Project Summary/Abstract The brain is composed of many densely interconnected regions, in constant communication and interaction with each other; cognition and behavior emerge from this interacting network. Yet, to date the vast majority of our knowledge regarding brain function, at cellular resolution, has come from studies focused on single brain regions. Understanding the joint cellular-resolution dynamics of large sets of brain regions remains a major challenge for two main reasons. The first is (a) experimental: simultaneous recordings, from many regions across the brain, with sufficient power to identify neural dynamics at a fine timescale and cellular resolution remain extremely rare, meaning that the joint dynamics are rarely even measured in spiking datasets. The second obstacle is (b) analytical: even when recordings from multiple regions are available, analysis methods for understanding communication and interactions across many regions, at cellular resolution, are limited and still under development. We propose to address both of these challenges head on. We will develop methods for chronic electrophysiological recordings from hundreds of neurons in each of tens of regions across the brain. We expect to record from 6,000 to 12,000 neurons simultaneously in well-trained, freely-moving rats. We will also develop modeling and analysis methods to understand the interactions between brain regions contained in that data. We will do this in the context of elucidating the multi-region interactions underlying the accumulation of sensory evidence for decision-making. Moreover, to begin to understand to what extent our findings generalize to other types of decision-making, we will also apply our methods to a value-based reward-guided task, and compare across the two tasks.