Be concrete to be comprehended: consistent imageability effects in semantic dementia for nouns, verbs, synonyms and associates.

There are two contrasting views on the nature of comprehension impairment in semantic dementia: (a) that it stems from degradation of a pan-modal "hub" that represents core conceptual knowledge or (b) that it results from degradation of modality-specific visual feature knowledge. These theories make divergent predictions regarding comprehension of concrete versus abstract words in the disorder. The visual hypothesis predicts that concrete words should be particularly impaired because they depend heavily on visual information. In contrast, the pan-modal hub hypothesis holds that all types of knowledge are affected but predicts less severe impairment of concrete words because they have richer and more detailed semantic representations than abstract words. We investigated concreteness effects in the comprehension of six SD patients. Across nouns, verbs, synonymous and associative relationships, a clear and consistent pattern emerged: concrete words were always comprehended more successfully than abstract words. These findings extend those of previous studies and suggest that conceptual impairment in SD is not confined to concepts that rely on visual information. Instead, all types of knowledge are affected by the progressive deterioration of modality-invariant representations (required for coherent pan-modal concepts). Concrete words succumb less quickly by virtue of their richer and more detailed semantic representations.