 Expected results: The investigators predict that, compared to placebo, MB will: i improve working memory retention in a delayed match-to-sample task by memory performance and enhanced fMRI responses in the prefrontal cortex and parietal lobes, ii improve episodic memory as determined by fMRI activation in the hippocampus, medial temporal lobes and prefrontal cortex iii reduce reaction time in a psychomotor vigilance test and enhance fMRI responses within a cortical sustained attention network iv improve CBF and v improve fMRI connectivity in default mode and visuospatial and memory networkssubnetworks. The fMRI and behavioral performance effects on memory will be greater in the MCI and mild AD groups than in the healthy aging group. The effects will be greater in the MCI and AD groups than in the control groups. 