Entorhinal-hippocampal connections and object memory in the rat: acquisition versus retention

Investigations of the neurobiology of memory using experimental animals have modeled many of the characteristic features of amnesia seen in human clinical populations. To examine long-term memory, however, animal models of amnesia often employ extended measures of acquisition, which stand in contrast to the retention measures used with humans. To determine the role of entorhinal-hippocampal circuitry on both information acquisition and long-term retention, rats with bilateral transections of the angular bundle were trained on three object discrimination problems and then retrained two weeks later to measure retention. Animals with discrete lesions of the angular bundle, which disrupted perforant path connections from the entorhinal cortex to the hippocampus and efferent hippocampal-cortical projections, acquired the object discrimination problems normally but showed a marked deficit in retention. These findings are important because they indicate that the role of entorhinal-hippocampal connections may be limited to maintaining some types of information (e.g., single object discriminations) for retention. This dissociation, moreover, suggests that behavioral paradigms that include a measure of retention may be particularly important for characterizing the mnemonic functions of the hippocampal/parahippocampal region.