Elsevier

Medical Hypotheses

Volume 62, Issue 5, May 2004, Pages 733-739
Medical Hypotheses

Mirror writing ability is genetic and probably transmitted as a sex-linked dominant trait: it is hypothesised that mirror writers have bilateral language centres with a callosal interconnection

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Abstract

Mirror writing is the ability to write from right to left, reversing each letter so that when held to a mirror the script appears normal. There is no information on the prevalence of this trait but a suggestion was received that it may be hereditary. A newspaper survey was carried out to discover the approximate prevalence and whether a hereditary factor is involved.

The results indicated a prevalence of 1 in 6500. There is strong evidence that the trait is hereditary and is associated with non-right-handedness'. It is hypothesised that mirror writers may comprise a very small group of people who not only have bilateral language centres but also have an interconnecting pathway between these centres via the corpus callosum.

The surprising genetic nature of a trait with no obvious evolutionary consequences is discussed. The data can best be explained on the basis of the trait being the phenotypic expression of an X-linked dominant gene, which does have evolutionary consequences. Similarities are noted between the proposed genetics of mirror writing and those of synaesthesia and of a few pathological X-linked dominant syndromes such as Rett syndrome. Other similarities and contrasts between mirror writing and synaesthesia are noted.

Introduction

Little is known about mirror writing or mirror writers. The trait involves the ability to write fluently and cursively from right to left (in English, or from left to right in e.g., Arabic) reversing every letter so that the script appears normal when held to a mirror. There are no references to any surveys of mirror writing in the literature of medicine or psychology. However a Sydney family exists where mirror writers are present in three generations. On being told of this, it seemed to me that the trait must either be hereditary or be much more common than appeared to be the case.

It is known from the results of Wada testing that around 97% of right-handers and 70% of left-handers are left lateralised for language while 30% of left-handers have anomalous cerebral lateralization [1]. Of these 30% of left-handers, 3/5 have bilateral or diffuse cerebral language representation, and the other 2/5 are fully right lateralized. The results of the recently introduced cheaper, non-invasive method of assessing lateralization, namely functional transcranial Doppler ultrasonography [2], [3] are broadly similar. 1.8% (i.e., 3/5 of 3%) of right-handers are also presumed to have bilateral or diffuse lateralization – note that 1.8% of right-handers is a numerically equal number of individuals to 18% of left-handers assuming a prevalence of left-handedness in the community of 10%. Thus 3.6% of the population have bilateral representation of language, and it seemed to me that mirror writers must belong to this group and that mirror writing would therefore be associated with non-right-handedness. Mirror writing ability would appear to require the presence of a language centre (for speech, language, reading and writing) in both cerebral hemispheres with an active communicating pathway via the corpus callosum.

In the literature on handedness, the term left-handedness is used synonymously with non-right-handedness'. In this article the term `ambidextrous' implies not only the ability to write With either hand (although one is normally preferred over the other) but also the ability to write with both hands simultaneously, writing normal script with one hand and mirror script with the other.

A newspaper survey was conducted to ascertain the approximate frequency of the mirror writing trait and to investigate its genetics. The survey was based on the premise that mirror readers would be mirror writers, a premise which the results of this survey show to be valid.

Section snippets

Method

On 6/10/01 a short mirror-printed Aesop's Fable (Fig. 1) was published in a provincial Queensland newspaper (the Mackay Daily Mercury), the readership being estimated at 65,000 (sales figure multiplied by 3.5). Anyone over the age of 10 who could read the fable with ease (a 1 min time limit was suggested) was invited to obtain and complete a questionnaire. Left-handers who wrote backward-sloping ticks (

) were particularly invited to take note as it was thought that this might be a pointer to

Results

The results are summarised in Table 1. Ten genuine mirror writers were identified. These 10 index cases have 45 living FDRs of whom 12 are mirror writers; the FDRs were asked to complete the same questionnaire but with the addition of some `Press Bloopers' (Fig. 1) because some index subjects complained about the outdated fuzzy font of the Aesop's Fable, printed in 1922. The total of 22 mirror writers (10 index subjects and 12 FDRs) ranged in age from 12 to 78. Three were male (M) and 19 female

Discussion

In 1999, Schott [4] published an account of the little that is known about MW in the Lancet. Schott quotes Macdonald Critchley, the eminent neurologist from the early part of the last century, as suggesting an association between mirror writing and left-handedness, stammering and dyslexia. He also noted that mirror writing is not uncommon in young children when learning to read and write; it is interesting to speculate that lateralization of language functions may not be fully developed at this

Acknowledgements

I am grateful to Dr Helen Leonard, Clinical Senior Lecturer, Centre for Child Research, UWA, Perth WA, and Medical co-ordinator Australian Rett Syndrome Study, for her help with references and information on Rett syndrome.

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