AbstractAwake humans constantly extract conceptual information from a flow of perceptual inputs. Category membership (e.g., is it an animate or inanimate thing?) is a critical semantic feature used to determine the appropriate response to a stimulus. Semantic representations are thought to be processed along a posterior-to-anterior gradient reflecting a shift from perceptual (e.g., it has eight legs) to conceptual (e.g., venomous spiders are rare) information. One critical region is the anterior temporal lobe (ATL): patients with semantic variant primary progressive aphasia (svPPA), a clinical syndrome associated with ATL neurodegeneration, manifest a deep loss of semantic knowledge.Here, we test the hypothesis that svPPA patients, in the absence of an intact ATL, perform semantic tasks by over-recruiting areas implicated in perceptual processing. We acquired MEG recordings of 18 svPPA patients and 18 healthy controls during a semantic categorization task. While behavioral performance did not differ, svPPA patients showed greater activation over bilateral occipital cortices and superior temporal gyrus, and inconsistent engagement of frontal regions.These findings indicate a pervasive reorganization of brain networks in response to ATL neurodegeneration: the loss of this critical hub leads to a dysregulated (semantic) control system, and defective semantic representations are compensated via enhanced perceptual processing.
Corticobasal Degeneration Presenting as Non-Fluent/Agrammatic Primary Progressive Aphasia: A Case Report