 This study will evaluate acute symptomatic improvements in cognitive performance in healthy elderly participants with mild-to-moderate Alzheimers disease. A four-week pilot study will first evaluate the CogState computerized cognitive battery in participants who are not receiving treatment. After completion of the pilot study, participants will be randomized to receive Placebo, Donepezil 5 mg, MK-0249 7.5 mg, and MK-0249 25 mg. After each drug administration the participants will be given the CogState computerized cognitive battery to assess cognitive performance. The primary hypotheses are that cognitive performance in participants with Alzheimers disease, or in healthy participants, is improved by the administration of a single oral 5 mg dose of Donepezil as compared to placebo. 