this article. Because there is no visible cue in the hidden-platform water maze task, it would not help the animal find the platform. 3. Route system. Routes stored in the hippocampus can be written out to the cortex, so that directions necessary to reach a goal are associated with local views. This is the system detailed in section 2.5 (see also section 4.3). This system requires training for each step the animal must take; it cannot learn to associate local views with directions to distant goals without hippocampal help (through route replay). The Role of the Hippocampus 97 If there were a way to show the animal the route to the goal, it might be possible to train the route system even without a hippocampus. Whishaw, Cassell, and Jarrard (1995) and Schallert, Day, Weisend, and Sutherland (1996) both showed ways to train the route system directly and found that animals could learn to solve the water maze even with hippocampal lesions. Whishaw et al. (1995) trained animals with fimbria/fornix lesions to find a visible platform and then removed the visible platform. These animals concentrated their search where the platform had been. Schallert et al. (1996) used animals with kainate-colchicine hippocampal lesions. The animals were first trained with a large platform that filled almost the entire maze. Once the animals could reach that platform reliably, it was shrunk trial by trial until it was the same size as a typical platform in a water maze task. Again, the animals could learn to solve the water maze without a hippocampus. 4.3 Where Is the Route System? Although the data are not yet conclusive, we suggest that the most likely candidate for anatomical instantiation of the route system is from posterior parietal to posterior cingulate cortex. There is a lot of evide...

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ABSTRACT: It has been argued that the role of the hippocampus in memory is time-limited: during aperiod of memory consolidation, other brain regions such as the neocortex are said to acquire the ability to supportmemoryretentionandretrievalontheirown.Analternativeview is that retention and retrieval of memory for autobiographical episodes depend on the hippocampal complex, regardless of the age of the memory. We examined the participation of the hippocampal complex in a functionalmagneticresonanceimaging(fMRI)studyinwhichparticipants were asked to recollect autobiographical events that occurred either within the last 4years or more than 20 years ago. We found equivalent levels of hippocampal activation in both conditions in all participants (N �10). In addition, activation in neocortical regions did not differ as a function of the age of the memory, even though most of the recent memories recalled were less than 2years old and the remote memories morethan35yearsold.Theresultssupportthenotionthatthehippocampal complex participates in retention and recovery of even very old autobiographicalmemories,andplaceboundaryconditionsontheoriesof memory consolidation. Hippocampus 2001;11:707–714. ©2001 Wiley-Liss, Inc. KEY WORDS: hippocampus; consolidation; autobiographical memory; functional MRI; retrieval; medial temporal lobe

this paper, we report findings from structural neuroimaging, functional neuroimaging and cognitive psychological testing in a case of isolated retrograde amnesia with a unique and potentially illuminating lesion localization. We investigated two issues: (i) the neuropathology of isolated retrograde amnesia and (ii) the processes supporting preserved anterograde mnemonic function in isolated retrograde amnesia

Abstract. Consolidation has been implemented in two ways: as straight rehearsal of patterns or as pseudorehearsal, in which pseudoitems are created by sampling attractors or input–output combinations from the network. Although both implementations have been investigated by several authors, few have explored how it is decided which pattern or pseudoitem is consolidated. Controlling consolidation is not trivial, as it is susceptible to a corruption. In runaway consolidation, one or two patterns monopolize all consolidation resources and come to dominate the entire network. Runaway consolidation is analysed, and three solutions are explored. Suppressing transmission in the connections in which consolidation takes place is shown to work best. Placing bounds on connections or unlearning attractors also alleviates runaway consolidation, though less effectively so.

well characterized. Of particular relevance are the reversal properties of plasticity mechanisms found in the circuit. In vitro studies are now characterizing these

Since Marr’s (1971) classic work, theories of the hippocampus have been guided by four assumptions. 1. The hippocampus rapidly and automatically acquires information. 2. The hippocampus is a limited capacity storage system and consequently is subject to relatively rapid forgetting. 3. Information originally stored in the hippocampus becomes permanently stored in the cortex and independent of the hippocampus. 4. The hippocampus plays a critical role in the genesis of the cortical memories. The last three assumptions form the basis of what is called the standard theory of systems consolidation (Squire and Alvarez 1995). The major premise of this theory is that there is a dynamic reorganization in the location of memories as a function of their age. As new memories are formed, old memories get moved from the hippocampus to cortex. In its strong form, this theory predicts that, provided enough time, memories once dependent on the hippocampus can survive even if the hippocampus is completely removed. Numerous studies of humans and other animals with damage to the hippocampus have been conducted to evaluate this view. Recently, Bontempi, Frankland, and their colleagues (Bontempi

There is growing evidence that the brain regions involved in encoding an episode are partially reactivated when that episode is later remembered. That is, the process of remembering an episode involves literally returning to the brain state that was present during that episode. This article reviews studies of episodic and associative memory that provide support for the assertion that encoding regions are reactivated during subsequent retrieval. In the first section, studies are reviewed in which neutral stimuli were associated with different modalities of sensory stimuli or different valences of emotional stimuli. When the neutral stimuli were later used as retrieval cues, relevant sensory and emotion processing regions were reactivated. In the second section, studies are reviewed in which participants used different strategies for encoding stimuli. When the stimuli were later retrieved, regions associated with the different encoding strategies were reactivated. Together, these studies demonstrate not only that the encoding experience determines which regions are activated during subsequent retrieval but also that the same regions are activated during encoding and retrieval. In the final section, relevant questions are posed and discussed regarding the reactivation of encoding regions during retrieval.

State-dependent EEG in the hippocampus (HPC) has traditionally been divided into two activity patterns: theta, a large-amplitude, regular oscillation with a bandwidth of 3–12 Hz, and large-amplitude irregular activity (LIA), a less regular signal with broadband characteristics. Both of these activity patterns have been linked to the memory functions subserved by the HPC. Here we describe, using extracellular field recording techniques in naturally sleeping and urethane-anesthetized rats, a novel state present during deactivated stages of sleep and anesthesia that is characterized by a prominent large-amplitude and slow frequency (�1 Hz) rhythm. We have called this activity the hippocampal slow oscillation (SO) because of its similarity and correspondence with the previously described neocortical SO. Almost all hippocampal units recorded exhibited differential spiking behavior during the SO as compared with other states. Although the hippocampal SO occurred in situations similar to the neocortical SO, it demonstrated some independence in its initiation, coordination, and coherence. The SO was abolished by sensory stimulation or cholinergic agonism and was enhanced by increasing anesthetic depth or muscarinic receptor antagonism. Laminar profile analyses of the SO showed a phase shift and prominent current sink-source alternations in stratum lacunosum-moleculare of CA1. This, along with correlated slow oscillatory field and multiunit activity in superficial entorhinal cortex suggests that the hippocampal SO may be coordinated with slow neocortical activity through input arriving via the temporo-ammonic pathway. This novel state may present a favorable milieu for synchronization-dependent synaptic plasticity within and between hippocampal and neocortical ensembles. Key words: synchrony; state; sleep; memory; urethane; non-REM

that eventually led to the independent discipline of experimental psychology. In its early years, experimental psychology was concerned primarily with the study of Montreal, Quebec H3A 2B4 sensation, but by the turn of the century the interests