'Pre-semantic' cognition in semantic dementia: Six deficits in search of an explanation.

On the basis of a theory about the role of semantic knowledgein the recognition and production of familiar words andobjects, we predicted that patients with semantic dementiawould reveal a specific pattern of impairment on six differenttasks typically considered ‘‘pre-’’ or ‘‘non-’’ semantic: readingaloud, writing to dictation, inflecting verbs, lexical decision,object decision, and delayed copy drawing. The prediction wasthat all tasks would reveal a frequency-by-typicality interaction,with patients performing especially poorly on lower-frequencyitems with atypical structure (e.g., words with an atypicalspelling-to-sound relationship; objects with an atypical featurefor their class, such as the hump on a camel, etc). Of 84 criticalobservations (14 patients performing 6 tasks), this predictionwas correct in 84/84 cases; and a single component in a factoranalysis accounted for 87% of the variance across seven measures:each patient’s degree of impairment on atypical items inthe six experimental tasks and a separate composite score reflectinghis or her degree of semantic impairment. Errors alsoconsistently conformed to the predicted pattern for both expressiveand receptive tasks, with responses reflecting residualknowledge about the typical surface structure of each domain.We argue that these results cannot be explained as associatedbut unrelated deficits but instead are a principled consequenceof a primary semantic impairment.