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Summary
A plethora of studies in rodents have described spatially tuned neurons, including place cells in the hippocampus and grid cells in the medial entorhinal cortex (MEC), suggesting a crucial role of the hippocampal formation in spatial navigation (1). Human studies have, in turn, shown that the hippocampal formation is involved in declarative memory (memories of facts and events) (2). What, then, is the function of the hippocampus? Is it involved in memory or in spatial navigation, or does it have a more general function that encompasses both? Several studies have shown that place cells remap, changing the location at which they respond, following geometrical changes in the environment, and that they can be modulated by nonspatial factors, according to the animals' specific tasks (3). These findings highlight the coding of cognitive components, challenging the notion that place cells represent an invariant spatial map. Although grid cells also realign with physical changes in the environment (4), the geometrical structure of their fields has been considered to provide a more invariant spatial representation. But do grid cells encode cognitive aspects as well? On pages 1443 and 1447 of this issue, Boccara et al. (5) and Butler et al. (6), respectively, show that this is the case, providing compelling evidence that grid cells are modulated by reward location. Complementing these studies, Baraduc et al. (7) showed that variations in spatially tuned neurons in the monkey hippocampus are accompanied by a “schema” of the task. Together these studies provide insights on how neurons in the hippocampal formation go beyond purely spatial representations and are modulated by cognitive factors.
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