Neural signatures of reading-related orthographic processing in braille

Blind readers use a tactile reading systems consisting of raised dot arrays: braille/. How does the human brain implement reading by touch? The current study looked for signatures of reading-specific orthographic processes in braille, separate from low-level somatosensory responses and semantic retrieval. Of specific interest were responses in posterior parietal cortices (PPC), because of their role in high-level tactile perception. Congenitally blind, proficient braille readers read real words and pseudowords by touch while undergoing fMRI. We leveraged the system of contractions in English braille, where one or more braille cells can represent combinations of English print letters (e.g., "ing" , "one" ), making it possible to separate physical and uncontracted letter-length. All words in the study consisted of 4 braille cells, but their corresponding Roman spellings varied from 4 to 7 letters (e.g., "con-c-er-t" . contracted: 4 cells; uncontracted: 7 letters). We found that the bilateral supramarginal gyrus (SMG) in the PPC increased its activity as the uncontracted word length increased. By contrast, in the hand region of primary somatosensory cortex (S1), activity increased as a function of a low-level somatosensory feature: dot-number per word. The PPC also showed greater response to pseudowords than real words and distinguished between real and pseudowords in multi-voxel-pattern analysis. Parieto-occipital, early visual and ventral occipito-temporal, as well as prefrontal cortices also showed sensitivity to the real-vs-pseudoword distinction. We conclude that PPC is involved in sublexical orthographic processing for braille, possibly due to brailles tactile modality. Significance statementBlind readers use tactile reading systems of raised dot arrays: braille. To identify signatures of orthographic processing for reading by touch, and dissociate it from tactile and linguistic process, we leveraged the system of contractions in English braille, where one or more braille characters represents combinations of English print letters. Blind proficient braille readers read real words and pseudowords during fMRI scans. While all words consisted of 4 braille characters, the uncontracted spelling ranged from 4-7 letters. Activity in bilateral-posterior-parietal cortices, just posterior to primary-somatosensory cortex, increased with uncontracted word length, independent of tactile complexity (number of raised dots per word). By contrast, primary-somatosensory activity increased with tactile complexity. The posterior-parietal cortices contribute to tactile reading.