Language Processing in Frontotemporal Dementia: A Brief Review

Abstract

Frontotemporal dementia (FTD) is a neurodegenerative condition that presents with a number of distinct behavioral phenotypes. Here we review languageprocessing deficits in three subgroups of FTD patients: progressive nonfluent aphasia (PNFA), semantic dementia (SD), and nonaphasic FTD patients with a disorder of social and executive functioning (SOC/EXEC). These three clinical subgroups have contrasting patterns of regional cortical atrophy that can be linked to their language impairments. PNFA patients’ disease includes left ventral inferior frontal cortex, resulting in impaired grammatical processing. SD patients demonstrate a profound impairment for semantic knowledge related to atrophy of the left temporal lobe. SOC/EXEC patients’ frontal atrophy tends to be more right lateralized and is associated with declines in executive functioning. SOC/EXEC patients’ limited executive resources impact language processing in a variety of ways, including slowed grammatical processing and impaired narrative discourse. FTD patients therefore provide converging evidence regarding dissociable components of language processing and their neuroanatomical bases. Understanding single words requires extensive perceptual, lexical, and semantic processing (Price 1998). Comprehending a sentence poses additional requirements of knowing the rules governing relationships between words and applying these words during processing (Friederici 2002), and engaging in a conversation also depends on the ability to plan a narrative and exchange information in an organized and interactive manner with others. These processes are subserved by a distributed network of brain regions that have been elucidated largely through studies of stroke patients and functional neuroimaging in healthy adults. More recently, converging evidence regarding the neuroanatomical bases for language processing comes from patients with focal patterns of cortical atrophy, such as those observed in frontotemporal dementia (FTD). 1 Frontotemporal dementia encompasses a spectrum of syndromes of varying etiologies that result in relatively focal damage to the frontal and temporal lobes. It occurs primarily in older adults, although its average age of onset is earlier than that of Alzheimer’s disease (Haase 1977; Brun and Gustafson