Dyslexia is a specific learning disability that primarily affects reading and spelling, despite average or above-average intelligence. It stems from a neurological difference that makes processing written language difficult. While the core challenge is phonological (difficulty manipulating language sounds), a common visible characteristic is the confusion of letters and symbols, known as reversals. Reversals are frequent in early literacy but persist longer and more intensely for those with dyslexia.
The Most Common Letter Reversals
The most frequent visual confusions involve letters that are mirror images of each other, creating directional or rotational errors. The pair ‘b’ and ‘d’ represents the most common letter reversal, where the stem is flipped horizontally. This mirror-image confusion is a hallmark of the reversal phenomenon.
Another pair frequently mixed up due to rotational similarity is ‘p’ and ‘q’, which are the same shape rotated in space. Less common rotational confusions include ‘m’ and ‘w’, or ‘n’ and ‘u’, which are mistaken when turned upside down. These errors demonstrate difficulty recognizing that the orientation of a visual symbol changes its meaning.
The Cognitive Basis for Mirror Imaging
The reason for these specific visual confusions stems from a natural and highly adaptive brain function called “mirror invariance” or “mirror generalization.” This function allows the visual system to treat an object and its mirror image as identical, which is incredibly useful for navigating the physical world. For example, a chair is still recognized as a chair whether it is seen facing left or facing right, because its identity does not depend on its orientation.
Reading, however, requires the brain to suppress this natural tendency, as the orientation of a letter is what defines it—a ‘b’ becomes a ‘d’ when flipped. The difficulty in overriding this mirror-image equivalence is a significant factor in the visual processing struggles of many individuals with dyslexia. Research suggests that those with dyslexia may fail to fully suppress this symmetry generalization, especially during the rapid visual processing required for reading. This means the brain continues to see mirror-image letters as having the same underlying shape, making it harder to establish the directional discrimination necessary for fluent literacy.
Confusion Beyond Letters
The directional processing challenge extends beyond letters, affecting other symbolic systems like numbers and sequences. Number reversals are common, with visually similar pairs like ‘6’ and ‘9’ frequently confused due to their rotational relationship. Children may also struggle with multi-digit numbers, transposing the order, such as reading or writing ’12’ as ’21’.
Reversals can also impact whole words, involving the transposition of letters within a sequence. For example, “was” might be read or written as “saw.” This indicates difficulty maintaining the correct left-to-right sequencing of symbols, suggesting a broader challenge with orthographic processing.
Distinguishing Dyslexic Confusion from Normal Development
It is important to understand that letter and number reversals are a normal part of early childhood development for all children. Occasional reversals are common in preschool and kindergarten as the young brain learns that written symbols are directional. For neurotypical learners, this confusion generally resolves by the time they reach age seven or eight.
The key distinction for identifying potential dyslexia is the persistence and severity of these errors past this typical developmental window. If a child continues to exhibit frequent reversals after age eight, it warrants closer attention, particularly if combined with other reading difficulties, such as slow reading speed or trouble with spelling and decoding. Reversals alone are not a definitive sign of dyslexia, but when coupled with a core difficulty in phonological processing, they indicate an underlying learning difference.