Visual, motor & attentional aspects of dyslexia
A causal link between visual attention span and reading acquisition
Sylviane Valdois and Marie-Line Bosse
The question has been hotly debated whether developmental dyslexia resulted from a language problem (a phonological disorder) or a visual impairment. We have introduced the concept of visual attention (VA) span to account for the poor reading outcome of a subset of dyslexic children who show preserved phonological skills. It has been shown that the VA span is reduced in a subgroup of dyslexic children and that this disorder relates to atypical activation of the superior parietal lobules. VA span abilities further contribute to reading performance in both dyslexic and non-dyslexic children, independently of their phonological skills. However, the available data are not strong evidence for a causal relationship. We will report data from a longitudinal study carried out on 130 children who were assessed twice in kindergarten and at the end of 1st grade. Their VA span, phonological skills, verbal short-term memory, letter name and letter sound knowledge, and reading abilities were measured in kindergarten and considered as potential predictors of their reading performance one year later. Structural equation models showed that pre-reading VA span accounts for a significant and proper amount of variance in reading one year later, after controlling for the other predictive factors. Our findings show that VA span abilities in prereaders predict future reading acquisition, thus suggesting a causal link between poor VA span and poor reading outcome in developmental dyslexia.
Spatial attention and learning to read: Evidence from a 3-years longitudinal study
Sandro Franceschini, Simone Gori and Andrea Facoetti
Developmental dyslexia is a neurobiological disorder that affects about 10% of the children. Although impaired auditory and speech sound processing is widely assumed to characterize dyslexic individuals, emerging evidence suggests that dyslexia could arise from a more basic cross-modal letter-to-speech sound integration deficit. Nevertheless, letters must be precisely selected from irrelevant and cluttering letters by rapid shifting of visual attention before the correct letter-to-speech sound integration is applied. Thus, is prereading visual parietal-attention functioning able to explain future reading emergence and development? The present 3-years longitudinal study shows that prereading attentional shifting ability - assessed by serial search performance and spatial cueing facilitation - captures not only future basis of reading skills (i.e., rapid letter naming and pseudoword length effect) but also words and text reading abilities in grades 1 and 2 after controlling for speech-sound processing as well as nonalphabetic crossmodal mapping. Our results provide evidence that visual spatial attention efficiency in preschoolers specifically predicts future reading acquisition, suggesting new approaches for early identification and a more efficient prevention of developmental dyslexia.
Reading unstable words in dyslexia: inefficiency of saccade-vergence neuroplasticity
We have recently shown that saccades from dyslexic teenagers during reading are abnormally disconjugate; their eyes are drifting disconjugately during fixations causing vergence errors and highly variable fixation disparity. Dyslexics are thus confronted to unstable letters interfering with reading. Are these problems a consequence of reading difficulty? We think not, as similar abnormalities exist for saccades to single targets. We suggest that the motor learning mechanisms controlling saccade-vergence interaction remain inefficient in dyslexia. Here we examine whether variability of fixation disparity increases during the 5min of reading test (due to fatigue or reading difficulty). No time effect was found neither for dyslexics nor for controls, suggesting that the differences between groups are constitutive. In another study we measure disconjugacy of saccades and fixations in a mindless reading task: the text is transformed to X’s except a target letter C in the middle of each string. Dyslexic teenagers are requested to fixate successively each letter C. The results show again abnormal disconjugacy, similar to that during text reading. Thus, the deficit of vergence control causing saccade and fixation disconjugacy seems to be primary and needs to be addressed first. Whether reading difficulty especially over long periods accentuates disconjugacy needs further investigation
The magnocellular theory of visual dyslexia
Of the 10% of children who find it unexpectedly difficult to learn to read fluently despite normal intelligence, health and education (developmental dyslexia), many have impaired development of visual magnocellular neurones. This impairs their ability to see letters and words properly. Magnocellular neurones are responsible for directing visual attention and eye movements during reading, hence for accurately sequencing letters. This new understanding of the visual processing problems in dyslexia has enabled the development of novel and effective remedial treatments, such as coloured filters and fixation training. Impaired development of magnocells is partly genetic, partly associated with autoimmunity and aggravated by lack of essential micronutrients, in particular omega-3 fatty acids derived from oily fish.