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The hippocampus and remote autobiographical memory
No full textIn Newsdesk (August, 2005),1 new evidence for the neuroanatomy of remote memory was reported. On the basis of the findings of the US team lead by Larry Squire,2 remote autobiographical memory was suggested to be independent of the medial temporal lobe but dependent on the neocortex. By contrast with previous hypotheses, this new proposal predicts that after damage to the medial temporal lobe only recent autobiographical memories should be impaired in neurological patients, whereas loss of both recent and old autobiographical memories implies additional damage in the neocortex. However, there is evidence not included in the Newsdesk article, that is problematic for this new prediction.
Two patients, NT and VC, were previously reported to have lesions restricted to the medial temporal lobe and exhibited loss of remote memories extending for decades. Patient NT presented with extensive and ungraded retrograde amnesia after a right temporal lobectomy.3 This patient had substantial difficulty recalling autobiographical memories dating to childhood. The neuropathological investigations revealed clear-cut sclerosis of the unresected left hippocampus, but all other cortical areas, including the previously removed right temporal lobe, were normal. Thus, it is tempting to conclude that her severe remote memory loss was a consequence of her bilateral hippocampus damage.
Detailed cognitive testing of the severely amnesic patient VC reinforces this conclusion. [4] and [5] On all retrograde memory tests, including the standard autobiographical memory interview, his results were equally poor over all periods tested: he had no autobiographical recollection from any period of his life. Qualitative MRI, MRI volumetry, voxel based morphometry, spectroscopy, and functional MRI showed that the primary abnormality was located in the hippocampus bilaterally. Only MRI volumetry identified a slight decrease of the left parahippocampal volume, but functional neuroimaging showed that this region was active in VC during memory retrieval. Therefore, investigations of VC suggest that the hippocampus is crucial for remembering one's personal past. This finding is consistent with those from other lesion and neuroimaging studies.6
The discrepancy in findings between patients such as NT and VC and those reported by Squire's team may depend on important differences in the patients' severity of amnesia. For example, Bayley and colleagues2 draw attention to the test results of patients (EP and GP) with selective damage to the temporal medial lobe who obtained maximum scores on the childhood portion of the autobiographical memory interview (9/9), by contrast with the very impaired score of VC (1/9). However, EP and GP's performance on other standard memory tests was only mildly impaired, whereas on similar tests VC barely could score any points.
We suggest, therefore, that questions regarding the neuroanatomy of remote memory, and particularly the role of the medial lobe and hippocampus, are far from resolved. Further studies of amnesic patients with well-documented and restricted lesions are needed to ascertain the critical anatomical structures affected in remote memory
The space for memory in posterior parietal cortex: Re-analyses of bottom-up attention data
No full textThe ventral (VPC) and dorsal sectors of the posterior parietal cortex are long known to mediate bottom-up and top-down attention to the external space. Because these regions also are implicated in retrieval of episodic memories, we proposed they also mediate attention to the internal (memory) space. One objection to this Attention to Memory hypothesis is that parietal regions involved in directing attention to percepts and memory are spatially adjacent but not overlapping, suggesting that different neural mechanisms are involved in each. This misalignment is most pronounced in VPC. Here, we re-examine fMRI data, and show that (1) different VPC subregions are associated with different aspects of bottom-up attention to the external space, (2) only VPC subregions showing invalid cue (but not oddball) effects overlap with those associated with episodic memory retrieval, leading us to conclude that (3) the same regions that signal unexpected percepts also signal unexpected memories. These findings are consistent with the 'overarching view' of VPC as deploying bottom-up attention during both perception and episodic memory retrieval, and suggest that the degree of anatomical convergence across the two domains depends on the correspondence between the specific bottom-up attention demands of perceptual and memory tasks
Cognitive contributions of the ventral parietal cortex: an integrative theoretical account.
No full textAlthough ventral parietal cortex (VPC) activations can be found in a variety of cognitive domains, these activations have been typically attributed to cognitive operations specific to each domain. In this article, we propose a hypothesis that can account for VPC activations across all the cognitive domains reviewed. We first review VPC activations in the domains of perceptual and motor reorienting, episodic memory retrieval, language and number processing, theory of mind, and episodic memory encoding. Then, we consider the localization of VPC activations across domains and conclude that they are largely overlapping with some differences around the edges. Finally, we assess how well four different hypotheses of VPC function can explain findings in various domains and conclude that a bottom-up attention hypothesis provides the most complete and parsimonious account
Response to Nelson et al.: ventral parietal subdivisions are not incompatible with an overarching function.
No full texthttp://www.sciencedirect.com/science/article/pii/S136466131200153
Top-down and bottom-up attention to memory: a hypothesis (AtoM) on the role of the posterior parietal cortex in memory retrieval.
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Top-down and bottom-up attention-to-memory: Mapping functional connectivity in two distinct networks that underlie cued and uncued recognition memory.
No full textThe objective of this study was to examine the functional connectivity of brain regions active during cued and uncued recognition memory to test the idea that distinct networks would underlie these memory processes, as predicted by the attention-to-memory (AtoM) hypothesis. The AtoM hypothesis suggests that dorsal parietal cortex (DPC) allocates effortful top-down attention to memory retrieval during cued retrieval, whereas ventral parietal cortex (VPC) mediates spontaneous bottom-up capture of attention by memory during uncued retrieval. To identify networks associated with these two processes, we conducted a functional connectivity analysis of a left DPC and a left VPC region, both identified by a previous analysis of task-related regional activations. We hypothesized that the two parietal regions would be functionally connected with distinct neural networks, reflecting their engagement in the differential mnemonic processes. We found two spatially dissociated networks that overlapped only in the precuneus. During cued trials, DPC was functionally connected with dorsal attention areas, including the superior parietal lobules, right precuneus, and premotor cortex, as well as relevant memory areas, such as the left hippocampus and the middle frontal gyri. During uncued trials, VPC was functionally connected with ventral attention areas, including the supramarginal gyrus, cuneus, and right fusiform gyrus, as well as the parahippocampal gyrus. In addition, activity in the DPC network was associated with faster response times for cued retrieval. This is the first study to show a dissociation of the functional connectivity of posterior parietal regions during episodic memory retrieval, characterized by a top-down AtoM network involving DPC and a bottom-up AtoM network involving VPC
Top-down and bottom-up attention to memory are dissociated in posterior parietal cortex: neuroimagingand and neuropsychological evidence.
No full textFunctional Interplay Between Posterior Parietal Cortex and Hippocampus During Detection of Memory Targets and Non-targets
Full text linkPosterior parietal cortex is frequently activated during episodic memory retrieval but its role during retrieval and its interactions with the hippocampus are not yet clear. In this fMRI study, we investigated the neural bases of recognition memory when study repetitions and retrieval goals were manipulated. During encoding participants studied words either once or three times, and during retrieval they were rewarded more to detect either studied words or new words. We found that (1) dorsal parietal cortex (DPC) was more engaged during detection of items studied once compared to three times, whereas regions in the ventral parietal cortex (VPC) responded more to items studied multiple times; (2) DPC, within a network of brain regions functionally connected to the anterior hippocampus, responded more to items consistent with retrieval goals (associated with high reward); (3) VPC, within a network of brain regions functionally connected to the posterior hippocampus, responded more to items not aligned with retrieval goals (i.e., unexpected). These findings support the hypothesis that DPC and VPC regions contribute differentially to top-down vs. bottom-up attention to memory. Moreover, they reveal a dissociation in the functional profile of the anterior and posterior hippocampi
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