The dissociability of lexical retrieval and morphosyntactic processes for nouns and verbs: A functional and anatomoclinical study

Nouns and verbs can dissociate following brain damage, at both lexical retrieval and morphosyntactic processing levels. In order to document the range and the neural underpinnings of behavioral dissociations, twelve aphasics with disproportionate difficulty naming objects or actions were asked to apply phonologically identical morphosyntactic transformations to nouns and verbs. Two subjects with poor object naming and 2/10 with poor action naming made no morphosyntactic errors at all. Six of 10 subjects with poor action naming showed disproportionate or no morphosyntactic difficulties for verbs. Morphological errors on nouns and verbs correlated at the group level, but in individual cases a selective impairment of verb morphology was observed. Poor object and action naming with spared morphosyntax were associated with non-overlapping lesions (inferior occipitotemporal and fronto-temporal, respectively). Poor verb morphosyntax was observed with frontal-temporal lesions affecting white matter tracts deep to the insula, possibly disrupting the interaction of nodes in a fronto-temporal network.