The nature of the visual deficits in developmental dyslexia

Facoetti, A. 1, 2 & Vidyasagar, T. 3

1 Developmental & Cognitive Neuroscience Lab, General Psychology Department, University of Padova, Italy
2 Developmental Neuropsychology Unit, "E. Medea" Scientific Institute, Bosisio Parini, Lecco, Italy
3 Department of Optometry & Vision Sciences and Melbourne Neurosciences Institute, University of Melbourne, Australia

Causal connections between multi-sensory attention and learning to read, Andrea Facoetti.

Although impaired phonological processing is assumed to characterize dyslexic individuals, emerging evidence suggests that dyslexia could arise from a more basic cross-modal letter-to-speech sound integration deficit. Before the correct letter-to-speech sound integration is applied, letters have to be precisely selected from cluttering letters and global spoken word have to be segmented in their speech-sound units by efficient orienting of visual and auditory attention, respectively. Spatial attention deficits in children with dyslexia might impair their ability to focus on each successive letter in a visual word, and an extra-large letter spacing could help reading in dyslexic children. We showed that this simple manipulation of letter spacing substantially improved text reading efficiency on the fly (without any training) in a large, unselected sample of Italian and French dyslexic children. We demonstrated that only 12 hours of playing action video games-not involving any direct phonological or orthographic training-drastically improve the reading abilities of children with dyslexia. Action video games training could improve the efficiency of the Magnocellular-Dorsal pathway, because only action video games require an extraordinary speed in terms of transient events and moving objects, and an emphasis on peripheral processing. To test this prediction, we measured text reading and attentional skills in two matched groups of adult poor readers before and after a motion perceptual learning or an active control training. We found that only the group treaded with 20 hours motion perceptual learning improved their reading abilities. Attentional skills also improved during Magnocellular-Dorsal pathway training. These results showed that dorsal-attention pathway improvement can directly translate into better reading abilities, providing a new and fast remediation of dyslexia. Importantly, we showed that pre-reading visual and auditory attentional orienting (assessed by spatial cueing facilitation and temporal order judgment), in addition to speech-sound processing, and cross-modal mapping, captures about 60% of the future reading acquisition skills. All these results demonstrate the causal role of multi-sensory selective attention in reading acquisition, and suggest new approaches for early identification and efficient prevention of dyslexia.

A neural framework for impairment in visual attention being a core deficit in developmental dyslexia, Trichur Vidyasagar.

Reading is a relatively recent activity in the history of Homo sapiens. It almost certainly uses neural mechanisms, which had already evolved over many hundreds of millennia for a different purpose. What may these be? It is becoming apparent that a crucial stage in reading are visual attentional mechanisms that usually help us to select and process only that small fraction of sensory information which is relevant at a particular instant, from among a world of ?noisy? sensory inputs. A careful consideration of all the known neural pathways needed for reading a text opens up the possibility that the core deficit could be at any one of a number of stages in the visual system that apply the above process of attentional selection to reading. The cortical region that orchestrates such selective attention is the posterior parietal cortex, which seems to direct a spotlight of attention on to earlier visual areas. Such gating of sensory inputs rapidly and at a fine spatial scale by the feedback pathways from the dorsal stream to earlier visual areas is a fundamental process in reading and any deficit in this process can potentially lead to poor reading abilities. Such deficit could be either in the visual magnocellular inputs that project to the dorsal stream or in the dorsal stream areas themselves. It is also conceivable that even the severe phonological impairments commonly associated with dyslexia could at least partially, if not totally, be a simple downstream consequence of the deficit in visuo-spatial attention. However, there is also the possibility that a general impairment in spatio-temporal processing of sensory information could lead to both the visuo-spatial and phonological difficulties and the dyslexic symptoms may often have a multifactorial cause.