Sparse Representation in the Human Medial Temporal Lobe

Waydo, Stephen; Kraskov, Alexander; Quiroga, Rodrigo Quian; Fried, Itzhak; Koch, Christof

doi:10.1523/jneurosci.2101-06.2006

Cited by 154 publications

(182 citation statements)

References 20 publications

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“…However, these studies involved tasks in which memories were acquired over an extended period (allowing for the development of place fields), not tasks in which multiple memories were formed in rapid succession on a single trial. The same is true of prior evidence for sparse coding of semantic memory in the human medial temporal lobe (11). Our findings suggest that, as has long been predicted, rapidly formed episodic memories are supported by a sparse distributed code in the human hippocampus.…”

Section: Discussionsupporting

confidence: 87%

“…Previous work with humans has suggested that the representation of semantic memory in the hippocampus is relatively sparse (11). In addition, one study (10) found that episodic memory of a particular video clip (tested using recall) was preceded by the selective reactivation of the same neuron that had reliably responded to the presentation of that clip on an earlier test of semantic memory.…”

Section: Discussionmentioning

confidence: 99%

“…The results of these studies suggest that long-established semantic memories may be represented by fewer than 1% of neurons in the hippocampus (11). However, neurocomputational theories are concerned with the representation of episodic memories.…”

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confidence: 97%

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Sparse and distributed coding of episodic memory in neurons of the human hippocampus

Wixted¹,

Squire

Jang

et al. 2014

Proc. Natl. Acad. Sci. U.S.A.

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Neurocomputational models hold that sparse distributed coding is the most efficient way for hippocampal neurons to encode episodic memories rapidly. We investigated the representation of episodic memory in hippocampal neurons of nine epilepsy patients undergoing intracranial monitoring as they discriminated between recently studied words (targets) and new words (foils) on a recognition test. On average, single units and multiunits exhibited higher spike counts in response to targets relative to foils, and the size of this effect correlated with behavioral performance. Further analyses of the spike-count distributions revealed that (i) a small percentage of recorded neurons responded to any one target and (ii) a small percentage of targets elicited a strong response in any one neuron. These findings are consistent with the idea that in the human hippocampus episodic memory is supported by a sparse distributed neural code.recognition memory | intracranial recording | amygdala T he hippocampus is known to play a fundamental role in declarative memory (1-4), but it is not known how mnemonic information is coded by the activity of individual hippocampal neurons. At least three different coding schemes have been considered: a localist coding scheme, a fully distributed coding scheme, and a sparse distributed coding scheme (5). In a localist coding scheme, an individual neuron (sometimes referred to as a "grandmother cell") codes only one memory, and each memory is coded by the activity of only one neuron. In a fully distributed coding scheme, each memory is coded instead by a pattern of activity across many hippocampal neurons. Falling between these two extremes is a sparse distributed coding scheme in which each memory is coded by the activity of a small proportion of hippocampal neurons, and each neuron contributes to the representation of only a few memories. Sparse distributed coding has long been hypothesized to be the most efficient way for hippocampal neurons to encode episodic memories (remembering events) in rapid succession without overwriting previously stored memories (6-8).Most prior work concerned with the coding of declarative memory in the human hippocampus has focused on the neural representation of semantic memories (remembering facts), such as memory for famous people or landmarks (9, 10). The results of these studies suggest that long-established semantic memories may be represented by fewer than 1% of neurons in the hippocampus (11). However, neurocomputational theories are concerned with the representation of episodic memories. The purpose of our study was to test predictions of these neurocomputational theories about how episodic memories are represented by neurons of the hippocampus.The representation of episodic memory in the hippocampus typically has been investigated using recognition procedures. In recognition, the task is to discriminate between familiar items presented earlier in the experimental session (targets) and novel items not previously presented (foils). An episodic memory signal...

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Section: Discussionsupporting

confidence: 87%

Section: Discussionmentioning

confidence: 99%

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Sparse and distributed coding of episodic memory in neurons of the human hippocampus

Wixted¹,

Squire

Jang

et al. 2014

Proc. Natl. Acad. Sci. U.S.A.

