Literacy modulates engagement of the right inferior frontal gyrus in phonological processing of spoken language

Neuroimaging studies comparing literate and illiterate participants have revealed how left-lateralized language and vision networks come to show selectivity for processing written words. However, learning to read may also change the brain's processing of spoken language. We recruited older adults from São Paulo, Brazil, who are classified as "functionally illiterate" from a lack of formal education. They were compared with both age-matched and younger highly educated adults to discern effects of both age and education. Participants completed a word-monitoring task in which they listened to an extended narrative and responded with a button press whenever they heard a specific target word. This task was performed in the native language Portuguese and in an unknown language, Japanese, along with a low-level baseline tone-detection task. We hypothesized that illiterate participants would be unable to perform the unknown language version due to undeveloped abilities to detect and extract phonological sequences from speech, related to phonological awareness skills developed by reading. This was confirmed, with unknown-language word monitoring performance selectively correlated with functional literacy. In functional magnetic resonance imaging (fMRI), a positive age effect revealed that literate older adults (N = 21) recruited sensorimotor networks to a larger degree than younger adults equated on education (N = 23), in both languages. A positive education effect specific to the unknown language revealed that illiterate older participants (N = 15) failed to recruit the right inferior frontal gyrus, homolog to Broca's area. These results suggest that the right inferior frontal gyrus (rIFG) plays a crucial role in explicit phonological analysis, an ability developed through years of formal education and literacy.