People often develop fearful responses to things, but have no conscious control over the fear. 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.

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.

Inclusion Criteria:

• age between 18 and 50
• minimum of high school education
• fluent in English
• normal vocabulary

Exclusion Criteria:

• any significant medical or psychiatric comorbidity (e.g. asthma, diabetes, hypertension, depression, high anxiety)--subjects must be in excellent health such that they would be classified as American Society of Anesthesiologists Physical Status Class I
• deficit in vision or hearing that would impede the study
• allergies to any of the study drugs, to soybeans, or to eggs
• a history of head trauma
• a family history of major psychiatric illness
• body mass index > 30 kg/m2
• a recent history of recreational drug use
• prior exposure to the study materials
• pregnancy
• a personal or family history of any porphyria
• failure to exhibit a skin conductance response to deep inspiration
• the ability to read Chinese characters
• assessment by the investigators that the subject may be unable to cooperate or comply with the study requirements.