fMRI evidence for dual routes to the mental lexicon in visual word recognition.

Event-related fMRI was used to investigate lexical decisions to words of high and low frequency of occurrence and to pseudowords. The results obtained strongly support dual-route models of visual word processing. By contrasting words with pseudowords, bilateral occipito-temporal brain areas and posterior left middle temporal gyrus (MTG) were identified as contributing to the successful mapping of orthographic percepts onto visual word form representations. Low-frequency words and pseudowords elicited greater activations than high-frequency words in the superior pars opercularis [Brodmann's area (BA) 44] of the left inferior frontal gyrus (IFG), in the anterior insula, and in the thalamus and caudate nucleus. As processing of these stimuli during lexical search is known to rely on phonological information, it is concluded that these brain regions are involved in grapheme-to-phoneme conversion. Activation in the pars triangularis (BA 45) of the left IFG was observed only for low-frequency words. It is proposed that this region is involved in processes of lexical selection.