Localisation of function for noun and verb reading: Converging evidence for shared processing from fMRI activation and reaction time

Some researchers have argued in favour of verbs primarily activating the left frontal operculum (FO) in the dorsal stream, and left middle temporal (MT) region in the ventral stream, and that nouns primarily activate the left inferior temporal (IT) region in the ventral stream. Others have suggested that the activation representing noun and verb processing involves a shared neural network. We explored these hypotheses through the naming of identical, homonymous, separately cued nouns (the bat) and verbs (to bat) presented in word format using a modified naming task that ensured participants were treating the target as the appropriate part of speech (POS). Using homonymous homographs for both the noun and verb referents provides for an optimally controlled comparison given the target stimuli and responses are physically identical. Experiment 1 was a functional magnetic resonance imaging (fMRI) experiment that showed that the majority of activation was shared by both the noun and verb naming conditions, across both the ventral and dorsal streams, including the regions suggested by previous researchers as unique to verbs (FO, MT) or nouns (IT). In contrast, there was little unique activation attributable to noun processing, and practically no unique activation attributable to verb processing. This experiment, which supports a spatially shared ventral and dorsal network for noun and verb naming, was the impetus for new hypotheses involving the sharing of processes in time. Experiment 2 showed an overadditive interaction on naming reaction time (RT) between POS and bigram frequency, which provided converging evidence that the shared processing for nouns and verbs involves sublexical processing, whereas an overadditive interaction between POS and word frequency provided converging evidence that the shared processing also involves orthographic lexical access. As such, our study provides converging fMRI and RT evidence that noun and verb reading predominantly share processing along both the ventral-lexical and dorsal-sublexical reading streams.