Examining the episodic-semantic interaction during future thinking - A reanalysis of external details

While traditional analyses of autobiographical construction tend to focus on the 'internal' or episodic details of the narrative, contemporary studies employing fine-grained scoring measures reveal the 'external' component to contain important information relevant to the individual's life story. Here, we used the recently developed NExt scoring protocol to explore profiles of external details generated by patients with Alzheimer's disease (AD) (n = 11) and semantic dementia (SD) (n = 13) on a future thinking task. Overall, distinct NExt profiles were observed for future events in AD and SD. Specifically, AD patients provided significantly more Specific Episode external details compared with Controls. Using voxel-based morphometry, these increased external details within future narratives related to grey matter intensity in medial and lateral frontal regions in AD. By contrast, SD patients displayed an elevation of Specific Episode, Extended Episode, and General Semantic details during future simulation relative to Controls, which related to grey matter intensity of medial and lateral parietal regions. Our findings suggest that the compensatory external details generated during future simulation comprise an array of episodic and semantic details that vary in terms of specificity and self-relevance, which may be differentially affected depending on the locus of underlying neuropathology in dementia. Adopting a fine-grained approach to external details helps to characterise the interplay between episodic and semantic content during future stimulation and suggests potentially differential vulnerability and preservation of distinct components of the constructed narrative in clinical disorders.