Dissociating Stimulus-driven Semantic and Phonological Effects during Reading and Naming

The aim of the present study was to dissociate the neural correlates of semantic and phonological processes during word reading and picture naming. Previous studies have addressed this issue by contrasting tasks involving semantic and phonological decisions. However, these tasks engage verbal short term memory and executive functions that are not required for reading and naming. Here, 20 subjects were instructed to overtly name written words and pictures of objects while their neuronal responses were measured using functional Magnetic Resonance Imaging (fMRI). Each trial consisted of a pair of successive stimuli which were either semantically-related (e.g. ROBIN-nest), phonologically-related (e.g. BELL-belt), unrelated (e.g. KITE-lobster) or semantically and phonological identical (e.g. FRIDGE-fridge). In addition, a pair of stimuli could either be presented in the same modality (word-word or picture-picture) or a different modality (word-picture or picture-word). We report that semantically-related pairs modulate neuronal responses in a left-lateralized network, including the pars orbitalis of the inferior frontal gyrus, the middle temporal gyrus, the angular gyrus and the superior frontal gyrus. We propose that these areas are involved in stimulus-driven semantic processes. In contrast, phonologically-related pairs modulate neuronal responses in bilateral insula. This region is therefore implicated in the discrimination of similar, competing phonological and articulatory codes. The above effects were detected with both words and pictures and did not differ between the two modalities even with a less conservative statistical threshold. In conclusion, this study dissociates the effects of semantic and phonological relatedness between successive items during reading and naming aloud.