The contribution of neurodegenerative diseases to the modelling of semantic memory: A new proposal and a review of the literature.

This paper provides a focused review of the literature on semantic impairment in semantic dementia (SD) and Alzheimer's disease (AD). An attempt is made to interpret the most relevant phenomena in the light of a new model of semantic memory. This model comprises a language-based component (disrupted in SD and AD), which supports our ability to establish reliable token vs. type relationships in the service of propositional thinking, and a philogenetically older sensorimotor component, which is needed to categorize our environment in a more implicit way. Extant neuropsychological models of semantic memory are also reviewed and compared with the new model in terms of their ability to explain the observed phenomena and to deal with the problem of establishing token vs. type relationships starting from inconsistent cross modal input representations and arbitrary category boundaries.