It is estimated that by 2016, nearly 50% of HIV-positive individuals in the US will be aged 50 or older, and up to 60% of those will experience some degree of cognitive impairment as they age. The purpose of this study is to evaluate the contribution of the neuronal cholinergic receptor system to the cognitive impairments seen in adults aging with chronic HIV Infection. By using anti-cholinergic challenge drugs to reversibly "stress" cognitive functioning, the investigators hope to understand whether the presence of the HIV virus in the brain impairs the neural system necessary for normal cognition, more than would be expected from normal cognitive aging.