Reversal of the concreteness effect in semantic dementia.

Patients with semantic dementia (SD) have a striking impairment in semantic memory, but the basis for this deficit is unclear. We examined semantic memory for concrete and abstract verbs with a two-alternative, forced-choice measure of lexical semantic associative knowledge. Patients with SD had significantly greater difficulty with concrete verbs (z = -3.33) than with abstract verbs (z = -2.05), a "reversal of the concreteness effect" that was present in a majority of individual patients. The subgroup of SD patients with imaging had significant cortical thinning in the anterior and inferolateral portions of the temporal lobes. These areas of visual association cortex may be important for storing and processing visual features for word meaning. Moreover, poor performance with concrete relative to abstract verbs correlated with cortical thinning of the right anterior temporal lobe in SD, suggesting that this region may contribute to storing and processing visual semantic features. These observations raise the possibility that degraded visual feature knowledge contributes in part to the impaired comprehension of concrete words in SD.