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Varieties of Semantic Deficit

This chapter reviews neuropsychological literature on comprehension impairments. Evidence from patients suggests that semantic cognition involves multiple components which can be separately damaged, leading to semantic deficits which play out in qualitatively different ways. This chapter reviews patterns of behavior seen in semantic dementia (SD), a progressive degenerative disorder leading to loss of stored knowledge, and access deficits, including semantic aphasia (SA) and Wernicke’s aphasia (WA), where knowledge is not lost but inaccessible. The chapter argues for a modality and category free “hub” of knowledge, degraded in SD. Additionally SA patients with access deficits are limited in their executive use of knowledge—with inflexible understanding across modalities. The chapter focuses on current areas of debate, highlighting brain regions implicated in different aspects of semantic cognition, and poses unanswered questions within the semantic cognition literature.