Mirror-Image Equivalence and Interhemispheric Mirror-Image Reversal

Mirror writing of digits: Is there a difference between boys and girls?

The neurological process of mirror generalization in memory, also known as mirror symmetrization, presents a real dilemma for typically developing 5- to 6-year-olds when learning to write characters (digits and letters). Should they write the digit 3 oriented to the left, that is correctly, or to the right, which leads to its mirror image ε? It has been anecdotally suggested that boys are more prone to mirror-writing than girls, but there is no scientific evidence for this idea. The present article gathers data from 691 children in the upper section of the French école maternelle (age between 5 and 6 ½), who each wrote the digits 0 to 9 four times under dictation and not necessarily in their natural order. Both simple and complex (mixed-effects linear regression) statistical comparisons on the percentages of digit reversal, show a substantial difference: girls produce more mirror reversals than boys. And yet the reversal curves as a function of the digits are quite similar between the two sexes (r = .97). It has been proposed that mirror reversal of characters results from the left orientation of some of them (e.g., 3, 7, J, Z), that is, in an orientation contrary to the direction of writing in our Western cultures. The present investigation shows that (1) this character orientation hypothesis (choosing to write characters in the same orientation as sentence writing) better explains reversals than the counterclockwise hypothesis (children are trained to draw circles counter-clockwise to prepare for attached cursive writing); (2) the study of the stability of reversals additionally supports the explanation of mirror writing by the left orientation of the digits (1, 2, 3, 7 and, less obviously, 9); but (3) neither of the preceding findings (left-right orientation and stability) provided a convincing explanation for the aforementioned gender difference.

Similarities and differences in the neural correlates of letter and speech sound integration in blind and sighted readers

Learning letter and speech sound (LS) associations is a major step in reading acquisition common for all alphabetic scripts, including Braille used by blind readers. The left superior temporal cortex (STC) plays an important role in audiovisual LS integration in sighted people, but it is still unknown what neural mechanisms are responsible for audiotactile LS integration in blind individuals. Here, we investigated the similarities and differences between LS integration in blind Braille (N = 42, age range: 9-60 y.o.) and sighted print (N = 47, age range: 9-60 y.o.) readers who acquired reading using different sensory modalities. In both groups, the STC responded to both isolated letters and isolated speech sounds, showed enhanced activation when they were presented together, and distinguished between congruent and incongruent letter and speech sound pairs. However, the direction of the congruency effect was different between the groups. Sighted subjects showed higher activity for incongruent LS pairs in the bilateral STC, similarly to previously studied typical readers of transparent orthographies. In the blind, congruent pairs resulted in an increased response in the right STC. These differences may be related to more sequential processing of Braille as compared to print reading. At the same time, behavioral efficiency in LS discrimination decisions and the congruency effect were found to be related to age and reading skill only in sighted participants, suggesting potential differences in the developmental trajectories of LS integration between blind and sighted readers.

Preventing the Development of Dyslexia: A Reply to Mather

Because reading/writing is a fundamental tool for children's development, the main failure in its learning-developmental dyslexia-gives rise to many attempts to remediate. A recent remedy proposed by Mather (2022), published in Perceptual and Motor Skills [129(3), p. 468], is impressive through its radical nature and the extent of its consequences. It consists of delaying the teaching of writing to the age of 7-8 years, whereas, at present, most children in Western or comparable cultures learn to write even before compulsory school (generally at age six). In this article, I present a set of arguments whose addition and possible interaction lead, if not to reject, at least to restrict Mather's proposal. My arguments show both the inefficiency of Mather's proposal through two observational studies, its practical inapplicability in contemporary society, the importance of learning to write at least in the first year of elementary school, and the stinging past failure of a math reform of similar scope (i.e., learning to count). I also question the neurological theory underlying Mather's proposal, and, finally, I point out that, even if delaying learning to write were limited to students who Mather expects (at age six) to experience future dyslexia, this remedy would be inapplicable and probably ineffective.

