Impact of language proficiency and orthographic transparency on bilingual word reading: an fMRI investigation

The purpose of the present functional magnetic resonance imaging (fMRI) investigation was to examine how language proficiency and orthographic transparency (letter-sound mapping consistency) modulate neural activity during bilingual single word reading. Spanish-English bilingual participants, more fluent in their second language (L2; English) than their native language (L1; Spanish), were asked to read words in the two languages. Behavioral results showed that participants were significantly slower in reading words in their less proficient language (Spanish) than in their more proficient language (English). fMRI results also revealed that reading words in the less proficient language yielded greater activity in the articulatory motor system, consisting of supplementary motor area/cingulate, insula, and putamen. Together, the behavioral and fMRI results suggest that the less practiced, hence less proficient, language requires greater articulatory motor effort, which results in slower reading rates. Moreover, we found that orthographic transparency also played a neuromodulatory role. More transparent Spanish words yielded greater activity in superior temporal gyrus (STG; BA 22), a region implicated in phonological processing, and orthographically opaque English words yielded greater activity in visual processing and word recoding regions, such as the occipito-parietal border and inferior parietal lobe (IPL; BA 40). Overall, our fMRI results suggest that the articulatory motor system is more plastic, hence, more amenable to change because of greater exposure to the L2. By contrast, we propose that our orthography effect is less plastic, hence, less influenced by frequency of exposure to a language system.