A Simplified Hippocampal Model That Learns and Uses Three Kinds of Context

Abstract

Context plays a critical role in cognition. Previously Hirsh ('74),Kesner & Hardy (83), and Gray ('82) proposed that the hippocampus couldlearn context. Presented here is a simple hippocampal model that learnsand uses three types of context: a context coming from the past, acontext coming from the present, and a context concerned with thefuture. In all three of these situations, context is encoded by neuronsthat fire in a way analogous to hippocampal place cells. When thesefiring patterns do not appear, the network seems incapable of solvingcontext dependent problems.In psychology, the importance of context arises before the turn of thecentury (Boring, `50), in particular Titchner advocated a critical rolefor context in perception. Context is important to the networks thatlearn language in the schemes of Pollack ('90), Elman ('90), Jordan('86), and Meter ('92). Although not part of the usual terminology,context is at the heart of frames and schemas used by other cognitivepsychologists. More to the point here, context learning seems to be partof hippocampal function (Hirsh, `74, Kesner & Hardy, `83, Gray, `82).Context learning is compatible with the Cohen and Eichenbaum theory offlexible memory (Cohen, `84; Eichenbaum et al., `92). Hirsh places theuse of context at the center of proper encoding and recall of long-termmemory. Context specifies the location of long-term memory storage. Inthis view, context is equivalent to episodic memory. Moreover, episodicmemory associates disparate objects and events from single experiences;unfortunately, it is a lack of episodic memory that so hampers patientslike H. M. and R. B. And, finally, even though not an explicit part ofO'Keefe and Nadel's ('78) cognitive mapping theory (but see Nadel andWillner, `80), a coding that is analogous to hippocampal place cells (wecall them context cells) - a coding that can be used to get from point Ato point B - are context-based codes when viewed within the function ofour model of the hippocampus.To appreciate the role of context in memory, picture this one situation.You go to the hippocampal conference at Grand Cayman and for the firsttime you meet John Smith, a scientist from Seattle. A year or two lateryou visit the NIH and you see a vaguely familiar face; it's John Smithbut you cannot remember his name. (As always striving for politeness aswell as wishing to avoid embarrassment, you struggle to come up with aname to match the face.) If you can only remember the place, thecircumstances, the episode where you met him, then you will have achance of remembering the name. You well up a vague association of theconference room where you met and at the same time comes the hotel, thebeach, and then ... `'John, what a surprise seeing you here! How areyou?''In other words, the storage of unique events is intimately associatedwith the surrounding circumstances (context). Of course, the idea ofcontext-dependent memory is a couple of thousand years old asexemplified by the Roman's method of loci for memorizing long speeches.One sequence (the speech) is learned by associating it with anothersequence of patterns (the sequence of statues you pass as you walkthrough a well-known museum) by using each successive statue and itslocus as the context for successive words and phrases in the speech.Less grandiose forms of context are useful in many other types ofcognitive processes. Thus, many cortical regions would need to producecontext codes. But, context-based codes do seem particularly importantfor hippocampal functions including setting up appropriately retrievablestores of memories.