The cognitive neuropsychology of action semantics: A review

The conceptual knowledge that mediates our ability to use familiar objects, understand viewed actions, and engage in communication about actions is often termed "action semantics". The underlying format, cognitive organization, and neural substrates of these representations are matters of active scientific investigation. This review synthesizes the large and diverse literature on action semantics in individuals with neurological disorders characterized by prominent motor deficits (e.g., Parkinson's disease and Amyotrophic lateral sclerosis), as well as those for whom motor deficits are often less prominent (e.g., Alzheimer's disease, Frontotemporal Dementia, stroke). Research in these two groups of disorders is strikingly "siloed" and offers many contradictory findings with respect to whether action semantic representations are abstract (i.e., "disembodied") or grounded in sensory and motor features that reflect the way knowledge was acquired. Findings across these populations also disagree as to whether action semantic representations are organized somatotopically or with a semantic feature-based architecture, and mediated by anterior motor-related or relatively posterior sensory-related brain regions. Many of these disparate findings can be reconciled with consideration of a multidimensional, multimodal representational architecture mediated by a distributed left-lateralized network of brain regions. We provide suggestions for specific methodological approaches for research with neurological populations that may further our understanding of the format, organization, and neural substrates of action semantics.