Hippocampal, but not parahippocampal, damage in a case of dense retrograde amnesia: a pathological study

Entorhinal cortex and consolidated memory

The entorhinal cortex is thought to support rapid encoding of new associations by serving as an interface between the hippocampus and neocortical regions. Although the entorhinal-hippocampal interaction is undoubtedly essential for initial memory acquisition, the entorhinal cortex contributes to memory retrieval even after the hippocampus is no longer necessary. This suggests that during memory consolidation additional synaptic reinforcement may take place within the cortical network, which may change the connectivity of entorhinal cortex with cortical regions other than the hippocampus. Here, I outline behavioral and physiological findings which collectively suggest that memory consolidation involves the gradual strengthening of connection between the entorhinal cortex and the medial prefrontal/anterior cingulate cortex (mPFC/ACC), a region that may permanently store the learned association. This newly formed connection allows for close interaction between the entorhinal cortex and the mPFC/ACC, through which the mPFC/ACC gains access to neocortical regions that store the content of memory. Thus, the entorhinal cortex may serve as a gatekeeper of cortical memory network by selectively interacting either with the hippocampus or mPFC/ACC depending on the age of memory. This model provides a new framework for a modification of cortical memory network during systems consolidation, thereby adding a fresh dimension to future studies on its biological mechanism.

Human amnesia and the medial temporal lobe illuminated by neuropsychological and neurohistological findings for patient E.P

We present neurohistological information for a case of bilateral, symmetrical damage to the medial temporal lobe and well-documented memory impairment. E.P. developed profound memory impairment at age 70 y and then was studied for 14 y He had no capacity for learning facts and events and had retrograde amnesia covering several decades. He also had a modest impairment of semantic knowledge. Neurohistological analysis revealed bilaterally symmetrical lesions of the medial temporal lobe that eliminated the temporal pole, the amygdala, the entorhinal cortex, the hippocampus, the perirhinal cortex, and rostral parahippocampal cortex. The lesion also extended laterally to involve the fusiform gyrus substantially. Last, the superior, inferior, and middle temporal gyri were atrophic, and subjacent white matter was gliotic. Several considerations indicate that E.P.'s severe memory impairment was caused by his medial temporal lesions, whereas his impaired semantic knowledge was caused by lateral temporal damage. His lateral temporal damage also may have contributed to his extensive retrograde amnesia. The findings illuminate the anatomical relationship between memory, perception, and semantic knowledge.

The role of the hippocampus in long-term memory: is it memory store or comparator?

Several attempts have been made to reconcile a number of rival theories on the role of the hippocampus in long-term memory. Those attempts fail to explain the basic effects of the theories from the same point of view. We are reviewing the four major theories, and shall demonstrate, with the use of mathematical models of attention and memory, that only one theory is capable of reconciling all of them by explaining the basic effects of each theory in a unified fashion, without altogether sacrificing their individual contributions. The key issue here is whether or not a memory trace is ever stored in the hippocampus itself, and there is no reconciliation unless the answer to that question is that there is not. As a result of the reconciliation that we are proposing, there is a simple solution to several outstanding problems concerning the neurobiology of memory such as: consolidation and reconsolidation, persistency of long term memory, novelty detection, habituation, long-term potentiation, and the multifrequency oscillatory self-organization of the brain.

The neuroscience of remote memory

Recently, there has been renewed interest in the organization and neurobiology of remote memory, and the pace of work in this area has accelerated. Yet the recent literature does not suggest that a consensus is developing, and there is disagreement about both facts and their interpretation. This article undertakes a comprehensive review of the three kinds of evidence that have been most prominent in recent discussion: studies of retrograde amnesia in memory-impaired patients who have well-characterized lesions, neuroimaging of healthy volunteers, and work with experimental animals including lesion studies, imaging and mouse genetics. The available evidence tells a coherent story and leads to some straightforward conclusions about the neuroscience of remote memory.

