What's new with the amnesic patient H.M.?

Perceptual fluency, semantic familiarity and recognition-related familiarity: an electrophysiological exploration

Scalp recorded event-related potentials (ERPs) were used to examine the neuronal activity associated with perceptual fluency, semantic familiarity and recognition-related familiarity. We assume that ERP differences between first and second presentations of non-famous faces in an implicit memory condition reflect perceptual fluency, ERP differences between first presentations of famous and non-famous faces reflect semantic familiarity (i.e., familiarity arising from semantic memory retrieval), and early ERP differences between first and second presentations of non-famous and famous faces in an explicit recognition memory task reflect recognition-related familiarity. Semantic familiarity elicited a broadly distributed effect between 200 and 300 ms after stimulus onset, possibly representing the activation of face recognition units. Between 300 and 450 ms, frontal effects were observed for semantic familiarity and recognition-related familiarity, while perceptual fluency was associated with a centro-parietally focused effect. Thus, familiarity arising from the retrieval of semantic information and recognition-related familiarity depend at least partly on the same neuronal circuits, while these are dissociable from those mediating perceptual fluency.

The case of K.C.: contributions of a memory-impaired person to memory theory

K.C. has been investigated extensively over some 20 years since a motorcycle accident left him with widespread brain damage that includes large bilateral hippocampal lesions, which caused a remarkable case of memory impairment. On standard testing, K.C.'s anterograde amnesia is as severe as that of any other case reported in the literature, including H.M. However, his ability to make use of knowledge and experiences from the time before his accident shows a sharp dissociation between semantic and episodic memory. A good deal of his general knowledge of the world, including knowledge about himself, is preserved, but he is incapable of recollecting any personally experienced events. In displaying such "episodic amnesia," which encompasses an entire lifetime of personal experiences, K.C. differs from many other amnesic cases. Here, we document for the first time the full extent of K.C.'s brain damage using MRI-based quantitative measurements. We then review the many investigations with K.C. that have contributed to our understanding not only of episodic and semantic memory but also to the development of other aspects of memory theory. These include the distinction between implicit and explicit memory, the prospect of new learning in amnesia, and the fate of recent and remote memory for autobiographical and public events, people, and spatial locations.

Puzzling thoughts for H. M.: can new semantic information be anchored to old semantic memories?

Researchers currently debate whether new semantic knowledge can be learned and retrieved despite extensive damage to medial temporal lobe (MTL) structures. The authors explored whether H. M., a patient with amnesia, could acquire new semantic information in the context of his lifelong hobby of solving crossword puzzles. First, H. M. was tested on a series of word-skills tests believed important in solving crosswords. He also completed 3 new crosswords: 1 puzzle testing pre-1953 knowledge, another testing post-1953 knowledge, and another combining the 2 by giving postoperative semantic clues for preoperative answers. From the results, the authors concluded that H. M. can acquire new semantic knowledge, at least temporarily, when he can anchor it to mental representations established preoperatively.

Failure to acquire new semantic knowledge in patients with large medial temporal lobe lesions

We examined new semantic learning in two profoundly amnesic patients (E.P. and G.P.) whose lesions involve virtually the entire medial temporal lobe (MTL) bilaterally. The patients were given five tests of semantic knowledge for information that could only have been acquired after the onset of their amnesia in 1992 and 1987, respectively. Age-matched and education-matched controls (n = 8) were also tested. On tests of recall, E.P. and G.P. each scored 10% correct on a test of 20 easy factual questions (controls = 90%), 2% and 4% correct on 55 questions about news events (controls = 85%), and 0% and 4% correct on a test of 24 famous faces. On three tests of recognition memory for this same material, the patients scored at chance levels. Similarly, the patients were unable to judge whether persons who had been famous for many decades were still living or had died during the past 10 years (E.P. = 53%; G.P. = 50%; controls = 73%; chance = 50%). Lastly, neither patient E.P. nor patient G.P. could draw an accurate floor plan of his current residence, despite having lived there for 10 years and 1 year, respectively. The results demonstrate that the capacity for new semantic learning can be absent, or nearly absent, when there is virtually complete damage to the MTL bilaterally. Accordingly, the results raise the possibility that the acquisition of conscious (declarative) knowledge about the world cannot be supported by structures outside the MTL, even with extended exposure. Published 2004 Wiley-Liss, Inc.

