Nouns and verbs in the brain: grammatical class and task specific effects as revealed by fMRI

The wide variety of techniques and tasks used to study the neural correlates of noun and verb processing has resulted in a body of inconsistent evidence. We performed a functional magnetic resonance imaging (fMRI) experiment to detect grammatical class effects that generalize across tasks. A total of 12 participants undertook a grammatical-class switching task (GCST), in which they were presented with a noun (or a verb) and were asked to retrieve the corresponding verb (or noun), and a classical picture naming task (PNT) widely used in the previous aphasiological and imaging literature. The GCST was explicitly designed to ensure control over confounding variables, such as stimulus complexity or imageability. Conjunction analyses of the haemodynamic responses measured in the two tasks indicated a shared verb-related activation of a dorsal premotor and posterior parietal network, pointing to a strong relationship between verb representation and action-oriented (visuo-)spatial knowledge. On the other hand, no brain area was consistently associated with nouns in both tasks. Moreover, there were task-dependent differences between noun and verb retrieval both at behavioural and at physiological level; the grammatical class that elicited the longest reaction times in both tasks (i.e., verbs in the PNT and nouns in the GCST) triggered a greater activation of the left inferior frontal gyrus. Therefore, we suggest that this area reflects a general increase in task demand rather than verb processing per se.