The fate of old memories after medial temporal lobe damage

Damage to the hippocampal region and related medial temporal lobe structures (perirhinal, entorhinal, and parahippocampal cortices) impairs new learning (anterograde amnesia) as well as memory for information that was acquired before the damage occurred (retrograde amnesia). We assessed retrograde amnesia with the Autobiographical Memory Interview (AMI) and with a news events test in six patients with damage limited primarily to the hippocampal region (H group) and two patients with large medial temporal lobe lesions (MTL group). On the news event test, the H group exhibited temporally limited retrograde amnesia covering approximately 5 years. On the same test, the MTL group exhibited an extensive retrograde amnesia covering decades. Nevertheless, performance was relatively spared for very remote time periods. On the AMI, all patients had intact remote autobiographical memory. Because our patients with hippocampal lesions, as well as our patients with large MTL lesions, performed normally on the AMI, patients who perform poorly on the same test presumably have damage beyond the hippocampus and related structures in the medial temporal lobe. The findings emphasize the difference in the extent of retrograde amnesia associated with hippocampal lesions and large MTL lesions.

The cognitive neuroscience of remote episodic, semantic and spatial memory

The processes and mechanisms implicated in retention and retrieval of memories as they age is an enduring problem in cognitive neuroscience. Research from lesion and functional neuroimaging studies on remote episodic, semantic and spatial memory in humans is crucial for evaluating three theories of hippocampal and/or medial temporal lobe-neocortical interaction in memory retention and retrieval: cognitive map theory, standard consolidation theory and multiple trace theory. Each theory makes different predictions regarding first, the severity and extent of retrograde amnesia following lesions to some or all of the structures mentioned; second, the extent of activation of these structures to retrieval of memory across time; and third, the type of memory being retrieved. Each of these theories has strengths and weaknesses, and there are various unresolved issues. We propose a unified account based on multiple trace theory. This theory states that the hippocampus is needed for re-experiencing detailed episodic and spatial memories no matter how old they are, and that it contributes to the formation and assimilation of semantic memories and schematic spatial maps.

Effects of Temporal Lobe Epilepsy on Retrograde Memory

PURPOSE

In a previous investigation (Lah et al., 2004), we found deficits in retrograde memory in patients who had undergone temporal lobectomy (TL). In this study, we set out to determine whether such deficits are present before surgery in patients with temporal lobe epilepsy (TLE).

METHODS

Memory for public and autobiographic facts and events was assessed in patients with focal left-sided (n=15) or right-sided (n=14) TLE and healthy control subjects (n=15). The impact of epilepsy and underlying cognitive deficits on retrograde memory also was examined.

RESULTS

Patients with left TLE demonstrated retrograde memory deficits across domains. Patients with right TLE showed defective recall only in the autobiographic domain. Young age at onset (younger than 14 years) was associated with greater difficulties in recall of famous events, and patients receiving polytherapy had significantly reduced recall of autobiographic events compared with those receiving monotherapy. In most cases, deficient memory for the past was associated with impairments in other cognitive skills, especially language abilities.

CONCLUSIONS

In unoperated-on patients with TLE, we found deficits in retrograde memory that were similar to those seen after TL, with the pattern of deficits being influenced by side of lesion, anticonvulsant medication, and word-finding deficits. Unlike patients tested after right TL, patients with right TLE did not have difficulty recalling details of famous events, which raises the possibility that right TL results in a decline in this aspect of retrograde memory.

Recollection and familiarity in dense hippocampal amnesia: A case study

In the amnesia literature, disagreement exists over whether anterograde amnesia involves recollective-based recognition processes and/or familiarity-based ones depending on whether the anatomical damage is restricted to the hippocampus or also involves adjacent areas, particularly the entorhinal and perirhinal cortices. So far, few patients with well documented anatomical lesions and detailed assessment of recollective and recognition performance have been described. We report a comprehensive neuroanatomical assessment and detailed investigation of the anterograde memory functions of a previously described severe amnesic patient (VC). The results of four previously published neuroradiological investigations (resting PET, qualitative MRIs, volumetric MRI and functional MRI) together with the results of two new investigations (voxel-based morphometry and magnetic resonance spectroscopy) are presented. The consistent finding across these different qualitative and quantitative examinations of VC's brain has shown that there is primarily structural and functional abnormality located selectively in the hippocampus bilaterally. Marked impairments in both verbal and non-verbal recall and recognition standardized memory tests were documented in the context of VC's intact cognitive profile and normal semantic memory. The results of five new experimental recognition memory tests tapping recollection and familiarity using verbal, topographical (buildings and landscapes) and unknown human faces memoranda revealed striking differential effects according to the type of stimuli used. A receiver operating characteristic analysis revealed that VC's recollective- and familiarity-based recognition processes were well preserved for unknown human faces. In contrast, recollective-based recognition for verbal and topographical material was at floor. Familiarity-based recognition was also impaired, significantly below controls for verbal and buildings memoranda and quite weak, although not reaching significance, for landscapes. These data suggest that the hippocampus is involved in recollective processes of verbal and topographical stimuli. It also plays an appreciable role in familiarity processes for these stimuli. However, recollection and familiarity of human faces appear not to depend on this region.