Medial temporal lobe structures are needed to re-experience remote autobiographical memories: evidence from H.M. and W.R

The nature and extent of retrograde amnesia in patients with medial temporal lobe (MTL) lesions is currently under debate. While some investigators propose a temporally limited role for the MTL in episodic and semantic memory, others claim that MTL structures are needed for episodic memories of one's entire lifetime, and that only semantic memory becomes independent of the MTL. To address this issue, we tested two amnesic patients, H.M. and W.R., with bilateral MTL lesions on a series of remote memory tests that together distinguished episodic memory from semantic memory performance. Notably, we used a new method to assess autobiographical memory that measured the degree of re-experiencing of personal happenings from the past. Both patients showed relatively spared semantic memory, but severe impairment on measures of autobiographical memory, with no temporal gradient. Our data support the view that MTL structures play a significant role in recalling specific personal episodes, not only from the recent past but from the distant past as well.

Theta rhythm of navigation: Link between path integration and landmark navigation, episodic and semantic memory

Five key topics have been reverberating in hippocampal-entorhinal cortex (EC) research over the past five decades: episodic and semantic memory, path integration ("dead reckoning") and landmark ("map") navigation, and theta oscillation. We suggest that the systematic relations between single cell discharge and the activity of neuronal ensembles reflected in local field theta oscillations provide a useful insight into the relationship among these terms. In rats trained to run in direction-guided (1-dimensional) tasks, hippocampal cell assemblies discharge sequentially, with different assemblies active on opposite runs, i.e., place cells are unidirectional. Such tasks do not require map representation and are formally identical with learning sequentially occurring items in an episode. Hebbian plasticity, acting within the temporal window of the theta cycle, converts the travel distances into synaptic strengths between the sequentially activated and unidirectionally connected assemblies. In contrast, place representations by hippocampal neurons in 2-dimensional environments are typically omnidirectional, characteristic of a map. Generation of a map requires exploration, essentially a dead reckoning behavior. We suggest that omnidirectional navigation through the same places (junctions) during exploration gives rise to omnidirectional place cells and, consequently, maps free of temporal context. Analogously, multiple crossings of common junction(s) of episodes convert the common junction(s) into context-free or semantic memory. Theta oscillation can hence be conceived as the navigation rhythm through both physical and mnemonic space, facilitating the formation of maps and episodic/semantic memories.

A comparison and evaluation of the predictions of relational and conjunctive accounts of hippocampal function

Relational and conjunctive memory theory each postulate that the hippocampus participates in the formation of long-term memory representations comprised of associations between multiple elements. The goals of the current work were to clarify and contrast these theories by outlining the nature of the representations that are spared vs. impaired following hippocampal damage according to each theoretical perspective. Relational theory predicts that hippocampal lesions will impair performance on tasks that require the formation of new long-term representations in which distinct elements must be regarded in relation to all other elements. Representations that remain intact despite hippocampal damage include separate representations of distinct individual elements or multiple stimuli fused into a static "blend" such as several elements viewed from one vantage point. Additionally, the relational account predicts that rapid incidental online processing of the relations can be achieved through structures other than the hippocampus, but this information will not be stored. In contrast, conjunctive theory predicts that hippocampal damage will impair the rapid formation of unitary representations that contain features of elements and their relative relationships bound in an inflexible manner. Deficits in the rapid formation of these conjunctive representations result in impaired performance on tasks that require rapid incidental stimulus binding. However, intact formation of conjunctive representations can occur over multiple trials in the service of problem solving. Using these theoretical frameworks, recent findings from the human and nonhuman animal literature are reexamined in order to determine whether one theory better accounts for current findings. We discuss empirical studies that serve as "critical experiments" in addressing the relational vs. conjunctive debate, and find that the predictions of relational theory are supported by existing findings over those from the conjunctive account.

