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

Two distinct mechanisms are often considered necessary to account for gener ation of the past-tense of English verbs: a lexical associative process For irregular forms like speak --> spoke, and a rule-governed process ('add -e d') for regular and novel forms like talk --> talked and wug --> wugged. An alternative account based on a parallel-distributed processing approach pr oposes that one complex procedure processes all past-tense types. In this a lternative 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 limit ed evidence has been available concerning the relationship between knowledg e of word meaning and verb-tense processing. The study reported here evalua ted the past-tense verb abilities of ii patients with semantic dementia, a neurodegenerative condition characterised by degraded semantic knowledge. W e predicted and confirmed that the patients would have essentially normal a bility to generate and recognise regular land novel past-tense Forms, bur 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 s ignificantly correlated with the degree of comprehension impairment as meas ured 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. (C) 2001 Published by Elsevier Science Ltd.