The mammalian hippocampus plays critical roles in navigation and episodic memory. The most prominent hippocampal neural correlate of these behaviors is “place cell” activity: during spatial exploration, a subset of excitatory principal neurons in the hippocampus fire in restricted regions of the environment (termed ‘‘place fields’’). At the population level, place cells form internal maps of the external environment, which are thought to also support hippocampal memory functions. Indeed, place cell maps have been shown to dynamically reorganize in response to novel sensory cues, changes in the multi-sensory environmental context, and in particular, in response to salient features of the environment, such as appetitive reward signals during goal- oriented spatial learning. It is widely assumed that afferent excitatory inputs to the hippocampus carry critical information required for these navigational and memory-related changes in hippocampal network dynamics. Two subregions of the Entorhinal Cortex (EC) have been particularly implicated in this process by providing dual input streams to the hippocampus. Specifically, the medial EC (MEC) is thought to be involved in primarily processing spatial information related to global contextual reference frame, while the lateral EC (LEC) is thought to primarily process information related to individual items and locations based on a local reference frame. Nevertheless, a detailed understanding of the circuit level mechanisms of entorhinal cortical influences on hippocampal population dynamics is still lacking with conflicting experimental results showing varying degree of deficits in navigational and memory behaviors following EC manipulations. This deficiency largely stems from the lack of available information on (1) the specific type of information conveyed directly by identified EC afferents to the hippocampus, (2) the experience-dependent changes in information carried by EC projections, and (3) changes in EC signaling in response to relative novelty and salience of the sensory environment. The MEC and LEC direct projections to pyramidal cells (CA1PCs) in the mouse dorsal hippocampal CA1 area offers a tractable circuit to bridge this knowledge gap by leveraging chronic, subcellular-resolution two-photon functional imaging of identified EC afferents and CA1PCs during head-fixed virtual reality navigation and learning. These experiments will test the hypothesis that CA1PCs receive direct spatial, sensory and salience-related information from identified EC inputs which instruct CA1PCs population dynamics depending on behavioral task demands. Aim 1 will use chronic calcium imaging of EC axonal inputs in CA1 to characterize what information is carried by the MEC and LEC direct projections to CA1 and analyze how their activity changes over time related to novelty and behavioral salience. Aim 2 will employ simultaneous calcium imaging from EC axons and CA1 pyramidal cells (CA1PCs), as well as optogenetic mani...