We review lesion and neuroimaging evidence on the role of the hippocampus, and other structures, in retention and retrieval of recent and remote memories. We examine episodic, semantic and spatial memory, and show that important distinctions exist among different types of these memories and the structures that mediate them. We argue that retention and retrieval of detailed, vivid autobiographical memories depend on the hippocampal system no matter how long ago they were acquired. Semantic memories, on the other hand, benefit from hippocampal contribution for some time before they can be retrieved independently of the hippocampus. Even semantic memories, however, can have episodic elements associated with them that continue to depend on the hippocampus. Likewise, we distinguish between experientially detailed spatial memories (akin to episodic memory) and more schematic memories (akin to semantic memory) that are sufficient for navigation but not for re-experiencing the environment in which they were acquired. Like their episodic and semantic counterparts, the former type of spatial memory is dependent on the hippocampus no matter how long ago it was acquired, whereas the latter can survive independently of the hippocampus and is represented in extra-hippocampal structures. In short, the evidence reviewed suggests strongly that the function of the hippocampus (and possibly that of related limbic structures) is to help encode, retain, and retrieve experiences , no matter how long ago the events comprising the experience occurred, and no matter whether the memories are episodic or spatial. We conclude that the evidence favours a multiple trace theory (MTT) of memory over two other models: (1) traditional consolidation models which posit that the hippocampus is a time-limited memory structure for all forms of memory; and (2) versions of cognitive map theory which posit that the hippocampus is needed for representing all forms of allocentric space in memory.

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Functional magnetic resonance imaging was used to study brain regions implicated in retrieval of memories that are decades old. To probe autobiographical memory, family photographs were selected by confederates without the participant's involvement, thereby eliminating many of the variables that potentially confounded previous neuroimaging studies. We found that context-rich memories were associated with activity in lingual and precuneus gyri independently of their age. By contrast, retrosplenial cortex was more active for recent events regardless of memory vividness. Hippocampal activation was related to the richness of re-experiencing (vividness) rather than the age of the memory per se. Remote memories were associated with distributed activation along the rostrocaudal axis of the hippocampus whereas activation associated with recent memories was clustered in the anterior portion. This may explain why circumscribed lesions to the hippocampus disproportionately affect recent memories. These findings are incompatible with theories of long-term memory consolidation, and are more easily accommodated by multiple-trace theory, which posits that detailed memories are always dependent on the hippocampus.

Anterograde amnesia following hippocampal damage involves the loss of the capacity to form new declarative memories but leaves nondeclarative memory processes intact. Current theories of declarative memory suggest the existence of two complementary memory systems: a hippocampal-based system that specializes in rapid acquisition of specific events and a neocortical system that slowly learns through environmental statistical regularities and requires the initial support of the hippocampal system. Contrary to this notion, we demonstrate a neurocognitive mechanism that enables rapid acquisition of novel arbitrary associations independently of the hippocampus. This mechanism has been dubbed "fast mapping" (FM) and is believed to support the rapid acquisition of vocabulary in children as young as 16 mo of age. We used FM to teach novel word-picture associations to four profoundly amnesic patients with hippocampal system damage. Patients were able to acquire arbitrary associations through FM normally, despite profound impairment on a matched standard associative memory task. Most importantly, they retained what they learned through FM after a week's delay, when they were around chance level on the standard task. By contrast, two patients with unilateral damage to the left polar temporal neocortex were impaired on FM, suggesting that this cortical region is critical for associative learning through FM. Left perirhinal and entorhinal cortices might also play a role in learning through FM. Contrary to current theories, these findings indicate that rapid acquisition of declarativelike (relational) memory can be accomplished independently of the hippocampus and that neocortical plasticity can be induced rapidly to support novel arbitrary associations.consolidation | medial temporal lobe | neocortex | semantic memory M emory consolidation is a gradual, time-dependent, reorganization process by which memories become stable. Systems consolidation of declarative memory is thought to occur when memories that are initially supported by both the hippocampus and the neocortex become hippocampus-independent over periods of time that range from weeks to years (1-3). Systems consolidation is thought to be gradual because rapid acquisition of novel information by the neocortex would cause interference with existing knowledge structures ["catastrophic interference" (1)]. Therefore, novel associations always depend initially on the hippocampus, which specializes in rapid representation of novel associations or arbitrary relations. The hippocampus, in turn, supports the gradual changes in neocortical connections that allow for the incorporation of novel information into existing knowledge structures (1-4). We hypothesized that an exception to this rule might be observed in a process called "fast mapping" (FM) (5, 6), which supports the astounding ability of toddlers as young as 16 mo of age (7) to acquire rapidly vast numbers of novel word-referent associations. We predicted that adults with lesions to the hippocampus might be able to lea...

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The 'temporality' hypothesis of confabulation posits that confabulations are true memories displaced in time, while the 'strategic retrieval' hypothesis suggests a general retrieval failure of which temporal confusion is a common symptom. Four confabulating patients with rupture of an anterior communicating artery (ACoA) aneurysm, eight non-confabulating ACoA controls and 16 normal controls participated in three experiments designed to test the two hypotheses. In Experiment 1, participants were tested on two continuous recognition tasks, one requiring temporal context distinctions, previously shown to be sensitive to confabulation and another that only requires content distinctions. Both manipulations were sensitive to confabulation, but not specific to it. Temporal context and content confusions (TCCs and CCs) can be explained as failures to make fine-grained distinctions within memory. In Experiment 2, free recall of semantic narratives that require strategic retrieval but are independent of temporal context was used to induce confabulations associated with remote memory, acquired before the onset of amnesia. Confabulators produced significantly more errors. Thus, when retrieval demands are equated, confabulations can be induced in the absence of temporal confusions. Only confabulators conflated semantic content from different remote semantic narratives and introduced idiosyncratic content, suggesting that qualitatively different mechanisms are responsible for distortions due to normal memory failure and for confabulation. Lesion analyses revealed that damage to ventromedial prefrontal cortex is sufficient for temporal context errors to occur, but additional orbitofrontal damage is crucial for spontaneous confabulation. In Experiment 3, we tested whether failure in memory monitoring is crucial for confabulation. Recognition of details from semantic and autobiographical narratives was used to minimize the initiation and search components of strategic retrieval. Only confabulators made more false alarms on both tasks, endorsed even highly implausible lures related to autobiographical events and were indiscriminately confident about their choices. These findings support a strategic retrieval account of confabulation of which monitoring is a critical component. Post-retrieval monitoring has at least two components: one is early, rapid and pre-conscious and the other is conscious and elaborate. Failure of at least the former is necessary and sufficient for confabulation. Other deficits, including TCC and CC, may be required for spontaneous confabulations to arise. The confluence of different sub-components of strategic retrieval would determine the content of confabulation and exacerbate its occurrence.

Smaller hippocampal volume is not a necessary risk factor for developing PTSD and does not occur within 6 months of expressing the disorder. This brain abnormality might occur in individuals with chronic or complicated PTSD.