Deficits in irregular past-tense verb morphology associated with degraded semantic knowledge

Two distinct mechanisms are often considered necessary to account for generation of the past-tense of English verbs: a lexical associative process for irregular forms like speak-->spoke, and a rule-governed process ("add -ed") for regular and novel forms like talk-->talked and wug-->wugged. An alternative account based on a parallel-distributed processing approach proposes that one complex procedure processes all past-tense types. In this alternative view, neuropsychological dissociations are explained by reduced input from word meaning that plays a greater role in successful generation of the past-tense for lower frequency irregular verbs, and by phonological deficits that disproportionately affect regular and novel forms. Only limited evidence has been available concerning the relationship between knowledge of word meaning and verb-tense processing. The study reported here evaluated the past-tense verb abilities of 11 patients with semantic dementia, a neurodegenerative condition characterised by degraded semantic knowledge. We predicted and confirmed that the patients would have essentially normal ability to generate and recognise regular (and novel) past-tense forms, but a marked and frequency-modulated deficit on irregular verbs. Across the set of 11 patients, the degree of impairment for the irregular past-tense was significantly correlated with the degree of comprehension impairment as measured by verb synonym judgements. These results, plus other features of the data such as the nature of the errors to irregular verbs, are discussed in relation to currently developing theories of the language system.