We also suggested that structures of the cortex responsible for saccade suppression and fixation control are immature in dyslexic children.
Indeed, our data showed that reading age-matched non-dyslexic children made smaller number of saccades than dyslexics suggesting that the reading experience does not explain the poor fixation reported in the present study in dyslexic children. These findings are in line with Clark et al. These authors acquired reading and spelling data longitudinally via a RMI scans before the onset of reading learning and after dyslexia was diagnosed.
SACCADIC - Definition and synonyms of saccadic in the English dictionary
They found that the visual area cortices were thinner in children who developed dyslexia. So, the cortical immaturity we suggested concerning the poor fixation in dyslexic children could be strictly linked with poor activity of SC and FEF neurons for a correct control of visual fixation.
In this line, Lovejoy and Krauzlis recorded the activity of SC in two monkeys after a muscimol injection and they reported a poor ability for them to focus their attention on a central fixation, suggesting that SC plays an important role in the selective attention. Krauzlis et al. According to these authors, SC activity leads to spatial attention regulation during overt orienting eye movements. Basing their analysis on Ignashchenkova et al. Thus, SC is important both in motor consequences as well as in visual-motor processing of attention. On the other hand, FEF neurons are known also to be involved in visual attention.
Moore and Fallah simulated the FEF area in two monkeys while the latter performed a spatial attention task.
The results showed that the enhancement of spatial attention is strongly related to the increase of FEF neurons activity and this has been confirmed in other studies on monkeys Moore and Fallah, as well as on healthy subjects and patients with FEF lesions Pierrot-Deseilligny et al. This study showed that the number of saccades in both chronological and reading age-matched non-dyslexic groups significantly decreased with age, suggesting that the quality of the fixation improves with age.
This finding is in line with previous studies exploring fixations in children populations Munoz et al. Thus, the frontal and prefrontal cortices involved in fixation abilities as well as in saccade suppressions are still developing in young children. Other neurophysiological and neurocognitive studies Sharpe and Zackon, ; Barkovich, ; Luna et al. Consequently we could make the hypothesis that the improvement of the visual fixation capabilities with age is related to a gradual and progressive maturation of such cortical structures until adolescence.
On the other hand, the improvement of the quality of visual fixation with age could also be correlated with the maturation of visual attention capabilities in children. Indeed, it is well known that the attentional functions are immature in children. Konrad et al. These networks involved right fronto-parietal regions for alerting, right temporo-parietal junction and right inferior frontal gyrus for orienting and reorienting, as well as anterior cingulate and lateral prefrontal cortex for executive attention.
The results showed a weak activity of the attention regions that were assessed but an important activity of the superior areas and the occipital cortex. The authors concluded that the attentional functions are not mature but are in transition in children. These results could explain the weak inhibition capabilities in children from 8 to 12 years old, as was reported previously by Bunge et al.
Based on all these findings, we suggested that the immaturity of these cortical areas in dyslexic children could lead to difficulties for them to focus their attention on the central target, leading to poor visual fixation performances compared with non-dyslexic age-matched children. Finally, such poor fixation performance in dyslexic children could have a negative effect during reading given that during fixation period children extract lexical information from the words. So, such visual information processing could be responsible for longer fixations during reading, as reported by Bucci et al.
Our findings showed that dyslexic children exhibited many difficulties fixating a visual target and therefore triggered significantly more unwanted saccades than age-matched non-dyslexic children. An immaturity of cortical areas is most likely responsible for such poor fixation capabilities in dyslexic children. AT: Performed the experiments, analyzed the data, wrote the article.
The distinctive vertical heterophoria of dyslexics
MPB: Conceived and designed the experiments, performed the experiments, wrote the article. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The authors thank parents and children for their kind participation; Ms. Sandrine Larger for screening dyslexic children, and Paris Descartes Language Center for revising the English version of the manuscript. Ajrezo, L. Saccades improve postural control: a developmental study in normal children.
PLoS One 8:e Barkovich, A. Biscaldi, M. Human express saccade makers are impaired at suppressing visually evoked saccades. PubMed Abstract Google Scholar. Braun, D. Saccadic reaction time in patients with frontal and parietal lesions. Brain , — Bucci, M. The effect of a Stroop-like task on postural control in dyslexic children.
The effect of a cognitive task on the postural control in dyslexic children. The influence of oculomotor tasks on postural control in dyslexic children. Immaturity of binocular saccade coordination in dyslexic children: evidence from a reading and visual search study. PLoS One 7:e Bunge, S. Immature frontal lobe contributions to cognitive control in children: evidence from fMRI. Neuron 33, — Burman, D. PubMed Abstract.
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