The temporal context model in spatial navigation and relational learning: toward a common explanation of medial temporal lobe function across domains

The medial temporal lobe (MTL) has been studied extensively at all levels of analysis, yet its function remains unclear. Theory regarding the cognitive function of the MTL has centered along 3 themes. Different authors have emphasized the role of the MTL in episodic recall, spatial navigation, or relational memory. Starting with the temporal context model (M. W. Howard & M. J. Kahana, 2002a), a distributed memory model that has been applied to benchmark data from episodic recall tasks, the authors propose that the entorhinal cortex supports a gradually changing representation of temporal context and the hippocampus proper enables retrieval of these contextual states. Simulation studies show this hypothesis explains the firing of place cells in the entorhinal cortex and the behavioral effects of hippocampal lesion in relational memory tasks. These results constitute a first step toward a unified computational theory of MTL function that integrates neurophysiological, neuropsychological, and cognitive findings.

Prefrontal and hippocampal contributions to visual associative recognition: Interactions between cognitive control and episodic retrieval

The ability to recover episodic associations is thought to depend on medial-temporal lobe mnemonic mechanisms and frontal lobe cognitive control processes. The present study examined the neural circuitry underlying non-verbal associative retrieval, and considered the consequences of successful retrieval on cognitive control demands. Event-related fMRI data were acquired while subjects retrieved strongly or weakly associated pairs of novel visual patterns in a two-alternative forced choice associative recognition paradigm. Behaviorally, successful retrieval of strongly associated relative to weakly associated pairs was more likely to be accompanied by conscious recollection of the pair's prior co-occurrence. At the neural level, right ventrolateral prefrontal cortex (VLPFC) and hippocampus were more active during successful retrieval of Strong than of Weak associations, consistent with a role in visual associative recollection. By contrast, Weak trials elicited greater activation in right anterior cingulate cortex (ACC), which may detect conflict between the similarly familiar target and foil stimuli in the absence of recollection. Consistent with this interpretation, stronger ACC activity was associated with weaker hippocampal and stronger right dorsolateral PFC (DLPFC) responses. Thus, recollection of relevant visual associations (hippocampus and VLPFC) results in lower levels of mnemonic conflict (ACC) and decreased familiarity-based monitoring demands (DLPFC). These findings highlight the interplay between cognitive control and episodic retrieval.

Hippocampus

The hippocampus serves a critical role in declarative memory--our capacity to recall everyday facts and events. Recent studies using functional brain imaging in humans and neuropsychological analyses of humans and animals with hippocampal damage have revealed some of the elemental cognitive processes mediated by the hippocampus. In addition, recent characterizations of neuronal firing patterns in behaving animals and humans have suggested how neural representations in the hippocampus underlie those elemental cognitive processes in the service of declarative memory.

Evidence for semantic learning in profound amnesia: an investigation with patient H.M

Until recently, it seemed unlikely that any semantic knowledge could be acquired following extensive bilateral damage to the medial temporal lobes (MTL). Although recent studies have demonstrated some semantic learning in amnesic patients, questions remain regarding the limits of this capacity and the extent to which it relies on those patients' residual MTL function. The present study examined whether detailed, semantic memory could be acquired by a patient with no functioning hippocampus. We used cued recall and forced-choice recognition tasks to investigate whether the patient H.M. had acquired knowledge of people who became famous after the onset of his amnesia. Results revealed that, with first names provided as cues, he was able to recall the corresponding famous last name for 12 of 35 postoperatively famous personalities. This number nearly doubled when semantic cues were added, suggesting that his knowledge of the names was not limited to perceptual information, but was incorporated in a semantic network capable of supporting explicit recall. In forced-choice recognition, H.M. discriminated 87% of postmorbid famous names from foils. Critically, he was able to provide uniquely identifying semantic facts for one-third of these recognized names, describing John Glenn, for example, as "the first rocketeer" and Lee Harvey Oswald as a man who "assassinated the president." Although H.M.'s semantic learning was clearly impaired, the results provide robust, unambiguous evidence that some new semantic learning can be supported by structures beyond the hippocampus proper.

Hippocampal activation during transitive inference in humans

Studies in rodents have demonstrated that the integration and flexible expression of memories, necessary for transitive inference, depend on an intact hippocampus. To test this hypothesis in humans, we studied brain activation during the discrimination of a series of overlapping and non-overlapping arbitrary visual stimulus pairs. We report that transitive inference about overlapping pairs is associated with right anterior hippocampal activation, whereas recognition of non-overlapping stimulus pairs is associated with bilateral medial temporal lobe activation centered in the anterior parahippocampal gyrus. We conclude that immediate access to simple stimulus-stimulus relationships is mediated via the parahippocampal gyrus, whereas the flexible representation of memory requires the recruitment of the hippocampus.

