Early Detection of Semantic Memory Changes May Help Predict the Course of Alzheimer's Disease

Hsin-Te Chang, M. Chiu, M. Hua
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The earliest neuropathological changes in patients with AD may correlate more strongly with semantic memory than episodic memory [2,8,9]. Therefore, detecting early changes in the semantic memory of AD patients may be clinically important. Semantic memory impairment has been well-documented among patients with dementia of the Alzheimer type (DAT) [10-19] and among patients with prodromal stage of DAT (amnestic mild cognitive impairment, aMCI) [20-29]. Semantic memory deficits may occur early in the disease course of AD and therefore comprise useful markers of disease progression. However, the predictive value of semantic memory impairment remains controversial [20,22,27,29]. The previously observed inconsistencies may be attributable to distinct processes of semantic memory retrieval [10,30]. Many complex mental operations associated with semantic memory retrieval can be performed with minimal attentional capacity by dint of extensive practice. These types of mental operations are considered ‘automatic’. Conversely, other tasks that involve semantic memory retrieval require considerable attentional capacity to perform and are commonly referred to as ‘effortful’. AD patients display disproportionally poor performance on semantic tasks that require the effortful retrieval of semantic memories, whereas automatic retrieval tends to be better preserved [10]. Chang et al. [31]. Compared effortful and automatic retrieval of semantic memory among individuals with aMCI [32], DAT or subjective memory impairment (SMI) [1]. In this study, patients with DAT and aMCI-multiple domain (aMCI-md) [32] displayed poor performance on all semantic memory tasks. Conversely, patients with aMCI-single domain (aMCI-sd) [32] were found to have performed more poorly on a semantic memory task that required a relatively high degree of effortful retrieval. In addition, the risk of conversion from MCI to DAT (approximately 27 months after the first evaluation) among aMCI-sd patients who displayed poor performance on semantic memory tasks requiring effortful retrieval exceeded the risk faced by aMCI-sd patients who performed normally on the same tasks by more than two-fold in this study (50% vs. 23%). In contrast, aMCI-md patients who presented poor semantic memory in tasks that required automatic retrieval were at higher risk of conversion to DAT (58% vs. 38%). Nonetheless, the sensitivity of performance on the semantic memory task requiring high degree of effortful retrieval in predicting conversion from aMCI to DAT was relatively low compared to specificity in this study. Recent studies have suggested that combining performance on semantic memory tasks with biomarkers of AD may facilitate the prediction [22,28,33]. In addition, novel neurophysiological markers have been proposed by researchers to detect early neuropathological changes in AD and other cognitive disorders (e.g. increased excitability of motor cortex in transcranial magnetic stimulation studies [34-38]). The relatively low sensitivity of semantic memory performance in predicting DAT conversion among aMCI patients in Chang et al. [31] may be due to that the study did not incorporate the biomarkers of AD in the prediction. Moreover, combining other tasks requiring effortful semantic memory retrieval may also improve the sensitivity in predicting DAT conversion among aMCI patients [39-41].","PeriodicalId":15012,"journal":{"name":"Journal of Alzheimers Disease & Parkinsonism","volume":"56 1","pages":"1-2"},"PeriodicalIF":0.0000,"publicationDate":"2017-06-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"1","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"Journal of Alzheimers Disease & Parkinsonism","FirstCategoryId":"1085","ListUrlMain":"https://doi.org/10.4172/2161-0460.1000333","RegionNum":0,"RegionCategory":null,"ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"","JCRName":"","Score":null,"Total":0}
引用次数: 1

