Deficits in phonology and past tense morphology

Neuropsychological dissociations between regular and irregular past tense verb processing have been explained in two ways: (a) separate mechanisms comprising a rule-governed process for regular and a lexical-associative process for irregular verbs; (b) a single system drawing on phonological and semantic knowledge. The latter account invokes phonological impairment as the basis of poorer performance for regular than irregular past tense forms, due to greater phonological complexity of the regular past. In ten nonfluent aphasic patients, the apparent disadvantage for the production of regular past tense forms disappeared when phonological complexity was controlled. In a same-different judgement task on spoken words, all patients were impaired at judging regular present and past tense verbs like press and pressed to be different, but equally poor at the phonologically matched non-morphological discrimination between cress and crest. These results provide strong evidence for a central phonological deficit that is not limited to speech output nor to morphological processing; under such a deficit distinctions lacking phonological salience, as typified by regular past tense English verbs, became especially vulnerable.