Similar computational hierarchies for reading and speech in the occipital cortex of sighed and blind: converging evidence from fMRI and chronometric TMS

High-level perception results from interactions between hierarchical brain systems responsive to gradually increasing feature complexities. During reading, the initial evaluation of simple visual features in the early visual cortex (EVC) is followed by orthographic and lexical computations in the ventral occipitotemporal cortex (vOTC). While similar visual regions are engaged in tactile Braille reading in congenitally blind people, it is unclear whether the visual network maintains or reorganises its hierarchy for reading in this population. Combining fMRI and chronometric transcranial magnetic stimulation (TMS), our study revealed a clear correspondence between sighted and blind individuals (both male and female) on how their occipital cortices functionally supports reading and speech processing. Using fMRI, we first observed that vOTC, but not EVC, showed an enhanced response to lexical vs. non-lexical information in both groups and sensory modalities. Using TMS, we further found that, in both groups, the processing of written words and pseudowords was disrupted by the EVC stimulation at both early and late time windows. In contrast, the vOTC stimulation disrupted the processing of these written stimuli only when applied at late time windows, again in both groups. In the speech domain, we observed TMS effects only for meaningful words and only in the blind participants. Overall, our results suggest that, while the responses in the deprived visual areas might extend their functional response to other sensory modalities, the computational gradients between early and higher-order occipital regions are retained, at least for reading. Significance statement The sighted visual cortex hierarchically interprets visual signals, from simple visual features in the early visual cortex to complex features in higher-order visual areas. The blind visual cortex is known to respond to tactile and auditory information, but is a similar computational hierarchy used to process these signals? Here we showed that the blind visual cortex processes tactile reading in a spatiotemporal hierarchy strikingly similar to the hierarchy used by the sighted visual cortex to process visual reading. Intriguingly, the blind visual cortex seems additionally involved in the processing of spoken words. Our results suggest that the computational gradients between sensory-deprived early and higher-order areas are largely independent of visual experiences, despite their enhanced responses to crossmodal input.