Letters away from the looking glass: Developmental trajectory of mirrored and rotated letter processing within words

Abstract Discrimination of reversible mirrored letters (e.g., d and b) poses a challenge when learning to read as it requires overcoming mirror invariance , an evolutionary‐old perceptual tendency of processing mirror images as equivalent. The present study investigated when , in reading development, mirror‐image discrimination becomes automatic during visual word recognition. The developmental trajectory of masked priming effects was investigated from 2nd to 6th grade and in adults, by manipulating letter type (nonreversible; reversible) and prime condition (control; identity; mirrored; rotated). Standardized identity priming increased along reading development. Beginning readers showed mirror invariance during reversible and nonreversible letter processing. A mirror cost (slower word recognition in mirrored‐letter than identity prime condition) was found by 5th‐grade but only for reversible letters. By 6th grade, orthographic processing was no longer captive of mirror invariance. A multiple linear regression showed that letter representations, but not phonological processes or age, were a reliable predictor of the rise of mirror‐image discrimination in 2nd–4th‐graders. The present results suggest a protracted development of automatic mirror‐image discrimination during orthographic processing, contingent upon the quality of abstract letter representations. Research Highlights We traced the developmental trajectory of mirrored‐letter and rotated‐letter priming effects (e.g., i b ea and i p ea as primes of I D EA) in visual word recognition. Beginning readers (2nd–4th‐graders) showed mirror invariance and plane‐rotation sensitivity in orthographic processing, thus still being susceptible to the perceptual biases in charge in object recognition. A mirror cost was found in 5th‐graders but only for reversible letters; orthographic processing was no longer captive of mirror invariance by 6th‐grade. The automation of mirror‐image discrimination during orthographic processing depends on the quality of letter representations but not on phonological processes or age.