The neural correlates of verb and noun processing - A PET study

The hypothesis that categorical information, distinguishing among word clas ses, such as nouns, verbs, etc., is an organizational principle of lexical knowledge in the brain, is supported by the observation of aphasic subjects who are selectively impaired in the processing of nouns and verbs. The stu dy of lesion location in these patients has suggested that the left tempora l lobe plays a crucial role in processing nouns, while the left frontal lob e is necessary for verbs. To delineate the brain areas involved in the proc essing of different word classes, we used PET to measure regional cerebral activity during tasks requiring reading of concrete and abstract nouns and verbs for lexical decision. These tasks activated an extensive network of b rain areas, mostly in the left frontal and temporal cortex, which represent s the neural correlate of single word processing. Some left hemispheric are as, including the dorsolateral frontal. and lateral temporal cortex, were a ctivated only by verbs, while there were no brain areas more active in resp onse to nouns. Furthermore, the comparison of abstract and concrete words i ndicated that abstract word processing was associated with selective activa tions (right temporal pole and amygdala, bilateral inferior frontal cortex) , while no brain areas were more active in response to concrete words. Ther e were no significant interaction effects between word class and concretene ss. Taken together, these findings are compatible with the view that lexica l-semantic processing of words is mediated by an extensive, predominantly l eft hemispheric network of brain structures. Additional brain activations a ppear to be related to specific semantic content, or, in the case of verbs, may be associated with the automatic access of syntactic information.