The present thesis explores the electroencephalographic (EEG) patterns associated with attention and working memory functions in normal aging and in patients with moderate cognitive decline possibly precursor of Alzheimer's disease. Across five studies published between 2009 and 2015, we examine the surface EEG fluctuations generated by visual perception and cognitive processing in the form of 1) event-related potentials (ERPs), reflecting postsynaptic potentials of cortical neural populations, and 2) patterns of brain oscillations between 4 and 30 Hz, including theta (4-7.5 Hz), alpha (8-13 Hz) and beta (14-30 Hz) frequency bands, corresponding to rhythmic synchronizations of neuronal discharges in specific cell assemblies. Overall, these studies evidence various specific functional perturbations associated with cognitive activation in older adults and patients with cognitive deficits, demonstrating the capacity of electrophysiological indices to provide sensitive measures of brain function in normal aging and cognitive decline.