 Elsevier Science Inc.