Changes in social behavior are recognized as potential symptoms of behavioral-variant frontotemporal dementia (bvFTD) and semantic dementia (SD), yet objective ways to assess these behaviors in natural social situations are lacking. This study takes a truly social (or second-person) approach and examines changes in real-world social behavior in different dementia syndromes, by analyzing non-scripted social interactions in bvFTD patients (n = 20) and SD patients (n = 20), compared to patients with Alzheimer's disease (AD) (n = 20). Video recordings of 10-min conversations between patients and behavioral neurologists were analyzed for the presence of socially engaging (e.g., nodding, smiling, gesturing) and disengaging behavior (e.g., avoiding eye contact, self-grooming, interrupting). Results demonstrated disease-specific profiles, with bvFTD patients showing less nodding and more looking away than AD, and SD patients showing more gesturing than AD. A principal components analysis revealed the presence of four unobserved components, showing atypical disengaging patterns of behavior. Whole-brain voxel-based morphometry analyses revealed distinct neurobiological bases for each of these components, with the brain regions identified previously associated with behavior selection, abstract mentalization and processing of multi-sensory and socially-relevant information, in mediating socially engaging and disengaging behavior. This study demonstrates the utility of systematic behavioral observation of social interactions in the differential diagnosis of dementia.

社会行为的变化被认为是行为变异额颞叶痴呆（bvFTD）和语义性痴呆（SD）的潜在症状，但缺乏在自然社会情况下评估这些行为的客观方法。这项研究通过分析bvFTD患者（n  = 20）和SD患者（n  = 20）的非脚本化社交互动，采用了真正的社交（或第二人称）方法，并检查了不同痴呆综合征中现实世界社交行为的变化。相比，患者的阿尔茨海默氏病（AD）（ñ = 20）。分析了患者与行为神经病学家之间10分钟对话的视频记录，以了解是否存在社交活动（例如，点头，微笑，打手势）和脱离行为（例如，避免目光接触，自我修饰，打扰）。结果显示出特定疾病的特征，bvFTD患者比AD患者的点头更少，视线更多，而SD患者的手势则比AD多。主成分分析显示存在四个未观察到的成分，显示出行为的非典型分离模式。基于全脑体素的形态学分析揭示了这些成分各自的独特的神经生物学基础，而先前确定的大脑区域与行为选择，抽象思维和多感官和与社会相关的信息的处理相关，调解社交参与和疏离行为。这项研究证明了社交互动的系统行为观察在痴呆鉴别诊断中的作用。