�james l. mcclelland, timothy t. rogers, karalyn patterson, katia dilkina, and matthew lambon ralph 1 abstract We consider parallels in the development of conceptual knowledge and its disintegration in a neurological disorder. During development, children’s concepts become more differentiated. Developing children make striking errors, attributing properties to objects that they do not have and over-extending frequent category labels to other objects. Patients with semantic dementia (SD), a progressive condition affecting the anterior temporal lobes, show a loss of differentiation of concepts. Like developing children, they attribute properties to objects that they do not have and overextend frequent category labels. Such patients may draw four legs on a duck, or call an elephant a horse. We present a theoretical framework and computational models that explain these parallel phenomena. We also consider the disintegration of lexical knowledge, which parallels the deterioration of semantics in SD patients. We present a model that simulates these findings within a single system that processes and represents both lexical and semantic information. Interest in the nature of conceptual knowledge extends back at least to the ancient Greek philosophers. In recent years, there has been a wide range of different approaches to understanding the nature of conceptual knowledge, its development, and its neural basis. In most other work, however, these issues are not all treated together. Instead, workers in philosophy, adult experimental psychology, child development, and cognitive neuroscience have pursued related questions in relative ignorance of each other’s efforts. Even within cognitive neuroscience, there has been until recently a relative separation between approaches taken by neuropsychologists, who study the effects of brain disease on cognition in patients, and researchers who study the neural basis of conceptual knowledge i
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