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 contribute to the computational tasks that the brain performs? How are internally generated, self-organized network activities perturbed by external stimuli 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 hypotheses can only be tested by simultaneously monitoring the activities of many neurons and local field potentials in multiple regions of the intact brain at relevant temporal resolutions. To achieve our goal, we obtain large-scale electrophysiological recordings from neuronal ensembles in the hippocampus and related structures in rats and mice during resting conditions as well as while the animals perform various tasks. In addition, we combine in vivo electrophysiology and optogenetics to elucidate pathway- and cell-type-specific functions in information processing.