Previous research has demonstrated that young children produce mirror-image letter-reversals when printing their names in a leftward direction from the midline or right margin of their writing paper. This ability is postulated to be an epiphenomenon of a symmetric, proximal (arm) stage of motor development that ontogenetically precedes lateralization of fine-motor distal (finger) control-a stage in which each arm can be controlled by either side of the brain. From this view, canonical writing in right hemi-space is contralaterally mediated by the left hemisphere and mirror-writing in left hemi-space is ipsilaterally mediated by the right hemisphere. However, evidence of right hemisphere canonical letter processing in dyslexia suggests that this is not always the case. Concordantly, an early study corrected reversals by having children print these errors canonically in left hemi-space and then rightward across the midline into right hemi-space. To further understand this behaviour, the present study investigated mirror-writing in three schools (Public, Montessori, and Waldorf) each differing in how writing is introduced. It was hypothesised that there would be no school-differences in mirror-writing if printing had been learned before school-entry and that some children would produce reversals in right hemi-space that were canonically written in left-hemi-space (inverse reversals)-the opposite of the normal pattern. The results showed that 39% of the children demonstrated these inverse reversals. It is discussed how this unexpectedly high incidence may be foundational in the development of the phonologically proficient and deficient subtypes of dyslexia, spelling-dysgraphia, and the left-hand inverted writing posture.

No abstract

It has been argued that dyslexia may develop in strongly left eye dominant children through learning to write using ipsilateral, right hemisphere motor pathways. New light on this theory has been cast by recent findings of atypical enhanced corpus callosum white matter in children with dyslexia, reflecting right to left hemisphere communication that is resistant to intensive remedial reading intervention. Enhanced corpus callosum white matter is consistent with uninhibited right to left hemisphere ipsilateral mirror-motor innervation, manifested as frequent mirror-letter writing errors in children with dyslexia. Delaying writing instruction until 7-8 years of age may prevent these errors and as well as the development of dyslexia. During the 7-8 year age period, visual-proprioceptive integration enables a child to mentally map whole word visual images onto kinaesthetic/proprioceptive letter engrams (memory representations). Hypothetically, this process is facilitated by anterior commissure activity involving inter-hemispheric transfer of ipsilateral mirror-to-non mirror motor movement. This postulate, involving delayed writing instruction pending further maturation, also receives indirect support from the remarkable proficiency leap among second graders reading Hebrew as Hebrew involves a leftward orthography in which ipsilateral right to left hemisphere innervation is uninhibited. Additionally, and more directly, normal reading comprehension for learning English among children with agenesis of the corpus callosum suggests that letter-sound decoding is not the sole route to proficient reading comprehension. In this paper, I make recommendations for obtaining empirical evidence of premature writing as a cause of dyslexia.

The purpose of this theoretical analysis and synthesis is to indicate how left-eye sighting dominance may lead to reading failure through dysfunctional right hemisphere letter encoding. Differing compensatory strategies are postulated to lead to outcomes that include the development of the phonologically impaired and phonologically proficient subtypes of dyslexia as well as specific spelling disability. Evidence is presented indicating that these disorders might be prevented by delaying the introduction of letter writing until the age of 8 years. Early childhood speech categorization in children genetically at-risk of developing dyslexia is also considered from this perspective. Convergent support for this premature writing hypothesis is provided by a comparison with the development of the left-hand inverted writing posture.

A dual-task paradigm involving concurrent finger tapping and line orientation judgment was used to investigate brain processing differences in early adolescent good readers/poor spellers (dysgraphia), poor readers/poor spellers (dyslexia) and good readers/good spellers. Whereas all groups were similarly affected during the left-hand tapping condition, in the right-hand tapping condition the good spelling group displayed significantly less tapping disruption than both poor spelling groups, who did not differ significantly from each other. From these results, it can be inferred that individuals with dyslexia and dysgraphia share a left-hemisphere processing limitation that is not confined to written language. In light of other relevant research findings, I suggest that this limitation is due to the absence of a disembedding scanning mechanism for converting spatial arrays (e.g., spelling patterns) to temporal form-an impairment putatively caused by attempting to teach written language to children who are late in establishing left-hemisphere motor dominance.