Different reference frames on different axes: Space and language in indigenous Amazonians

Spatial cognition is central to human behavior, but the way people conceptualize space varies within and across groups for unknown reasons. Here, we found that adults from an indigenous Bolivian group used systematically different spatial reference frames on different axes, according to known differences in their discriminability: In both verbal and nonverbal tests, participants preferred allocentric (i.e., environment-based) space on the left-right axis, where spatial discriminations (like "b" versus "d") are notoriously difficult, but the same participants preferred egocentric (i.e., body-based) space on the front-back axis, where spatial discrimination is relatively easy. The results (i) establish a relationship between spontaneous spatial language and memory across axes within a single culture, (ii) challenge the claim that each language group has a predominant spatial reference frame at a given scale, and (iii) suggest that spatial thinking and language may both be shaped by spatial discrimination abilities, as they vary across cultures and contexts.

The visual-attention span deficit in developmental dyslexia: Review of evidence for a visual-attention-based deficit

The visual attention span (VAS) deficit hypothesis in developmental dyslexia posits that a subset of dyslexic individuals shows a multielement parallel processing deficit due to reduced visual attention capacity. However, the attention-based interpretation of poor performance on VAS tasks is hotly debated. The purpose of the present paper is to clarify this issue through a critical review of relevant behavioural and neurobiological findings. We first examine the plausibility of alternative verbal interpretations of VAS performance, evaluating whether performance on VAS tasks might reflect verbal short-term memory, verbal coding or visual-to-verbal mapping skills. We then focus on the visual dimensions of VAS tasks to question whether VAS primarily reflects visuo-attentional rather than more basic visual skills. Scrutiny of the available behavioural and neurobiological findings not only points to a deficit of visual attention in dyslexic individuals with impaired VAS but further suggests a selective endogenous attentional system deficit that relates to atypical functioning of the brain dorsal attentional network. The overview clarifies the debate on what is being measured through VAS tasks and provides insights on how to interpret the VAS deficit in developmental dyslexia.

Preventing the Development of Dyslexia: A Premature Writing Hypothesis

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.

Neural Kinesthetic Contribution to Motor Imagery of Body Parts: Tongue, Hands, and Feet

Motor imagery (MI) is assimilated to a perception-action process, which is mentally represented. Although several models suggest that MI, and its equivalent motor execution, engage very similar brain areas, the mechanisms underlying MI and their associated components are still under investigation today. Using 22 Ag/AgCl EEG electrodes, 19 healthy participants (nine males and 10 females) with an average age of 25.8 years old (sd = 3.5 years) were required to imagine moving several parts of their body (i.e., first-person perspective) one by one: left and right hand, tongue, and feet. Network connectivity analysis based on graph theory, together with a correlational analysis, were performed on the data. The findings suggest evidence for motor and somesthetic neural synchronization and underline the role of the parietofrontal network for the tongue imagery task only. At both unilateral and bilateral cortical levels, only the tongue imagery task appears to be associated with motor and somatosensory representations, that is, kinesthetic representations, which might contribute to verbal actions. As such, the present findings suggest the idea that imagined tongue movements, involving segmentary kinesthetic actions, could be the prerequisite of language.

Miswriting (Especially Mirror Writing) of the Digits: An Ecological Assessment Using ELFE Data

The French National Cohort of Children Study (Etude Longitudinale Française depuis l'Enfance—ELFE) tested the literacy and numeracy skills of 4- to 5-year-old typically developing children in the second year of école maternelle. Tasks were administered by more than 4,000 teachers at schools across France. One of the study's numeracy tasks required participants to write the number of ducks (up to 5) they had counted. Analyzing the digits written by 14,904 children showed that miswritings were much more common for the digit 3 than for the digit 4. This result is consistent with the right-orienting rule, which young children in Western cultures apply when they do not know a digit's orientation, and which leads them to write, for example, ε instead of 3.The nature and frequency of miswritten digits did not differ significantly between the 466 children presumed to have written with their left hand and the 3,531 children presumed to have written with their right hand. However, a logistic mixed-effects regression showed that two other factors—age of entry to école maternelle and very early smartphone use—had a (modest) influence on the percentage of digits that were miswritten.

