Episodic memory trace formation in the hippocampal system : a model of cortico-hippocampal interaction

We readily remember events and situations in our daily lives and acquire memories of speci c events by reading a newspaper, or watching a newscast. This ability to rapidly acquire \episodic" memories has been the focus of considerable research in psychology and neuroscience, and there is a broad consensus that the hippocampal system (HS), consisting of the hippocampal formation and neighboring cortical areas, plays a critical role in the encoding and retrieval of episodic memory. But how the HS subserves this mnemonic function is not fully understood. This report presents a computational model, smriti, that demonstrates how a cortically expressed transient pattern of activity representing an event can be transformed rapidly into a persistent and robust memory trace as a result of long-term potentiation within structures whose architecture and circuitry resemble those of the HS. Memory traces formed by the model respond to highly partial cues, and at the same time, reject similar but erroneous cues. During retrieval, these memory traces acting in concert with cortical circuits encoding semantic, causal, and procedural knowledge can recreate activation-based representations of memorized events. The model explicates the representational requirements of encoding episodic memories, and suggests that the idiosyncratic architecture of the HS is well matched to the representational problems it must solve in order to support episodic memory function. The model predicts the nature of memory de cits that would result from insult to speci c HS components and to cortical circuits projecting to the HS. It also identi es the sorts of memories that must remain encoded in the HS for the long-term, and helps delineate the semantic and episodic memory distinction. Copyright c 2001 by Lokendra Shastri