A maximum of eighty volunteers will receive sub-anesthetic doses of one of the four study drugs, or placebo, while performing a visual continuous recognition task ('CRT') of pictures with randomly varying affective load. Drug effect on affective modulation of working and early long-term memory behavior will be examined by analyzing which images are and are not recognized during the CRT. Drug effect on affective modulation of later long-term memory will be examined by analyzing which images are recognized several hours after presentation.