Paradoxical features of word finding difficulty in primary progressive aphasia.

Impaired word retrieval is a main symptom of primary progressive aphasia (PPA). The cognitive features of this impairment in PPA are poorly understood. We studied 12 patients with PPA (6 English-speaking and 6 Dutch-speaking), 7 patients with early-stage clinically probable Alzheimer's disease (PRAD), 5 patients with mild cognitive impairment (MCI), and 15 age-matched, cognitively intact, control subjects. Subjects had to name a picture (the probe), which was preceded by a written word (the prime) that could be the correct name of the picture, a noun belonging to the same semantic subcategory (related prime), a semantically unrelated noun (unrelated prime), or a pseudoword (neutral control). Naming latencies were longer in PPA and PRAD patients than in control subjects. Critically, the interaction between group and prime type was highly significant. PPA patients named the probe more slowly after a related compared with an unrelated prime. In contrast, PRAD patients, mild cognitive impairment patients, and healthy control subjects tended to name the probe faster when it was preceded by a related prime. The semantic interference effect in PPA generalized across languages and PPA subtypes. Selection among competing word forms sharing a same semantic field is abnormal in PPA. The semantic interference effect constitutes a positive distinguishing feature between PPA and PRAD.