Letter Reversals in Dyslexia: Is the Case Really Closed? A Critical Review and Conclusions

Reading is an automated act of turning symbols back into meaning. It runs on several processes at once, working in parallel, and the biggest of these sit either in the visual domain or in the domain of language comprehension. Other cognitive processes sit above them and keep the whole operation coordinated. Any account of dyslexia has to decide which of these processes it thinks goes wrong.

In this review we sort the available theories along a few lines. One line asks which subfunction is assumed to be damaged. A second asks at what level of processing the damage is supposed to take effect. A third asks whether the theory blames a single cause or several.

The field is messy, and the empirical record is messier. Studies contradict each other partly because the methods differ so widely, partly because researchers do not agree on what dyslexia actually is, and partly because experiments that chop the reading process into isolated pieces get interpreted in ways the data do not really support.

Against that background we re-examine the studies on reversal errors (confusing b and d, p and q, and the like) and reread them through the Functional Coordination Deficit Model. That model is multicausal. On its account, reversals are not dyslexia itself but a symptom seen in one subgroup of reading-disabled children, a sign that the coordination between visual decoding and phonological decoding has broken down.

The argument turns on a point about what graphemes are. To learn to read, a child has to treat letters as symbols rather than as objects. In ordinary visual object recognition, things that are mirror images of each other get represented by similar patterns of neural activity, and that generalization is useful: a cup is a cup whether its handle faces left or right. In reading it is a trap. A letter and its mirror image stand for different sounds, so the symmetry that helps elsewhere has to be shut off here.

When that suppression fails, visually symmetrical information leaks into the representation of the symbol. The link between what the eye sees and the sound it should map onto becomes ambiguous. Coordination between the visual and phonological sides falters, and learning to read gets harder.

Two conclusions follow, and both push against older views. Reversals are not evidence of a visual deficit, nor do they point to some incomplete dominance of one brain hemisphere over the other. And they are not the single defining mark of dyslexia; they are one symptom among others, present in some readers and not in all.