Memory traces of trace memories: neurogenesis, synaptogenesis and awareness

To associate events that are disparate in time, the brain must record, retain and perhaps even reflect on the individual events themselves. Aspects of such learning can be probed with trace conditioning, during which an animal learns to associate events that are temporally distant from one another. For decades, we have known that the formation of so-called trace memories (in which one stimulus is associated with a second stimulus that is discontinuous and later in time) depends on the hippocampal formation. Recent findings indicate that the hippocampus is crucial for the initial acquisition of trace memories but not for their expression or long-term storage. More recent findings implicate neurogenesis, synaptogenesis and awareness in the formation of trace memories.

Visual presentation of novel objects and new spatial arrangements of objects differentially activates the medial temporal lobe subareas in humans

A number of studies in rodents and monkeys report a distinction between the contributions of the hippocampus and perirhinal cortex to memory, such that the hippocampus is crucial for spatial memory whereas the perirhinal cortex has a pivotal role in perception and memory for visual objects. To determine if there is such a distinction in humans, we conducted a functional magnetic resonance imaging study to compare the medial temporal lobe responses to changes in object identity and spatial configurations of objects. We found evidence for the predicted distinction between hippocampal and perirhinal cortical activations, although part of the hippocampus was also activated by identification of novel objects. Additionally, an anterior-posterior activation gradient emerged inside the hippocampus and parahippocampal cortex. The anterior hippocampus, perirhinal cortex and anterior parahippocampal cortex are involved in perception of contextually novel objects, whereas the posterior hippocampus and posterior parahippocampal cortex are involved in processing of novel arrangements of familiar objects. These results demonstrate that there is a functional dissociation between processing of novel object identities and new spatial locations of objects among the subregions of medial temporal lobe structures in humans also.

The Neurobiology of Consolidations, Or, How Stable is the Engram?

Consolidation is the progressive postacquisition stabilization of long-term memory. The term is commonly used to refer to two types of processes: synaptic consolidation, which is accomplished within the first minutes to hours after learning and occurs in all memory systems studied so far; and system consolidation, which takes much longer, and in which memories that are initially dependent upon the hippocampus undergo reorganization and may become hippocampal-independent. The textbook account of consolidation is that for any item in memory, consolidation starts and ends just once. Recently, a heated debate has been revitalized on whether this is indeed the case, or, alternatively, whether memories become labile and must undergo some form of renewed consolidation every time they are activated. This debate focuses attention on fundamental issues concerning the nature of the memory trace, its maturation, persistence, retrievability, and modifiability.

Grundlagen und Empirie der Neuropsychologischen Therapie: Neuropsychotherapie oder Hirnjogging?

Zusammenfassung: Dieser Beitrag beschäftigt sich mit dem theoretischen Fundament und den empirischen Grundlagen neuropsychologischer Interventionen. Nach einem kurzen historischen Rückblick werden die konzeptionellen Grundlagen einer neuropsychologischen Behandlung dargestellt und erläutert. Neuropsychologische Interventionen werden drei Therapiesäulen zugeordnet: Restitution, Kompensation und integrierten Verfahren. Restitution bezieht sich auf Maßnahmen, mit denen gestörte neuronale Systeme wieder reaktiviert werden sollen. Kompensation beinhaltet den aktiven oder passiven Ausgleich von Funktionsdefiziten durch vorhandene Fähigkeiten. Hierzu zählt nicht nur das Lernen neuer Strategien und der Einsatz von Hilfsmitteln, sondern auch die Veränderung von Erwartungen, Lebenszielen und andere kognitive Anpassungen. Integrierte Verfahren stellen Verfahren (z. B. operante Techniken) anderer Psychotherapierichtungen dar, die bei einer neuropsychologischen Behandlung Anwendung finden können und auf die Bedürfnisse von Patienten mit organisch bedingten psychischen Störungen zugeschnitten sind. Die in dem Beitrag referierten Studien unterstreichen die Bedeutung der drei postulierten Therapiesäulen.

Can animals recall the past and plan for the future?

