People often develop fearful responses to things, but have no conscious control over the fear (e.g. phobias). This is a basic form of unconscious memory, called "fear conditioning." Intravenous anesthetic drugs have remarkable effects on conscious memory, but it is unknown whether they have similar effects on these unconscious fear memories.

To address this question, the investigators will study 114 healthy adult volunteer subjects. The subject is given a very low dose of an anesthetic drug intravenously (i.e. through the bloodstream). The dose is so low that the subject might not even be able tell if they are getting the drug. While they are receiving the drug, the subject will perform a series of memory tests and a fear conditioning experiment, which are set up like a very simple computer game. To create the "fear response", subjects will occasionally receive a mildly uncomfortable shock to their arm. The subject is able to determine the highest level of shock that they will receive.

The investigators are doing this study because the investigators wish to know exactly how the drugs affect the way people process fear and emotion. This knowledge might one day be used in the treatment of some psychiatric disorders.