The acute effects of Yohimbine on fear conditioning and the neural processing of emotional images, and on delayed extinction learning and memory for the images.

This study aims to examine sex differences in the effects of Yohimbine on the neural processing of, and delayed memory for, emotional images. Sex differences will also be explored in the effects of Yohimbine on fear conditioning, and whether this impacts the rate of extinction learning after a delay. Participants will be randomly assigned to recieve either Yohimbine or Placebo. Participants will first undergo a fear conditioning procedure in which they will be presented with coloured circles and a particular colour will be paired with a mild electrical stimulus. Rate of conditioning (or associative fear learning) will be measured via skin conductance responses across trials. Subsequently, in an fMRI scanning machine, all participants will view a series of emotional and neutral images. Two days later, participants will return and undergo a surprise memory test for the images. Following the memory tests, participants will undergo an extinction learning procedure in which skin conductance will be recorded across trials in which the same circles will be presented without any reinforcement (electrical stimulus). The hypothesis is that Yohimbine will enhance amydgala activation during the processing of emotional images, and be associated with superior memory for the emotional images. It is predicted that Yohimbine will enhance the rate of fear conditioning, particularly in women. It is also predicted that the administration of Yohimbine will result in strong amygdala activation during the processing of emotional images as well as the deactivation of prefrontal regions of the brain in participants with higher levels of endogenous cortisol. Finally, it is believed that participants in the Yohimbine group will experience a slower rate of extinction learning after the delay (and particularly in women). All participants and research staff will be blind to the drug contition of the subject until after the study is completed.