Neural processing of nouns and verbs: the role of inflectional morphology

Abstract

Dissociations of nouns and verbs following brain damage have been interpreted as evidence for distinct neural substrates underlying different aspects of the language system. Some neuroimaging studies have supported this claim by finding neural differentiation for nouns and verbs [Brain 122 (1999) 2337] while others have argued against neural specialisation [Brain 119 (1996) 159; Brain 124 (2001) 1619]. We suggest that one reason why these inconsistencies may have arisen is because the morphological structure of nouns and verbs has been ignored. In an event-related functional magnetic resonance imaging (fMRI) study we test the hypothesis that the neural processing of nouns and verbs differs when they are inflected. We contrasted the processing of regularly inflected nouns (dogs) with regularly inflected verbs (hitting), and found that the LIFG was more strongly activated in processing regularly inflected verbs compared to regularly inflected nouns. Moreover, regions of LIFG that were more active in the fMRI study for inflected verbs partially overlapped with the lesions in patients who have particular problems with verb morphology. Taken together with previous studies, these results suggest that noun and verb stems do not differ in terms of their representation, but when verbs are morphologically complex they differentially engage those neural systems which are involved in processes of morpho-phonology and syntax.

Cortical signatures of noun and verb production

“…As we have pointed out, however, the tasks for which verb processing correlated with left inferior prefrontal activity did not require access to grammatical information about stimulus words. Moreover, it is not even obvious that the ''verb'' stimuli in these studies should be interpreted unambiguously as verbs: both the English progressive form ending in -ing and the Italian infinitive, said to represent ''inflected'' verbs (16), are compatible with use as nouns in certain contexts (for example, the singing).…”

“…Few neuroimaging studies to date have investigated noun and verb retrieval by using tasks with comparable processing demands across the two categories. Some experiments comparing nouns and verbs have used word generation paradigms (12,25), which have been criticized for conflating processes involved in the production and retrieval of words of both categories (16,26). Of the studies in which noun and verb tasks were better matched, two [one conducted in English (16) and one in Italian (13)] have reported areas of greater activation for verbs, but no areas in which nouns were more activated.…”

“…Even in a recent study where an attempt was made to control for word form ambiguity, the stimuli included more than twice as many unambiguous nouns as verbs (16).…”

“…Although some of these experiments have used abstract as well as concrete word stimuli, thereby avoiding the simple conflation of semantic and grammatical categories, they have all nevertheless used tasks, like lexical decision (13-15) and word categorization (15,16), that emphasize nongrammatical aspects of word retrieval. Interestingly, these studies have not turned up consistent differences in patterns of cortical activity related to noun and verb processing.…”

“…Prior functional imaging studies of noun and verb processing (12)(13)(14)(15)(16) have been bedeviled by similar confounds. Although some of these experiments have used abstract as well as concrete word stimuli, thereby avoiding the simple conflation of semantic and grammatical categories, they have all nevertheless used tasks, like lexical decision (13)(14)(15) and word categorization (15,16), that emphasize nongrammatical aspects of word retrieval.…”