Abstract
Mirror-image confusions are common, especially in children and in some cases of neurological impairment. They can be a special impediment in activities such as reading and writing directional scripts, where mirror-image patterns (such as b and d) must be distinguished. Treating mirror images as equivalent, though, can also be adaptive in the natural world, which carries no systematic left-right bias and where the same object or event can appear in opposite viewpoints. Mirror-image equivalence and confusion are natural consequences of a bilaterally symmetrical brain. In the course of learning, mirror-image equivalence may be established through a process of symmetrization, achieved through homotopic interhemispheric exchange in the formation of memory circuits. Such circuits would not distinguish between mirror images. Learning to discriminate mirror-image discriminations may depend either on existing brain asymmetries, or on extensive learning overriding the symmetrization process. The balance between mirror-image equivalence and mirror-image discrimination may nevertheless be precarious, with spontaneous confusions or reversals, such as mirror writing, sometimes appearing naturally or as a manifestation of conditions like dyslexia.
Highlights
A common source of psychological disturbance is confusion between left-right mirror-images
The difficulty in discriminating mirror images seems to persist even through asymmetrical experience, most of us eventually overcome it. Especially those classified as dyslexic, it persists in mirrorwriting or in wrongly labeling letters such as b and d even though the experience of looking at script in books or the teacher’s copy is overwhelmingly asymmetrical. This suggests that the brain has a natural tendency to preserve symmetry in the face of asymmetrical experience, and so to treat mirror images as equivalent
Skeide et al (2016) found that a candidate gene for dyslexia, NRSN1, was associated with volume of the visual word form area (VWFA), and that NRSN1-associated volume determined before school could predict later dyslexia with over 70 percent accuracy
Summary
A common source of psychological disturbance is confusion between left-right mirror-images. Especially those classified as dyslexic (discussed later), it persists in mirrorwriting or in wrongly labeling letters such as b and d even though the experience of looking at script in books or the teacher’s copy is overwhelmingly asymmetrical This suggests that the brain has a natural tendency to preserve symmetry in the face of asymmetrical experience, and so to treat mirror images as equivalent. Rollenhagen and Olson (2000) recorded from pattern-selective cells in the inferotemporal cortex of the macaque, and again some cells responded more to left-right mirror images than to up-down mirror images, even when the stimuli appeared at different locations across the visual field They suggest that this mirror-image equivalence ‘‘is a direct correlate of lateral mirrorimage confusion as observed in perception’’ Input to the untrained hemisphere may either be compared to the reversed memory circuit in that hemisphere, or transferred veridically to the trained hemisphere for comparison with the nonreversed circuit
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