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Involvement of the hippocampus and medial entorhinal cortex in bridging discontinuous events


The medial entorhinal cortex (MEC) projects directly to the hippocampus. It is considered the major contributor of spatial information to the hippocampal place cells (1-3). However, it remains unclear to what extent MEC spatial firing is necessary for memory-guided behavior. We found that complete single or double bilateral lesions to the MEC and the hippocampus impaired rats in the delayed alternation task. Single lesioned rats partially improved their behavioral performance in delay trials across time. However, physiological analysis of CA1 cells in the MEC lesioned rats indicated less precision in firing, as well as less discrimination between left and right trials. Double lesioned rats did not present any behavioral improvement throughout trials, but in fact exhibited perseveration. These results could indicate that single lesions of either structure disrupt the function of the entorhino-hippocampal loop. Over time residual neurons of the intact structure are sufficient to compensate for the communication with the prefrontal cortex, as supported by partial behavioral recoveries. Double lesioned animals, however, are prevented from improving in memory related tasks.

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