Dissociations in processing derivational morphology: the right basal ganglia involvement.

In the neuropsychological literature, there is converging evidence for a dominant role of the left hemisphere in morphological processing. However, two right hemisphere patients were described with a clear dissociation between impaired derivational morphology and preserved inflectional processing. A recent fMRI experiment confirmed the involvement of right hemispheric areas in derivational processing and also suggested that the right basal ganglia contribute to deriving nouns from verbs. The present investigation was aimed at further demonstrating the role of the right hemisphere in derivational processing. Nine right brain damaged subjects were asked to perform different derivational tasks. Five out of nine subjects confirmed the previous data. They selectively failed only in deriving nouns from verbs (i.e. to observe-->observation), mostly substituting the derived noun with a frequent inflectional suffix of the verb paradigm (i.e. observed instead of observation). Lesion subtraction analysis revealed that the caudate nucleus and the corona radiata, are the subcortical structures associated with the morphological deficit. Anatomical commonalities were found between lesion site in these patients and activations in healthy subjects. An account of these results in terms of a distributed bi-hemispheric neural network in complex language tasks is offered.