The ability to interpret and use environmental context to inform cognitive processes and guide behavior is an essential function of the human brain. Accordingly, in everyday observation, objects are commonly linked to a contextual setting (e.g. stove-kitchen, boat-lake). Context is not simply a place but also includes characterizing features and objects (e.g. other appliances in the kitchen). These representations of context and object-context associations are a key component of many higher cognitive functions including perception, decision-making, and episodic memory. Notably, the use of context to guide cognition and behavior is disrupted in neurological and neuropsychiatric disorders. Despite the importance of contextual representations in cognitive function and neurological and neuropsychiatric disease, there are many open questions about where such representations are formed and stored and how they guide behavior. The parahippocampal cortex (PHC) in primates, or the homologous postrhinal cortex (POR) in rats, is thought to process spatial information received from the posterior parietal cortex, whereas the perirhinal cortex (PER) is known to process object information from the visual cortex. By one view, item and context information arrive separately to the hippocampus and are integrated there to form representations of events and episodes. By another view, item and spatial information are configured in the hippocampus to represent context. Recent studies in rats, monkeys and humans, however, provide evidence that representations of context may be formed in the PHC/POR, upstream of the hippocampus. Indeed, POR neurons have been shown to form representations that link objects to places, an essential component of complex representations of environmental context. This application is guided by the overarching hypothesis that the rat POR combines spatial information with object information received from the PER to form complex representations of context, including the spatial layout of objects and features in the local environment. The PI will employ experimental lesion, electrophysiology, and optogenetic approaches in behaving rats to test this hypothesis in three related aims: 1) experimental lesions will be used to confirm that connections between the POR and the PER are necessary for object-context recognition; 2) electrophysiology will be used to determine whether POR neurons encode object-context conjunctions and signal when a familiar object is encountered in a new context; and 3) electrophysiology and optogenetics will be used to examine whether object-context conjunctive coding in the POR relies on direct input from the PER. Representations of context are essential to form episodic memories and to make decisions based on context (e.g. how to behave in different social situations). Understanding how and where context is represented in the brain and how representations of context guide behavior will inform the basic neuroscience of episodic memory. In addition, understanding the neural basis of contextual processing is directly relevant to the study of mental illness and will steer us towards novel therapeutic approaches for the cognitive deficits in neuropsychiatric disease.