How does learning to read affect speech perception?

Behavioral studies have demonstrated that learning to read and write affects the processing of spoken language. The present study investigates the neural mechanism underlying the emergence of such orthographic effects during speech processing. Transcranial magnetic stimulation (TMS) was used to tease apart two competing hypotheses that consider this orthographic influence to be either a consequence of a change in the nature of the phonological representations during literacy acquisition or a consequence of online coactivation of the orthographic and phonological representations during speech processing. Participants performed an auditory lexical decision task in which the orthographic consistency of spoken words was manipulated and repetitive TMS was used to interfere with either phonological or orthographic processing by stimulating left supramarginal gyrus (SMG) or left ventral occipitotemporal cortex (vOTC), respectively. The advantage for consistently spelled words was removed only when the stimulation was delivered to SMG and not to vOTC, providing strong evidence that this effect arises at a phonological, rather than an orthographic, level. We propose a possible mechanistic explanation for the role of SMG in phonological processing and how this is affected by learning to read.