The ability of rats to take straight paths from a variety of start locations to a variety of goals suggests that their brains contain "cognitive maps" that are used in complex navigational tasks (43,72). The remarkable firing property of place cells - rapid firing only when the rat is in a particular part of its environment - suggests that they are part of the cognitive map. Two lines of evidence suggest that the hippocampus is a critical structure in rat navigation. First, hippocampal pyramidal neurons are place cells (45), and second, damage to hippocampal structures makes navigation behavior highly inefficient (34,65,51). Our past work has used computer-based data collection and reduction techniques to describe the firing properties of individual place cells (40), and to analyze the effects of environmental manipulations on place-cell activity (36,37,39,). The work proposed here extends the analysis of place cells into broader contexts. The most significant methodological change in the proposed work will be simultaneous recordings from sets of individual neurons. This is a powerful technique that permits direct analysis of how neuronal activity can be related to the activity of other neurons, as well as to sensory or behavioral variables. One common function suggested for the hippocampus is to determine environmental context. Indeed, it is frequently suggested that the memory deficits seen in humans with hippocampal lesions are due to a defect in contextual retrieval (13,78). The finding that the hippocampal place cells can have apparently independent representations for several environments, suggests that these environments represent contexts. The most extensive proposed study is to record from sets of neurons across multiple environments. The differences between the proposed study and previous work lie in our plan to record from large sets of neurons simultaneously and our plan to record these sets over a much broader range of environments. We hope to gain insight into the nature of environmental (contextual) representation. It has been proposed that the hippocampal representation is "map-like", meaning that the representation stores information about the spatial relationships among regions in the environment. We will test this idea by recording simultaneously from sets of neurons in ambiguous stimulus situations to see whether the firing fields maintain their spatial relationships with respect to each other. If they do, we will conclude that the representation has "cohesive" or "map-like" qualities. Finally, we will investigate the role of the hippocampus in navigational behavior directly. Place cells will be recorded during active navigation to see whether they signal the rat's perception of the location of the hidden goal. We will also continue investigations into a rat's navigational abilities when the hippocampus is lesioned.