Word-category specific deficits after lesions in the right hemisphere

A speeded lexical decision task was used to investigate word-category deficits in patients suffering from lesions in the right hemisphere and in neurological controls without cortical lesion. In all patients from one group (n = 12), the right frontal lobe was affected causing a left-sided hemiparesis. In the second group (n = 6), lesions primarily affected areas in the right inferior temporo-occipital lobes. Patients with motor deficits due to lesions in the spinal cord or in the periphery served as neurological controls (n = 9). Processing of three categories of words was investigated: verbs referring to actions (action verbs (acVs)); nouns with strong visual associations (visually-related nouns (viNs)); and nouns with both strong action and visual associations (bimodal nouns (biNs)). Stimulus categories were matched for word length and normalized lexical frequency. Error scores revealed a significant word category by patient group interaction. Patients with lesions in the right frontal lobe showed most severe deficits in processing action verbs, whereas those with lesions in their right temporo-occipital areas showed most severe deficits in processing visually-related nouns. Neurological controls did not show any differences between word categories. The double dissociation of the processing impairments seen in frontal versus temporo-occipital patients demonstrates that specific word-category deficits can arise from lesions in the right non-dominant hemisphere. An account for these results in terms of distributed neuronal systems representing words is offered. (C) 2003 Elsevier Science Ltd. All rights reserved.