A selective deficit for inflection production.

We report the case of an English-speaking aphasic patient (JP) with left posterior-frontal damage affecting the inferior frontal and precentral gyri. In speaking, JP was impaired with the regular inflections of nouns and pseudonouns, making errors like "pears" instead of pear or "door" for doors, while the spoken production of noun stems and irregularly inflected nouns (teeth) was preserved. JP's noun inflection errors stemmed from problems with inflection selection rather than a lack of understanding of concept numerosity or phonological deficit. Evidence that inflection deficits occur independently of semantic and phonological impairments supports accounts that propose dedicated neural substrates for morphological processes and raises a challenge for connectionist models that do not incorporate specific mechanisms for morphology. JP's results also demonstrated a lexical deficit selectively affecting the retrieval of verb stems and a more severe impairment for verb vs. noun inflections. JP's verb production deficit suggests a close interaction between inflectional and lexical processes probably reflecting the fact that English inflection choice in part depends on stem information stored in the lexicon.