Pathology of pure hippocampal sclerosis in a patient with dementia and Hodgkin's disease: the Ophelia syndrome

An archive autopsy case of a 50-year-old man who died of Hodgkin's lymphoma had a 4-year, 4-month history of dementia. After radiochemotherapy, the lymphoma subsided except for involvement of the spleen, but the dementia remained. Neuropathological examination revealed that the pathology was confined to the hippocampus, both hippocampi showing sclerosis without inflammation. Neurons of sector cornu ammonis (CA) 1 were completely lost whereas moderate neuron loss was also observed in sectors CA3 and 4, and the dentate gyrus. Neurons of sector CA2 were relatively well preserved and the subiculum was intact. There was no evidence of global hypoxia, or of neurodegenerative disorders with pathological changes affecting the hippocampus. Although there was a long preneoplastic history, and no inflammatory changes were found at autopsy, the present case of hippocampal sclerosis could be included in the category of paraneoplastic limbic encephalitis associated with Hodgkin's lymphoma or the Ophelia syndrome.

Retrieval of autobiographical memory in Alzheimer's disease: Relation to volumes of medial temporal lobe and other structures

The representation of autobiographical memory is distributed over a network of brain structures, with the medial temporal lobe (MTL) at its epicenter. Some believe that, over time, all memories become independent of their MTL component ("consolidation theories"). Others have suggested that this is true only of semantic memory, while episodic aspects of autobiographical memories are dependent on the MTL for as long as they exist, such as multiple trace theory (MTT). In the present study, the volumes of 28 brain regions, including the MTL, and their relation to autobiographical memory were investigated in a group of patients with Alzheimer's disease with varying degrees of retrograde memory loss as assessed by the Autobiographical Memory Interview (AMI). We used the multivariate analysis method of partial least squares (PLS) to assess patterns of atrophy that can lead to retrograde amnesia. We found that different aspects of autobiographical memory were associated with different patterns of tissue loss. Personal semantics were related to a pattern of bilateral anterior and posterior lateral temporal cortex degeneration, more pronounced on the left, as well as right frontal degeneration. Autobiographical event memory ("episodic") was associated with combined atrophy in bilateral MTL and anterior lateral temporal neocortex, more pronounced on the right. This pattern was invariant for memories from childhood, early adulthood, and recent memories, in line with the predictions of MTT, suggesting that MTL tissue is crucial for retrieval of episodic memories regardless of their age.

Preclinical research on stress, memory, and the brain in the development of pharmacotherapy for depression

We have reviewed two areas of research on stress, memory, and synaptic plasticity which may be relevant toward understanding the neurobiology of major depressive disorder (MDD). First, we have presented the view that the hippocampus (HC) and prefrontal cortex (PFC) function jointly as a memory system which enables multitask processing (working memory) and consolidation of contextual information. The amygdala, by contrast, is necessary for the consolidation of emotional memories. Cognitive and neurophysiological studies have shown that HC-PFC processing is impaired, and amygdaloid processing is enhanced, by stress and in anxiety and mood disorders, including MDD. Second, we have reviewed research on the behavioral and neurophysiological actions of tianeptine, an antidepressant that is known to block the adverse effects of chronic stress on hippocampal morphology. Recent work has shown that acute tianeptine enhances cognitive and electrophysiological measures of HC-PFC functioning without interfering with the emotion-induced enhancement of amygdaloid functioning in rodents. We conclude with a synthesis of the preclinical and clinical literature on stress, memory, and tianeptine with the proposal that tianeptine should enhance HC-PFC memory-related processing in people under stress.