Mirror writing and reversing single letters in stroke patients and normal elderly

Mirror writing (MW) refers to the production of individual letters or whole word strings in reversed direction. When held to a mirror, these letters or words can be read normally. We observed MW in a considerable number of stroke patients. Of the 86 patients screened 15 (17.5%) showed at least one instance of mirror writing in any of the tasks. Both right (14% of 36 patients) and left (20% of 50 patients) hemisphere damaged patients produced reversed letters only when writing with their left hand, respectively the contralesional and ipsilesional hand. The dissociated performance between the two hands in brain damaged patients is relevant to the interpretation of MW because, unlike all other peripheral dysgraphias, MW affects the non-dominant hand only. Importantly, healthy elderly also showed MW solely when writing with their left hand (6.9% of 86 participants). MW in controls was less frequent but qualitatively similar to that observed in brain damaged patients. This finding is consistent with the motor interpretation of MW that assumes an inability to transform the stored letter forming programmes for left hand writing. However, several cases have been reported in the literature of a more pervasive form of MW whereby patients mirror reverse entire words or sentences. This pattern has been observed in children learning to write but it has never been observed in healthy adult volunteers. We propose that the diagnosis of MW should be limited to the reversal of whole words, multi-digit numbers and full sentences, which reveal a disorder in coding the correct direction of writing rather than an inability to accomplish the correct spatial orientation of single letters.