On the instantiation of grammatical categories in the brain

The expressive capacity of language derives from grammatical rules, which combine items from a fixed set of meaningful elements (morphemes and words) into sentences of theoretically unbounded complexity. How the brain retrieves words from memory, and then assigns them to grammatical operations of the appropriate type, is a key question in the neurobiology of language. These three papers explore the neural regions engaged in operations that apply to nouns and verbs, the principal grammatical categories in every human language. The first paper reports the case of a brain-damaged subject, R.C., who is more impaired at producing grammatical forms of words and pseudo-words used as verbs ( he judges, he wugs ) than of the same words used as nouns ( the judges, the wugs ). This pattern constitutes the first clear demonstration that grammatical knowledge about verbs can be selectively impaired following brain damage. A comparison of R.C.'s neuropsychological profile with that of a patient who shows similar difficulties with nouns suggests that nouns and verbs are processed by separate neural systems with components in the left frontal lobe. The latter two papers use functional magnetic resonance imaging to identify cortical regions that are hemodynamically active when normal subjects produce nouns or verbs in the context of short phrases. The second paper describes four regions activated selectively during either noun or verb trials. Interestingly, these regions do not correspond to the areas associated with grammatical processing in patients with aphasia. Rather, they appear to be engaged in representing essential features of meaning for nouns and verbs. The third paper replicates this pattern of "core" regions, but also shows that the characteristics of activation in these regions distinguish them from regions likely to be involved in grammatical processing. A brain area engaged in a given grammatical operation, like producing a word as a verb, can be identified by a monotonic decrease in activation as the operation is repeated. By contrast, regions involved in retrieving information about word meaning do not show adaptation across stimuli. These results suggest that distinct neural systems are involved in processing semantic and grammatical aspects of knowledge about nouns and verbs.