Rhythmic brain activity governs behaviour through the coordination of large numbers of nerve cells within and amongst specialised brain regions. Of particular importance is the formation or recall of everyday memories, which requires the synchronised action of millions of nerve cells of the temporal lobe within about a tenth of a second. In mammals, including humans, such synchronisation is observed as a 'slow' oscillating electrical rhythm measured by electroencephalography (EEG). Embedded within each cycle of the slow EEG signal, higher frequency oscillations emerge in relation to cognitive processes. Brain disorders including dementia and age-related memory impairments are accompanied by perturbation of these brain rhythms, thus highlighting their biological importance. The mechanisms for establishing and maintaining such rhythmic brain activity at various time scales, and the specific roles of the hundreds of nerve cell varieties that cooperate to deliver such a feat of function, remain to be defined. Brain rhythmicity creates sequential "windows" of increased and decreased activity levels of large groups of nerve cells, which enables the cerebral cortex to encode and link actual sequences of real-life events that are represented on distinct oscillatory cycles. In the proposed project, we will exploit our discovery of three novel varieties of nerve cells for establishing their roles in rhythmic oscillatory neuronal activity and memory processing. The novel types of nerve cell are found in a subcortical area deep within the brain called the medial septum, and each type sends parallel projections to a select area or areas of the cerebral cortex that each plays a distinct role in the formation and recall of memories. These cooperative brain areas, including the hippocampus and entorhinal cortex, are the ones first affected by neurodegeneration in Alzheimer's disease. Using a novel technology for the molecular dissection of gene expression profiles of these and other nerve cells in the medial septum, we will provide a comprehensive definition of cell types in both the mouse and the human brain. Building on our recent discoveries, we will establish how the function of these types of nerve cells changes in a mouse model of Alzheimer's disease. We will then use external modulation of the activity of some of these specific pathways to test how to improve memory processing. This project will thus advance our understanding of the functional organisation of the mammalian brain in relation to memory processing.