Abstract

Copyright: © 2017 Chang HT, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Over the course of Alzheimer’s disease (AD), patients suffer from relentless progressive dementia. Characterization of at-risk stages of AD is crucial for targeted prevention of dementia [1]. Neuropathological evidence suggests that earliest AD affects declarative memory [24], which can be further categorized as either episodic memory or semantic memory [5,6]. Episodic memory deals with specific events of the past, whereas semantic memory deals with general knowledge of the world [6,7]. The earliest neuropathological changes in patients with AD may correlate more strongly with semantic memory than episodic memory [2,8,9]. Therefore, detecting early changes in the semantic memory of AD patients may be clinically important. Semantic memory impairment has been well-documented among patients with dementia of the Alzheimer type (DAT) [10-19] and among patients with prodromal stage of DAT (amnestic mild cognitive impairment, aMCI) [20-29]. Semantic memory deficits may occur early in the disease course of AD and therefore comprise useful markers of disease progression. However, the predictive value of semantic memory impairment remains controversial [20,22,27,29]. The previously observed inconsistencies may be attributable to distinct processes of semantic memory retrieval [10,30]. Many complex mental operations associated with semantic memory retrieval can be performed with minimal attentional capacity by dint of extensive practice. These types of mental operations are considered ‘automatic’. Conversely, other tasks that involve semantic memory retrieval require considerable attentional capacity to perform and are commonly referred to as ‘effortful’. AD patients display disproportionally poor performance on semantic tasks that require the effortful retrieval of semantic memories, whereas automatic retrieval tends to be better preserved [10]. Chang et al. [31]. Compared effortful and automatic retrieval of semantic memory among individuals with aMCI [32], DAT or subjective memory impairment (SMI) [1]. In this study, patients with DAT and aMCI-multiple domain (aMCI-md) [32] displayed poor performance on all semantic memory tasks. Conversely, patients with aMCI-single domain (aMCI-sd) [32] were found to have performed more poorly on a semantic memory task that required a relatively high degree of effortful retrieval. In addition, the risk of conversion from MCI to DAT (approximately 27 months after the first evaluation) among aMCI-sd patients who displayed poor performance on semantic memory tasks requiring effortful retrieval exceeded the risk faced by aMCI-sd patients who performed normally on the same tasks by more than two-fold in this study (50% vs. 23%). In contrast, aMCI-md patients who presented poor semantic memory in tasks that required automatic retrieval were at higher risk of conversion to DAT (58% vs. 38%). Nonetheless, the sensitivity of performance on the semantic memory task requiring high degree of effortful retrieval in predicting conversion from aMCI to DAT was relatively low compared to specificity in this study. Recent studies have suggested that combining performance on semantic memory tasks with biomarkers of AD may facilitate the prediction [22,28,33]. In addition, novel neurophysiological markers have been proposed by researchers to detect early neuropathological changes in AD and other cognitive disorders (e.g. increased excitability of motor cortex in transcranial magnetic stimulation studies [34-38]). The relatively low sensitivity of semantic memory performance in predicting DAT conversion among aMCI patients in Chang et al. [31] may be due to that the study did not incorporate the biomarkers of AD in the prediction. Moreover, combining other tasks requiring effortful semantic memory retrieval may also improve the sensitivity in predicting DAT conversion among aMCI patients [39-41].
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语义记忆变化的早期检测可能有助于预测阿尔茨海默病的病程
版权所有:©2017 Chang HT, et al。这是一篇根据知识共享署名许可协议发布的开放获取文章,该协议允许在任何媒体上不受限制地使用、分发和复制,前提是要注明原作者和来源。在阿尔茨海默病(AD)的病程中,患者会遭受持续进行性痴呆的折磨。阿尔茨海默病的高危阶段特征对于有针对性地预防痴呆症至关重要[1]。神经病理学证据表明,最早的AD会影响陈述性记忆[24],而陈述性记忆又可进一步分为情景记忆和语义记忆[5,6]。情景记忆处理的是过去的特定事件,而语义记忆处理的是对世界的一般认识[6,7]。AD患者最早的神经病理改变可能与语义记忆的相关性比情景记忆更强[2,8,9]。因此,检测阿尔茨海默病患者语义记忆的早期变化可能具有重要的临床意义。语义记忆障碍在阿尔茨海默型痴呆(DAT)患者[10-19]和DAT前驱期患者(遗忘性轻度认知障碍,aMCI)中有充分的文献记载[20-29]。语义记忆缺陷可能发生在阿尔茨海默病病程的早期,因此是疾病进展的有用标志。然而,语义记忆障碍的预测价值仍存在争议[20,22,27,29]。先前观察到的不一致性可能归因于不同的语义记忆检索过程[10,30]。许多与语义记忆检索相关的复杂心理操作可以通过广泛的练习以最小的注意力容量完成。这些类型的心理操作被认为是“自动的”。相反,其他涉及语义记忆检索的任务需要相当大的注意力才能完成,通常被称为“费力”。AD患者在需要费力检索语义记忆的语义任务中表现出不成比例的差,而自动检索往往保存得更好[10]。Chang等[31]。比较aMCI[32]、DAT或主观记忆障碍(SMI)个体语义记忆的费力和自动检索[1]。在本研究中,患有DAT和amci -多域(aMCI-md)的患者[32]在所有语义记忆任务中表现不佳。相反,amci -单域(aMCI-sd)患者[32]在语义记忆任务中表现更差,这需要相对高度的努力检索。此外,在本研究中,在需要费力检索的语义记忆任务中表现不佳的aMCI-sd患者中,从MCI转换为DAT的风险(大约在第一次评估后27个月)超过了在相同任务中表现正常的aMCI-sd患者所面临的风险两倍以上(50%对23%)。相比之下,在需要自动检索的任务中表现出较差语义记忆的aMCI-md患者转换为DAT的风险更高(58%对38%)。尽管如此,与本研究的特异性相比,需要高度努力检索的语义记忆任务在预测从aMCI到DAT转换方面的表现的敏感性相对较低。最近的研究表明,将语义记忆任务的表现与AD的生物标志物结合起来可能有助于预测[22,28,33]。此外,研究人员提出了新的神经生理标志物来检测阿尔茨海默病和其他认知障碍的早期神经病理变化(如经颅磁刺激研究中运动皮层兴奋性增加[34-38])。Chang等[31]在预测aMCI患者的数据转换时,语义记忆表现的敏感性相对较低,这可能是由于该研究没有将AD的生物标志物纳入预测。此外,结合其他需要费力语义记忆检索的任务也可能提高预测aMCI患者数据转换的敏感性[39-41]。
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