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“…In fact, the idea that the MTL represents information using a sparse code is one of the basic tenets of most contemporary memory models (17)(18)(19). Given that the MTL assigns a relatively small number of neurons to a specific stimulus (20), and that the number of stimuli in the environment is very large, does a feature such as personal relevance, which may be related to the incidental recollection of autobiographically significant information (3), make a stimulus more likely to elicit a selective excitatory response from a cell? Here, we address the question of whether individual neurons in the MTL show a preference for personally relevant pictures.…”

mentioning

confidence: 99%

Human medial temporal lobe neurons respond preferentially to personally relevant images

Viskontas

Quiroga

Fried

2009

Proc. Natl. Acad. Sci. U.S.A.

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People with whom one is personally acquainted tend to elicit richer and more vivid memories than people with whom one does not have a personal connection. Recent findings from neurons in the human medial temporal lobe ( epilepsy ͉ hippocampus ͉ memory ͉ familiarity ͉ single-unit recordings H umans are self-absorbed by nature. One virtually infallible method of enhancing memory is simply to relate the to-be-remembered information to one's self. The self-reference effect (1) is a well-documented encoding enhancement: people are more likely to remember items that are personally relevant, than items that have undergone some other deep or semantically elaborative encoding processes (2). One mechanism by which the self-referent effect might operate is via the incidental recollection of a rich network of information related to past experiences with that particular item. In addition, incidental recollection of related autobiographical associations can lead to performance advantages such as enhanced memory and speeded responding (3). This incidental recollection has been shown to occur in the context of identifying famous people (3), and has also been shown to involve the hippocampus (4, 5). Furthermore, famous faces and names have long been used to query the integrity of recent and remote semantic memory in patients with temporal lobe epilepsy (TLE) (3, 6-11). Finally, autobiographical memory retrieval consistently engages the medial temporal lobe (12), suggesting that cells in this region may respond differentially to faces that elicit autobiographical memory retrieval.Recently, recordings from the human medial temporal lobe (MTL) have shown that individual neurons can be highly selective in terms of the stimuli to which they respond (13). Out of a set of about 100 visual images across several categories, such as faces, animals, and landmarks, some cells showed robust responses to only a handful of pictures pertaining to a single conceptual category, such a particular famous person. This selectivity provides an important clue as to the mechanism by which the brain represents information currently in awareness (14). In animals, selective sparse coding has also been observed in recordings from the MTL (15, 16). In fact, the idea that the MTL represents information using a sparse code is one of the basic tenets of most contemporary memory models (17)(18)(19). Given that the MTL assigns a relatively small number of neurons to a specific stimulus (20), and that the number of stimuli in the environment is very large, does a feature such as personal relevance, which may be related to the incidental recollection of autobiographically significant information (3), make a stimulus more likely to elicit a selective excitatory response from a cell? Here, we address the question of whether individual neurons in the MTL show a preference for personally relevant pictures.In this study, patients with intracranial electrodes implanted for clinical reasons were shown photographs of varying personal relevance: previously unknown faces and...

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“…Indeed, even when a neuron in a given experiment responds to only one out of many images tested, it is assumed (quite plausibly) that it would fire to some other (untested) images (e.g. Waydo, Kraskov, Quiroga, Fried, & Koch, 2006). Accordingly, the most striking examples of selective neural responding are often taken as evidence for highly sparse and selective coding rather than for grandmother cells.…”

Section: Prefacementioning

confidence: 99%

Preface

Bowers¹

2017

Language, Cognition and Neuroscience

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Sparse Representation in the Human Medial Temporal Lobe

Cited by 154 publications

References 20 publications

Sparse and distributed coding of episodic memory in neurons of the human hippocampus

Sparse and distributed coding of episodic memory in neurons of the human hippocampus

Human medial temporal lobe neurons respond preferentially to personally relevant images

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