Selective impairments of nouns and verbs in aphasia: a word retrieval task in sentence context

INTRODUCTION Several authors described cases of dissociated impairment in naming nouns and verbs. There are four accounts of this dissociation: (i) patients may have purely lexical damage, which selectively affects verbs or nouns at a late stage of the linguistic processing (phonological or orthographic lexicons) (Rapp & Caramazza, 2002); (ii) the damage affects a lexical device, either at an ortographic-phonological modality-specific level (the lexeme; Levelt et al., 1999) or at a unitary lexical-syntactic level (the lemma) (Berndt et al., 1997); (iii) N-V dissociation arises from a semantic damage (Bird, Howard & Franklin, 2000); (iv) N-V dissociation is due to syntactic damage (Friedmann & Grodzinsky, 2000). To disentangle imageability and grammatical class effects, a new task was developed allowing to elicit nouns and verbs with identical imageability ratings in a sentence context. The results obtained will permit to address the following three questions: Does imageability play a role in determining N-V dissociation? If so, is imageability the unique cause of dissociation? If there is additional damage, at which level of linguistic processing does it take place? METHODS Twelve Italian aphasic patients and 11 normal controls participated in the study. Nouns and Verbs Retrieval in a Sentence Context (NVR-SC). Forty-five pairs of sentences denoting the same event, either using a noun or the corresponding verb (e.g. the evasion/to evade) were used. The first sentence was presented in complete form, while a gap was left in the second sentence, to be completed with the target word. For each pair of sentences two different conditions were employed, one triggering a verb and one triggering a noun. E.g. V-N: The prisoner was dreaming to evade The prisoner was dreaming the ……… N-V: The prisoner was dreaming the evasion The prisoner was dreaming to ……… The performance in the NVR-SC task was compared to that obtained on a classic picture naming task (50 nouns and 50 verbs). Statistical methods. Logistic regression analysis (LRA) was applied to the profiles of the patients, making it possible to study the effects of the lexical-semantic variables in univariate and multivariate linear models. RESULTS Picture naming task All patients with predominant verb deficit also have an imageability effect. In eight patients the grammatical class effect was no longer significant after introducing imageability in the statistical design (bivariate LRA). NVR-SC task Two of the verb-impaired patients in the picture naming task maintained the predominant verb deficit also in the NVR-SC task. In eight patients, the difference between nouns and verbs was no longer significant. In two patients, a paradoxical dissociation (V>N) emerged. Group analysis: performance on nouns and verbs across naming tasks (Figure 1) Patients named actions in the NVR-SC task better than in the picture naming task (58% correct versus 37%; p<.001). On the contrary, the naming of objects in the picture naming task was better than in the NVR-SC task (76% versus 61%; p<.05). DISCUSSION As expected, imageability effect is highly associated with noun-superiority. This result may have two explanations. (a) Since nouns are generally more imaginable than verbs, the imageability effect may cause noun-superiority. But, imageability alone cannot completely account for predominant verb impairment. In fact, four patients have predominant verb impairment even when imageability has been partialled out (bivariate LRA) and two patients are still dissociated in the NVR-SC task (where imageability of nouns and verbs was perfectly matched). Therefore, additional damage must be hypothesized. This would account for the fact that nouns are named better in the picture naming task, and verbs in the NVR-SC task. The former result is explained by the reduced imageability ratings; the latter may only be explained by localizing the additional damage at a central, lexical-syntactic level (i.e. the lemma). This explanation would account for the better performance on verbs in the NVR-SC task: the sentence frame and the correspondent noun may provide the patients with those information lost with the lemma damage, i.e. number of arguments and thematic roles. Nonetheless, this may account also for the fact that noun retrieval is not enhanced in the NVR-SC task: the thematic grid is not useful to retrieve nouns, since it is not a crucial aspect of noun lexical representation. In addition, aphasic patients may use a compensatory strategy to face their deficit. Since the thematic grid may be inferred from a mental image, this strategy will probably rely on visual representations of actions. Thus, the effectiveness of the compensatory strategy increases in relation to imageability (Luzzatti & Chierchia, 2002). (b) When left hemisphere language areas are completely damaged, lexical representations located in the right hemisphere emerge. This emergence is limited to high-frequency concrete nouns (Coltheart, 2000).