In a task designed to elicit the production of verbs, the patients known as AN-1033 and Boswell consistently produced the correct target words, performing no differently from normal controls. However, in a similar task designed to elicit the production of nouns, both patients performed quite defectively, and their scores were many SDs below those of controls. Language processing was otherwise normal--i.e., there were no impairments in grammar, morphology, phonetic implementation, or prosody; reading and writing were normal. In a third patient (KJ-1360), we obtained the reverse outcome--i.e., retrieval of common and proper nouns was preserved, but verb retrieval was defective. Together, the findings in the three patients constitute a double dissociation between noun and verb retrieval. In AN-1033 and Boswell, the lesions are located outside the so-called language areas (left frontoparietal operculum, posterior temporal region, inferior parietal lobule), where damage is associated with aphasia. The region of damage shared by the two patients is in left anterior and middle temporal lobe. This sector of left hemisphere contains systems for the retrieval of nouns that denote concrete entities. We propose that those systems are not essential for the retrieval of verbs and not involved in the vocal implementation of word forms. Those systems perform a two-way lexical-mediation role for concrete nouns and promote the reconstruction of a word form after the processing of sensory-motor characteristics of the entity denoted by that word. The findings in patient KJ-1360, whose lesion is in left premotor cortex, suggest that equivalent mediation systems for verbs are located in the left frontal region.
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