Finally, 5-HT2A receptors on cortical apical dendrites sit in a crucial location to influence cognitive processing and excitatory transmission. Since some 5-HT2A antagonists can lead to a down-regulation of 5-HT2A receptors and their altered distribution, as well as increased prefrontal cortical monoamine levels, neuropsychological testing to compare between and within subjects treated with M100907 and placebo may increase our understanding of the importance of serotonin receptors in cognition and depression. Subjects will be studied neuropsychologically before and twice following randomization. If M100907 enhances cognition in depression this would be an important advance given the high rates of impairment in functioning and residual cognitive difficulties observed in patients with depression.