Visual recognition of mirrored letters and the right hemisphere advantage for mirror-invariant object recognition

Unlike most objects, letter recognition is closely tied to orientation and mirroring, which in some cases (e.g., b and d), defines letter identity altogether. We combined a divided field paradigm with a negative priming procedure to examine the relationship between mirror generalization, its suppression during letter recognition, and language-related visual processing in the left hemisphere. In our main experiment, observers performed a centrally viewed letter-recognition task, followed by an object-recognition task performed in either the right or the left visual hemifield. The results show clear evidence of inhibition of mirror generalization for objects viewed in either hemifield but a right hemisphere advantage for visual recognition of mirrored and repeated objects. Our findings are consistent with an opponent relationship between symmetry-related visual processing in the right hemisphere and neurally recycled mechanisms in the left hemisphere used for visual processing of written language stimuli.