Mirror invariance dies hard during letter processing by dyslexic college students

Reversal errors (e.g., confusing b with d, or R with Я) are common in beginning readers and often persist in individuals with developmental dyslexia due to mirror invariance-an evolutionary-old perceptual tendency of processing mirror images as equivalent. This study investigated whether dyslexic adults still struggle with mirror-image discrimination when processing reversible letters (i.e., differing only by orientation; e.g., d, b, p) and nonreversible letters (i.e., differing also in shape; e.g., f, t, r). In a masked priming lexical decision task, one letter of the prime was manipulated by letter-type (reversible, nonreversible) and prime-condition: identity (e.g., judo, zero), control (judo, zero), mirrored-letter (jubo, zero), or rotated-letter (jupo, zero). Both dyslexic and neurotypical readers showed identity priming effects: faster recognition of target-words preceded by identity than control primes. Neurotypical readers also showed mirror and rotation costs, regardless of letter-type: slower word recognition after mirrored- or rotated-letter primes than an identity prime. In contrast, and for nonreversible letters only, dyslexics were as fast in recognizing target-words preceded by identity as by mirrored-letter primes (qualified by Bayesian statistics). These findings suggest that, despite extensive reading experience, orthographic processing by dyslexic college students remains residually sensitive to mirror invariance.