Semantic impairment and past tense verb production: neighbourhood and frequency modulation of irregular past tense production
Harris L., Humphreys G.
© 2014, © 2014 Taylor & Francis.Background: An ongoing debate concerns whether two distinct processes are necessary to explain our production of regular and irregular past tense verbs. A key notion within a single-mechanism account is that the production of past tense verb forms is mediated by semantic knowledge and phonological support from verb neighbourhoods. There are currently very few studies that examine the effect of verb neighbourhoods within the irregular past tense on production. Aims: The study assessed the relative contributions of frequency, phonological support from neighbourhoods and semantic knowledge to the past tense verb production of JF, a patient with posterior cortical atrophy and impaired semantic knowledge. We also explored interactive effects between these factors, e.g., whether effects of phonological support from verb neighbourhoods (e.g., sleep–slept, weep–wept) were stronger when semantic knowledge about items was compromised. Methods & Procedures: We assessed irregular past tense verb production in JF and control participants using a series of verb production tests. Outcomes & Results: JF’s past tense verb production was predicted by the degree of regularity, frequency, and irregular phonological “neighbourhood” size, along also with the integrity of his semantic knowledge about the verbs. These factors interacted: the effect of phonological support from neighbourhoods was strongest where frequency, and semantic knowledge about individual items, was low. Conclusions: The data demonstrate that irregular past tense production is graded in this case. The effects of phonological verb neighbourhoods and semantic knowledge are consistent with a single-mechanism model. The findings emphasise effects of phonological neighbourhoods in supporting production of the English irregular past tense, and we argue that a single-mechanism model provides the most parsimonious explanation of the data.