Rats answer an unexpected question after incidental encoding

Abstract

SummaryA fundamental aspect of episodic memory is that retrieval of information can occur when encoding is incidental and memory assessment is unexpected [1][2][3][4]. These features are difficult to model in animals because behavioral training likely gives rise to well-learned expectations about the sequence of events. Thus, the possibility remains that animals may solve an episodic-memory test by using well-learned semantic rules without remembering the episode at memory assessment. Here we show that rats can answer an unexpected question after incidental encoding in a hippocampal-dependent manner, consistent with the use of episodic memory. Rats were initially trained to report about a recent event (food vs. no-food) and separately searched for food where there was no expectation of being asked about the presence of food. To test episodic memory, rats were given the opportunity to incidentally encode the presence or absence of food and were unexpectedly asked to report about the recent event. Temporary inactivation of the CA3 region of the hippocampus with bilateral infusions of lidocaine selectively eliminated the ability of rats to answer the unexpected, but not the expected, question. Our studies suggest that rats remember an earlier episode after incidental encoding based upon hippocampal-dependent episodic memory. Results and DiscussionAlthough events are not always known to be important when they occur, people can nonetheless remember details about such events. For example, eyewitness accounts rely on memories for incidental aspects of an earlier episode not known to be important when the episode occurred. Encoding occurs incidentally when apparently unimportant information is stored, but it is not known at the time of encoding that the information may subsequently be useful. By contrast, information is explicitly encoded when it is known that the information is needed later. When information is encoded for use in an upcoming, expected test of retention, it is possible that the explicitly encoded information is used to generate a planned action; according to this view, at the time of the test, the remembered action can occur successfully without remembering the earlier episode. Thus, it is difficult to determine if successful performance is based on a memory of the earlier episode or a planned action generated when information was explicitly encoded [1][2][3][4]. Indeed, it is possible that animals may have solved previous tests of episodic memory (e.g., [4][5][6][7]) by using well-learned semantic rules without remembering the episode. Formally, learned rules stored in semantic Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the conte...

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