According to the 'mental time travel hypothesis' animals, unlike humans, cannot mentally travel backwards in time to recollect specific past events (episodic memory) or forwards to anticipate future needs (future planning). Until recently, there was little evidence in animals for either ability. Experiments on memory in food-caching birds, however, question this assumption by showing that western scrub-jays form integrated, flexible, trial-unique memories of what they hid, where and when. Moreover, these birds can adjust their caching behaviour in anticipation of future needs. We suggest that some animals have elements of both episodic-like memory and future planning.

Successful recollection of remote autobiographical memories by amnesic patients with medial temporal lobe lesions

Current views about the organization of human memory make strikingly different predictions about the integrity of remote autobiographical memory following damage to the medial temporal lobe. We have carried out a detailed analysis of narrative content in memory-impaired patients for whom neuropsychological and neuroanatomical information is available. All eight patients were able to recall detailed memories from their early lives. The recollections of the patients and the recollections of 25 matched controls contained the same number of details (+/-5%) and were also similar by several other measures. The results support the view that autobiographical memories eventually become independent of the medial temporal lobe as time passes after learning. A number of other considerations suggest that the neocortex ultimately supports the capacity for recollecting remote autobiographical memory.

Impact of aging on hippocampal function: plasticity, network dynamics, and cognition

Aging is associated with specific impairments of learning and memory, some of which are similar to those caused by hippocampal damage. Studies of the effects of aging on hippocampal anatomy, physiology, plasticity, and network dynamics may lead to a better understanding of age-related cognitive deficits. Anatomical and electrophysiological studies indicate that the hippocampus of the aged rat sustains a loss of synapses in the dentate gyrus, a loss of functional synapses in area CA1, a decrease in the NMDA-receptor-mediated response at perforant path synapses onto dentate gyrus granule cells, and an alteration of Ca(2+) regulation in area CA1. These changes may contribute to the observed age-related impairments of synaptic plasticity, which include deficits in the induction and maintenance of long-term potentiation (LTP) and lower thresholds for depotentiation and long-term depression (LTD). This shift in the balance of LTP and LTD could, in turn, impair the encoding of memories and enhance the erasure of memories, and therefore contribute to cognitive deficits experienced by many aged mammals. Altered synaptic plasticity may also change the dynamic interactions among cells in hippocampal networks, causing deficits in the storage and retrieval of information about the spatial organization of the environment. Further studies of the aged hippocampus will not only lead to treatments for age-related cognitive impairments, but may also clarify the mechanisms of learning in adult mammals.

On the origins of autobiographical memory

Tolving argues that one form of explicit memory, autobiographical memory is uniquely human and has no nonhuman animal antecedents. We suggest that a form of memory used by humans and nonhuman animals, dead reckoning, shares a common limbic structure, including the cingulate cortex and hippocampus, and involves similar processes in recognition of self-action. Thus, it may be homologous to, and an antecedent of, autobiographical memory in humans.

Cellular and systems reconsolidation in the hippocampus

Cellular theories of memory consolidation posit that new memories require new protein synthesis in order to be stored. Systems consolidation theories posit that the hippocampus has a time-limited role in memory storage, after which the memory is independent of the hippocampus. Here, we show that intra-hippocampal infusions of the protein synthesis inhibitor anisomycin caused amnesia for a consolidated hippocampal-dependent contextual fear memory, but only if the memory was reactivated prior to infusion. The effect occurred even if reactivation was delayed for 45 days after training, a time when contextual memory is independent of the hippocampus. Indeed, reactivation of a hippocampus-independent memory caused the trace to again become hippocampus dependent, but only for 2 days rather than for weeks. Thus, hippocampal memories can undergo reconsolidation at both the cellular and systems levels.

Memory consolidation and the amygdala: a systems perspective

The basolateral region of the amygdala (BLA) plays a crucial role in making significant experiences memorable. There is extensive evidence that stress hormones and other neuromodulatory systems activated by arousing training experiences converge in regulating noradrenaline-receptor activity within the BLA. Such activation of the BLA modulates memory consolidation via BLA projections to many brain regions involved in consolidating lasting memory, including the hippocampus, caudate nucleus, nucleus basalis and cortex. Investigation of the involvement of BLA projections to other brain regions is essential for understanding influences of the amygdala on different aspects and forms of memory.