Semantic and lexical features of words dissimilarly affected by non-fluent, logopenic, and semantic primary progressive aphasia.

The findings are in line with a hierarchical mental lexicon structure with a conceptual (semantic) and a lexeme (word-form) level, such that a selective deficit at one of these levels of the mental lexicon manifests differently in lexical-semantic processing performance, consistent with the affected language-specific brain region in each PPA variant.