How Asymmetries Evolved: Hearts, Brains, and Molecules

Humans belong to the vast clade of species known as the bilateria, with a bilaterally symmetrical body plan. Over the course of evolution, exceptions to symmetry have arisen. Among chordates, the internal organs have been arranged asymmetrically in order to create more efficient functioning and packaging. The brain has also assumed asymmetries, although these generally trade off against the pressure toward symmetry, itself a reflection of the symmetry of limbs and sense organs. In humans, at least, brain asymmetries occur in independent networks, including those involved in language and manual manipulation biased to the left hemisphere, and emotion and face perception biased to the right. Similar asymmetries occur in other species, notably the great apes. A number of asymmetries are correlated with conditions such as dyslexia, autism, and schizophrenia, and have largely independent genetic associations. The origin of asymmetry itself, though, appears to be unitary, and in the case of the internal organs, at least, may depend ultimately on asymmetry at the molecular level.

Selective Inhibition of Mirror Invariance for Letters Consolidated by Sleep Doubles Reading Fluency

Mirror invariance is a visual mechanism that enables a prompt recognition of mirror images. This visual capacity emerges early in human development, is useful to recognize objects, faces, and places from both left and right perspectives, and is also present in primates, pigeons, and cephalopods. Notwithstanding, the same visual mechanism has been suspected to be the source of a specific difficulty for a relatively recent human invention-reading-by creating confusion between mirror letters (e.g., b-d in the Latin alphabet). Using an ecologically valid school-based design, we show here that mirror invariance represents indeed a major leash for reading fluency acquisition in first graders. Our causal approach, which specifically targeted mirror invariance inhibition for letters, in a synergic combination with post-training sleep to increase learning consolidation, revealed unprecedented improvement in reading fluency, which became two-times faster. This gain was obtained with as little as 7.5 h of multisensory-motor training to distinguish mirror letters, such as "b" versus "d." The magnitude, automaticity, and duration of this mirror discrimination learning were greatly enhanced by sleep, which keeps the gains perfectly intact even after 4 months. The results were consistently replicated in three randomized controlled trials. They not only reveal an extreme case of cognitive plasticity in humans (i.e., the inhibition in just 3 weeks of a ∼25-million-year-old visual mechanism), that allows adaptation to a cultural activity (reading), but at the same time also show a simple and cost-effective way to unleash the reading fluency potential of millions of children worldwide.

The role of the written script in shaping mirror-image discrimination: Evidence from illiterate, Tamil literate, and Tamil-Latin-alphabet bi-literate adults

Learning a script with mirrored graphs (e.g., d ≠ b) requires overcoming the evolutionary-old perceptual tendency to process mirror images as equivalent. Thus, breaking mirror invariance offers an important tool for understanding cultural re-shaping of evolutionarily ancient cognitive mechanisms. Here we investigated the role of script (i.e., presence vs. absence of mirrored graphs: Latin alphabet vs. Tamil) by revisiting mirror-image processing by illiterate, Tamil monoliterate, and Tamil-Latin-alphabet bi-literate adults. Participants performed two same-different tasks (one orientation-based, another shape-based) on Latin-alphabet letters. Tamil monoliterate were significantly better than illiterate and showed good explicit mirror-image discrimination. However, only bi-literate adults fully broke mirror invariance: slower shape-based judgments for mirrored than identical pairs and reduced disadvantage in orientation-based over shape-based judgments of mirrored pairs. These findings suggest learning a script with mirrored graphs is the strongest force for breaking mirror invariance.

The Battle between the Correct and Mirror Writings of a Digit in Children’s Recognition Memory

Recent research into character reversals in writings produced by occidental children has shown that they mainly reverse the left-oriented digits (1, 2, 3, 7, and 9) and therefore appear to apply a right-orienting rule. But do they produce similar reversal errors when asked to recognize the digits? In an experiment, based on eye-tracking observations of 50 children (Mage = 5.4 years), children had to point towards a target digit in a 2 × 2 matrix also containing three distractor digits, one of which was the mirror-reversed writing of the correctly written target digit. This recognition task led to a true “battle” in children’s memory between the two writings of the target digit. This battle is shown in the graphical abstract that represents a heat map from a sub-sample of children (on the left side) and the fixation points map from an individual child (on the right side). Rather than following the predicted right-orienting rule, the children’s responses appeared to be biased towards digits in the right-hand column of the 2 × 2 matrices (when the reversed target digit was not in the same column as the correctly written target digit). As a whole, these findings support the hypotheses that many 4- to 6-year-old’s representations of the digit writings are unoriented in their memory and that these children may adopt different solutions to overcome this lack of orientation depending on whether they write or read.

