Cognitive and neural mechanisms of face processing in frontotemporal dementia and primary progressive aphasia

Abstract

Face processing is an integral part of human social communication, relying on complex neural and cognitive networks. While the role of posterior (i.e., occipitotemporal) brain regions in face processing has been examined extensively, the contribution of anterior (i.e., frontotemporal) brain regions is less clear. Of relevance here, is the profile of frontal and temporal brain atrophy in frontotemporal dementia (FTD) and primary progressive aphasia (PPA) including, behavioural-variant FTD, semantic dementia, progressive nonfluent aphasia and logopenic progressive aphasia. A review of the literature demonstrates variable degrees of face processing impairment across these four syndromes, but with little investigation into the mechanisms underlying this impairment. The experimental studies of this thesis therefore employ novel experimental tasks in combination with neuroimaging to systematically assess face processing in FTD and PPA. Results arising from three separate studies demonstrate that behavioural-variant FTD and semantic dementia patients attend to and encode relevant facial cues, although an interaction between ‘late’ and ‘early’ face processing is evident (Chapters 3-5). Novel evidence of a widespread deficit affecting facial identity and expression processing in progressive nonfluent aphasia and logopenic progressive aphasia, moderated by cognitive ability, is also reported (Chapter 6). Together, findings demonstrate an interaction between anterior and posterior brain regions supporting face processing. Identification of the mechanisms contributing to overt deficits in FTD and PPA permits improved symptom characterisation. In addition, the investigations presented here provide a unique approach for clarifying the brain networks underlying complex behaviour and the functional, cognitive and social outcomes when disrupted.