A signal detection analysis of morphine effects on the response bias of rats in a two-shock discrimination task

Signal detection theory techniques were used to assess the extent to which morphine affects response bias of rats in a two-choice shock discrimination task. The ratio of low shock (“noise”) to high shock (“signal”) trials was varied to produce different response biases prior to drug administration. Morphine significantly reduced signal sensitivity (d′) under all three conditions without systematically affecting any of the measures of response bias assessed in this study. While consistent with most reports with respect to d′, these results conflict with a few previous reports of morphine disrupting performance more when the discriminative stimulus is the presence of shock than when it is the absence of shock.