Repetition suppression for mirror images of objects and not Braille letters in the ventral visual stream of congenitally blind individuals

Mirror-invariance is the cognitive tendency to perceive mirror-image objects as identical. Mirrored letters, however, are distinct orthographic units, and must be identified as different; mirror-invariance must be ‘broken’ to enable efficient reading. Consistent with this phenomenon, a small, localized region in the ventral visual stream, the Visual Word Form Area (VWFA), exhibits repetition suppression to both identical and mirror pairs of objects but only to identical, not mirror, pairs of letters (Pegado et al., 2011), a phenomenon named mirror ‘breaking”. The ability of congenitally blind individuals to ‘break’ mirror invariance for pairs of mirrored Braille letters has been demonstrated behaviorally (de Heering et al., 2018, Korczyk et al., 2024). However, its neural underpinnings have not yet been investigated. Here, in an fMRI repetition suppression paradigm, congenitally blind individuals (8 males and 10 females) recognized pairs of everyday objects and Braille letters in identical ('p' & 'p'), mirror ('p' & 'q'), and different ('p' & 'z') orientations. We found repetition suppression for identical and mirror pairs of everyday objects in the parietal and ventral-lateral occipital cortex, indicating that mirror-invariant object recognition engages the ventral visual stream in tactile modality as well. However, repetition suppression for identical but not mirrored pairs of Braille letters was found not in the VWFA, but in broad areas of the left parietal cortex and the lateral occipital cortex. These results suggest that reading-related orthographic processes in blind individuals depend on different neural computations that those of the sighted. Significance Statement Mirror-invariance is a perceptual bias to recognize mirrored objects as identical. Letters constitute a unique category of object: for example, ‘b’ and ‘d’ share identical shape yet must be identified as distinct entities to enable efficient reading. In our study, we investigated the neural underpinnings of tactile mirror-invariance in congenitally blind individuals and whether it was affected by tactile reading acquisition. We showed engagement of the parietal, occipital, and ventral visual regions in mirror-invariant tactile object recognition, indicating that this perceptual bias extends beyond the visual modality. Moreover, we found that unlike in the sighted, it was the parietal and lateral occipital cortex that showed neural signatures of breaking mirror-invariance for Braille letters in congenitally blind individuals, demonstrating, how following congenital visual deprivation, neural computations can be repurposed to meet novel task requirements.