Key insights into the organisation of semantic memory in the brain have been derived from semantic dementia, a subtype of frontotemporal dementia (FTD) associated with a degradation of conceptual knowledge following atrophy in the anterior temporal lobes (ATLs). Recent research in FTD has begun to focus on social-semantic knowledge, and its relationship to the known behavioural changes in this disorder. The aims of the thesis were to (i) determine how social-semantic knowledge is affected in FTD in relation to general conceptual knowledge, (ii) explore the roles of the left and right ATLs in social-semantic knowledge, and (iii) investigate the relation between social-semantic knowledge and behavioural changes in FTD. An additional theme that ran throughout the thesis was the use of a transdiagnostic approach to FTD.

First, I introduced a model of impaired social behaviour in FTD – controlled social-semantic cognition. In a formal review chapter, I proposed that the impaired social behaviour in semantic dementia results predominantly from a degradation of social-semantic knowledge, and that social-semantic knowledge is part of a broader conceptual system supported by a transmodal hub in the bilateral ATLs. In Chapter 3, I assembled a neuropsychological battery comprising social- and non-social-semantic tasks. Both batteries were run in a large cohort of people with FTD, as well as people with unilateral ATL resection for temporal lobe epilepsy (TLE). I found that social-semantic knowledge was degraded in parallel with general conceptual knowledge in FTD following bilateral ATL atrophy. In contrast unilateral ATL damage yielded a mild impairment for social and non-social concepts, with no left vs right differences. Chapter 4 focussed on a specific type of social concept – face/person knowledge. I compared the impact of bilateral ATL damage vs unilateral ATL damage on face recognition, person knowledge and general semantic memory. Bilateral ATL damage generated a severe impairment in both person knowledge and general semantic memory, whereas only a mild impairment was found after unilateral ATL damage. Strikingly this bilateral vs unilateral difference for semantic performance occurred despite similar levels of overall ATL volume. In Chapter 5, I used a battery of informant questionnaires to explore the range and nature of behavioural changes in FTD, and tested the hypothesis that social-semantic knowledge is related to impaired social behaviour in FTD. By recruiting the same FTD cohort as Chapter 3, I was able to explore the relation between behavioural changes and (i) neuropsychological performance, and (ii) grey matter volume. Behavioural changes were prevalent in both FTD subtypes and differed quantitatively (more frequent/severe in bvFTD) but not qualitatively. There was no association found between impaired social-semantic knowledge and behavioural changes, which may reflect informant questionnaires being more sensitive to prefrontal cortical-based behavioural changes rather than semantically driven behaviours.

In conclusion, my findings are in line with a theoretical framework whereby the bilateral ATLs underpin a transmodal representational hub critical for all aspects of conceptual knowledge, including social-semantic knowledge. My findings have important implications for FTD and for theories regarding the roles of the ATL in semantic memory.