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

Mirror-invariance effect describes 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 identical and mirror pairs of objects but only to identical pairs of letters. The ability of congenitally blind individuals to break mirror invariance for pairs of mirrored Braille letters has been demonstrated behaviorally. However, its neural underpinnings have not yet been investigated. Here, in an fMRI repetition suppression paradigm, congenially blind individuals (both sexes) 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 in the left parietal cortex and the lateral occipital cortex but not in the VWFA. These results suggest notable differences in reading-related orthographic processes between sighted and blind individuals, with the LOC region in the latter being a potential hub for letter-shape processing. Significance StatementMirror invariance is a tendency to recognize rotated objects as identical. Letters are unique shapes as people learn to recognize mirrored letters (e.g., b and d) as distinct objects. In our study, we investigated the neural underpinnings of tactile mirror invariance in congenitally blind individuals. We demonstrated 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, suggesting substantial differences between visual and tactile reading.