The spatiotemporal neural dynamics of Braille letter representations in individuals with congenital blindness

Experience shapes the visual brain, but the degree to which its functional organization is plastic remains unknown. A unique opportunity to detect the boundaries of functional plasticity is by measuring brain plasticity in individuals born with congenital blindness. We used Braille letter reading to probe the brains of blind individuals. Driven by the analogy to visual processing that proceeds from viewing condition-dependent to viewing condition-independent representations in recognition, we investigated how the Braille letter processing proceeds from a hand-dependent to a hand-independent representation. For this we measured fMRI (N=15) and EEG (N=11) while congenitally blind participants read Braille letters with either their left or right index finger. For both imaging modalities, we applied equivalent multivariate classification schemes: a) to assess hand-dependent letter representations we classified between letter pairs trained and tested within the same hand; b) to assess hand-independent letter representations we trained and tested classifiers across different hands. Finally, we integrated spatial and temporal information using EEG-fMRI representation fusion. The fMRI results indicated hand-dependent representations across somatosensory areas, intraparietal sulcus, insula, early visual cortex and ventral areas. Hand-independent information was present across early visual cortex, ventral areas, and insula. This reveals a transformation from hand-dependent to hand-independent representations along known pathways for both tactile object recognition (S1, S2, parietal cortex, insula) and sighted reading (early visual cortex, LFA, VWFA). The EEG results showed that hand-dependent representations emerged earlier than hand-independent representations. This suggests hierarchical processing of Braille letters with somatosensory, hand-dependent information being processed before rather high-level, hand-independent information. The integrated spatiotemporal analysis revealed that hand-dependent representations emerge before hand-independent representations in the same network, encompassing lateral occipital cortex and intraparietal sulcus. Together our findings reveal the spatio-temporal dynamics of Braille letter representations, indicating functional reorganization of the visual brain in mapping tactile sensory signals to meaning.