Grammatical distinctions in the left frontal cortex

Selective deficits in producing verbs relative to nouns in speech are well documented in neuropsychology and have been associated with left hemisphere frontal cortical lesions resulting from stroke and other neurological diso rders. The basis for these impairments is unresolved: Do they arise because of differences in the way grammatical categories of words are organized in the brain, or because of differences in the neural representation of actio ns and objects? We used repetitive transcranial magnetic stimulation (rTMS) to Suppress the excitability of a portion of left prefrontal cortex and to assess its role in producing nouns and verbs. In one experiment subjects g enerated real words; in a second, they produced pseudowords as nouns or ver bs. In both experiments, response latencies increased for verbs but were un affected for nouns following rTMS. These results demonstrate that grammatic al categories have a neuroanatomical basis and that the left prefrontal cor tex is selectively engaged in processing verbs as grammatical objects.