Influence of lexical and phonological variables on the production of verb tense in agrammatism.

Background: A key characteristic of the language of persons with post-stroke agrammatic aphasia (PSA-G) is difficulty with verb morphology, particularly for marking tense/aspect. The neurocognitive mechanisms underlying this deficit are poorly understood. Most prior investigations of this symptom have focused on testing syntactic theories of verb tense. But little is known about how lexical (morphophonological) factors influence the production of verb morphology. Investigating lexical factors will provide a deeper understanding of this deficit, in turn informing theories and future interventions of PSA-G. Aims: The first aim was to delineate the effects of lexical (lemma frequency, lexeme frequency, noun homonymy, and verb regularity) and phonological (phonological complexity) properties on the accuracy and error patterns of verb inflections in PSA-G. The second aim was to examine if substitution errors were influenced by any verb properties. Methods & Procedures: Sentences were elicited in three tenses (future, present and past) using three-picture sequences depicting an action before, during, and after completion. Accuracy and error patterns were examined in 20 English-speaking individuals with PSA-G. The effects of the following properties on accuracy were examined: two lexical frequency measures (lemma and lexeme), noun homonymy, regularity of the past tense, and number of phonemes in the verb form. Inaccurate verb tenses (substitution errors) were compared with the targets to examine the effects of lexical factors. Outcomes & Results: The overall tense production accuracy was 12%. Production of verb tense was more accurate for stimuli with higher lexeme frequency, noun homonymy, irregular past, and shorter phoneme length. Morphological paraphasias were predominantly Conclusions: In addition to their well-documented tense encoding deficit, agrammatic speakers experience computational limitations during sentence planning, as evidenced by the negative effect of phoneme length, positive effect of lexeme frequency and noun homphony on tense production accuracy. The overuse of commonly occurring verb forms (Verb + ing, verb stems) in morphological paraphasias allows speakers to advance the verb phrase to the next stage of the speech production pipeline (that is, articulatory planning). The present study supports a multi-source view of agrammatic language, which integrates the core morphosyntactic deficit of PSA-G with two non-syntactic components: (a) other linguistic processes inherent in sentence planning such as lexical retrieval and phonological encoding, and (b) computational limitations.