Possible Physical Basis of Mirror Symmetry Effect in Racemic Mixtures of Enantiomers: From Wallach’s Rule, Nonlinear Effects, B–Z DNA Transition, and Similar Phenomena to Mirror Symmetry Effects of Chiral Objects

Effects associated with mirror symmetry may be underlying for a number of phenomena in chemistry and physics. Increase in the density and melting point of the 50%L/50%D collection of enantiomers of a different sign (Wallach’s rule) is probably based on a physical effect of the mirror image. The catalytic activity of metal complexes with racemic ligands differs from the corresponding complexes with enantiomers as well (nonlinear effect). A similar difference in the physical properties of enantiomers and racemate underlies L/D inversion points of linear helical macromolecules, helical nanocrystals of magnetite and boron nitride etc., B–Z DNA transition and phenomenon of mirror neurons may have a similar nature. Here we propose an explanation of the Wallach effect along with some similar chemical, physical, and biological phenomena related to mirror image.

Bilaterally Symmetrical: To Be or Not to Be?

We belong to a clade of species known as the bilateria, with a body plan that is essentially symmetrical with respect to left and right, an adaptation to the indifference of the natural world to mirror-reflection. Limbs and sense organs are in bilaterally symmetrical pairs, dictating a high degree of symmetry in the brain itself. Bilateral symmetry can be maladaptive, though, especially in the human world where it is important to distinguish between left and right sides, and between left-right mirror images, as in reading directional scripts. The brains of many animals have evolved asymmetries, often but not exclusively in functions not dependent on sensory input or immediate reaction to the environment. Brain asymmetries in humans have led to exaggerate notions of a duality between the sides of the brain. The tradeoff between symmetry and asymmetry results in individual differences in brain asymmetries and handedness, contributing to a diversity of aptitude and divisions of labor. Asymmetries may have their origin in fundamental molecular asymmetries going far back in biological evolution.

Evolution of cerebral asymmetry

The human brain is often characterized in terms of a duality, with the left and right brains serving complementary functions, and even individuals are sometimes classified as either "left-brained" or "right-brained." Recent evidence from brain imaging shows that hemispheric asymmetry is multidimensional, comprised of independent lateralized circuits. Cerebral asymmetries, which include handedness, probably arise in phylogenesis through the fissioning of ancestral systems that divided and lateralized with increasing demand for specialization. They also vary between individuals, with some showing absent or reversed asymmetries. It is unlikely that this variation is controlled by a single gene, as sometimes assumed, but depends rather on complex interplay among several, perhaps many, genes. Hemispheric asymmetry has often been regarded as a unique mark of being human, but it has also become evident that behavioral and cerebral asymmetries are not confined to humans, and are widespread among animal species. They nevertheless exist against a fundamental background of bilateral symmetry, suggesting a tradeoff between the two. Individual differences in asymmetry, moreover, are themselves adaptive, contributing to the cognitive and behavioral specializations necessary for societies to operate efficiently.

A Synoptic and Theoretical Account of Character (Digits and Capital Letters) Reversal in Writings by Typically Developing Children

Reversing characters (digits and letters) when writing, and complete mirror writing, raise one of the oldest and most mysterious questions in developmental and educational psychology: Why do five-year-old children write symbols (e.g., ꓱ for E) they have neither learnt nor seen? Attempts to draw up a complete explanatory theory of character reversal in writings by typically developing children were long hindered by the existence of a seemingly satisfactory explanation (left-hand writing), the failure to bring together research in neuropsychology and educational psychology, and the failure to consider the shape and structure of the characters. The present paper remedies this situation by describing a new, comprehensive theory based on recent neuropsychological findings and extensive empirical observations. The theory assumes that a character’s orientation, detected in the early visual processing area, is deleted (or made inaccessible) by the mirror generalization process during transfer to memory. Consequently, there is a period, usually around age five, during which children have representations of the characters’ shapes but not their orientations. Hence, when asked to write a character, children have to improvise its orientation, and the orientation they choose (implicitly, non-consciously) is often derived from the writing direction in their culture.