Coordinated patterns of neural activity spanning broad spatial and temporal scales underlie information processing in the brain. How are these activity patterns generated by intrinsic cellular properties, synaptic interactions, neuromodulatory systems, and network dynamics? How do these activity patterns carry out computational tasks that the brain attempts to perform? How are internally generated, self-organized network activities perturbed by the external world to produce perception and memory? Our goal is to quantitatively characterize and mechanistically explain information processing in terms of neuronal activity dynamics. Possible mechanisms subserving these processes include precisely structured sequences of activity, synchrony within and across brain regions, and dynamic grouping of local and global neuronal assemblies by network oscillations. These possibilities are testable only if we can simultaneously monitor the activities of many neurons and local field potentials in multiple regions of the intact brain at relevant time resolutions. To achieve our goal, we will employ large-scale electrophysiological recordings of neuronal ensembles in the hippocampus and its related structures in rats and mice while they perform various tasks and while at rest. In addition, we will combine in vivo electrophysiology and optogenetics to elucidate pathway- and cell type-specific functions in information processing.