The purpose of this research is to understand how some of the drugs commonly used in anesthesia impair memory. We are particularly interested in whether the emotion associated with a memory influences how well these drugs are able to block memory. We are studying four commonly used drugs—propofol, thiopental, midazolam, and dexmedetomidine, all of which may have slightly differing effects. We will also study an inactive substance, called a placebo, that should have no effect. The results of this study will provide information that will be useful in understanding how memory works, how these drugs affect memory, and possibly why some people don't have their memory blocked as easily as others.