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Abstract:
Semantic dementia is a disorder in which patients demonstrate
a progressive deterioration of semantic knowledge, while other
cognitive faculties remain remarkably spared. Unlike many patients
with disturbed semantics, these patients rarely show
category-specific deficits. Nevertheless, the breakdown of semantic
knowledge in semantic dementia is structured. We provide data from
nine different tasks and several different patients, consistently
showing the same four phenomena: I) The preservation of general
relative to specific knowledge. II) Overgeneralization of
properties from highly familiar concepts to related instances. III)
Poorer performance with tasks involving words than those involving
picures. IV) Different kinds of errors in the domains of living and
nonliving things. These results are interpreted in the context of a
model which uses semantics to map between surface representations
in different modalities. We present simulation data from model
versions of the nine tasks used with the patients: picture naming,
naming to definition, word and picture definition, word and picture
sorting, word-to-picture matching, drawing, and delayed copying.
When input and output representations are constructed to match the
data from attribute-listing experiments with healthy controls, the
model provides a good fit to the patient data. Potential extensions
and implications of the model are also discussed.
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