Selective impairment of Nouns and Verbs in aphasia: The role of imageability revisited using a noun- and verb-retrieval task in a sentence context

Aphasic patients occasionally suffer from dissociated naming impairment for nouns and verbs. This phenomenon has been interpreted as an evidence of the separated organization of nouns and verbs in the mental lexicon. Nevertheless, Bird, Howard and Franklin (2000) suggested that the damage underlying noun-verb dissociation does not involve the lexical representations, but rather the corresponding semantic knowledge. Moreover, Bird and colleagues claimed that many dissociations reported in literature are merely caused by a strong imageability effect (Bird, Howard & Franklin, 2000; 2003). In fact, most authors used a picture naming task to assess patients’ naming ability and, due to the necessity to represent actions and objects with line drawings, nouns were frequently more imaginable than verbs (Luzzatti at al., 2002). To get around this drawback, we built up a new task which allows to test nouns and verbs with an almost identical imageability rate: the Nouns and Verbs Retrieval in a Sentence Context task (NVR-SC). The patients’ performance in this task has been compared to that obtained by the same patients in a standard picture naming task. We tested 10 aphasic patients with a selective verb deficit in the picture naming task. Two were still dissociated in the NVR-SC task, while 8 were not. The data indicate that imageability interacts with the retrieval of nouns and verbs, but cannot completely account for the noun-verb dissociation. Functional locus of the damage will be also discussed referring in particular to the lemma/lexeme dichotomy suggested by Levelt (Levelt et al., 1999).