University of Pennsylvania, Philadelphia, USA.
Cogn Neuropsychol. 2006;23(4):541-62. doi: 10.1080/02643290542000094.
Impaired semantic memory is ubiquitous in frontotemporal dementia (FTD), including patients with semantic dementia (SD), progressive nonfluent aphasia (PNFA) and nonaphasic FTD patients with a deficit in executive and social functioning (EXEC/SOC). One hypothesis attributes this to the degradation of specific categories of knowledge in semantic memory. This study explores the alternate hypothesis that impaired semantic memory in FTD can also reflect limitations in the categorisation processes that determine object meaning. Patients were taught a novel semantic category under two conditions: rule-based categorisation, where executive resources support the evaluation of specific features to determine category membership; and similarity-based categorisation, where category membership is determined by the overall resemblance of an item to a prototype or recalled exemplars. In the first experiment, patients learned a novel category composed of highly salient features. For SD patients, we found category membership judgment profiles following rule-based and similarity-based training that resembled the performance of control subjects. Categorisation was impaired following rule-based training in PNFA and EXEC/SOC patients. In the second experiment, we modified the category so that membership was determined by less salient features, thus increasing the burden on executive resources. Under these circumstances, SD patients' categorisation profiles continued to resemble those of control subjects, PNFA patients' category judgments were governed by feature salience, and EXEC/SOC patients' judgments were limited by impaired executive resources. These observations suggest that the semantic memory deficit in SD largely reflects degraded feature knowledge for familiar objects, while impaired semantic memory in PNFA and in EXEC/SOC patients largely reflects a deficit in the processes associated with semantic categorisation.
语义记忆损伤在额颞叶痴呆（FTD）中普遍存在，包括语义性痴呆（SD）、进行性非流利性失语（PNFA）和非失语性 FTD 患者，他们在执行和社会功能方面存在缺陷（EXEC/SOC）。一种假设将其归因于语义记忆中特定类别的知识退化。本研究探讨了另一种假设，即 FTD 中的语义记忆损伤也可能反映了确定物体意义的分类过程的限制。在两种条件下，对患者进行了新语义类别学习：基于规则的分类，其中执行资源支持对特定特征的评估以确定类别成员资格；基于相似性的分类，其中类别成员资格由项目与原型或回忆范例的整体相似性决定。在第一个实验中，患者学习了一个由高度显著特征组成的新类别。对于 SD 患者，我们发现基于规则和基于相似性的训练后的类别成员身份判断特征类似于对照组的表现。在 PNFA 和 EXEC/SOC 患者中，基于规则的训练后分类受损。在第二个实验中，我们修改了类别，使得成员资格由不太显著的特征决定，从而增加了执行资源的负担。在这些情况下，SD 患者的分类特征继续类似于对照组，PNFA 患者的类别判断由特征显著性决定，而 EXEC/SOC 患者的判断则受到受损执行资源的限制。这些观察结果表明，SD 中的语义记忆缺陷在很大程度上反映了熟悉物体特征知识的退化，而 PNFA 和 EXEC/SOC 患者的语义记忆损伤在很大程度上反映了与语义分类相关的过程缺陷。
