Reduced Visual Magnocellular Event-Related Potentials in Developmental Dyslexia

Electrophysiological signatures of visual temporal processing deficits in developmental dyslexia

Developmental dyslexia (DD) is a common neurodevelopmental disorder that affects reading ability despite normal intelligence and education. In search of core deficits, previous evidence has linked DD with impairments in temporal aspects of perceptual processing, which might underlie phonological deficits as well as inefficient graphemic parsing during reading. However, electrophysiological evidence for atypical temporal processing in DD is still scarce in the visual modality. Here, we investigated the efficiency of both temporal segregation and integration of visual information by means of event-related potentials (ERPs). We confirmed previous evidence of a selective segregation deficit in dyslexia for stimuli presented in rapid succession (<80 ms), despite unaffected integration performance. Importantly, we found a reduced N1 amplitude in DD, a component related to the allocation of attentional resources, which was independent of task demands (i.e., evident in both segregation and integration). In addition, the P3 amplitude, linked to working memory and processing load, was modulated by task demands in controls but not in individuals with DD. These results suggest that atypical attentional sampling in dyslexia might weaken the quality of information stored in visual working memory, leading to behavioral and electrophysiological signatures of atypical temporal segregation. These results are consistent with some existing theories of dyslexia, such as the magnocellular theory and the "Sluggish Attentional Shifting" framework, and represent novel evidence for neural correlates of decreased visual temporal resolution in DD.

The visual basis of reading and reading difficulties

Most of our knowledge about the neural networks mediating reading has derived from studies of developmental dyslexia (DD). For much of the 20th C. this was diagnosed on the basis of finding a discrepancy between children's unexpectedly low reading and spelling scores compared with their normal or high oral and non-verbal reasoning ability. This discrepancy criterion has now been replaced by the claim that the main feature of dyslexia is a phonological deficit, and it is now argued that we should test for this to identify dyslexia. However, grasping the phonological principle is essential for all learning to read; so every poor reader will show a phonological deficit. The phonological theory does not explain why dyslexic people, in particular, fail; so this phonological criterion makes it impossible to distinguish DD from any of the many other causes of reading failure. Currently therefore, there is no agreement about precisely how we should identify it. Yet, if we understood the specific neural pathways that underlie failure to acquire phonological skills specifically in people with dyslexia, we should be able to develop reliable means of identifying it. An important, though not the only, cause in people with dyslexia is impaired development of the brain's rapid visual temporal processing systems; these are required for sequencing the order of the letters in a word accurately. Such temporal, "transient," processing is carried out primarily by a distinct set of "magnocellular" (M-) neurones in the visual system; and the development of these has been found to be impaired in many people with dyslexia. Likewise, auditory sequencing of the sounds in a word is mediated by the auditory temporal processing system whose development is impaired in many dyslexics. Together these two deficits can therefore explain their problems with acquiring the phonological principle. Assessing poor readers' visual and auditory temporal processing skills should enable dyslexia to be reliably distinguished from other causes of reading failure and this will suggest principled ways of helping these children to learn to read, such as sensory training, yellow or blue filters or omega 3 fatty acid supplements. This will enable us to diagnose DD with confidence, and thus to develop educational plans targeted to exploit each individual child's strengths and compensate for his weaknesses.

Creativity, Eye-Movement Abnormalities, and Aesthetic Appreciation of Magritte’s Paintings

Dyslexic children have been shown to be more creative than their non-dyslexic counterparts. They have also been shown to have an abnormal oculomotor profile while viewing targets in free space, making vergence or saccadic eye movements while reading or when viewing Op art. They show a slower deceleration of their eye movements and a difficulty in coordinating their two eyes to obtain single fused vision in depth. Interestingly, their abnormal oculo-motor profile is exacerbated while reading more difficult texts. Given these differences, we postulate that dyslexics’ increased creativity may be related to their different eye movement control affecting how they perceive the world. Therefore, we decided to measure adolescent dyslexics’ creativity, oculomotor profile, and subjective responses while they viewed three paintings by Magritte. These were chosen to stimulate the perception of hidden conceptual spaces or stimulate conflict between the perception of the figural and textural content. For the first time to our knowledge, dyslexic adolescents were demonstrated to be more creative in terms of flexibility and fluidity than their non-dyslexic peers. Subjectively, while viewing the Magritte paintings, dyslexics reported fewer conceptual spaces and fewer hidden words than their non-dyslexic peers; thus, they confabulated less than non-dyslexics. Dyslexics also demonstrated an abnormal oculomotor profile similar to those that we have shown when reading, viewing randomized targets, and while perceiving illusions of depth in Op art paintings, in that they demonstrated difficulty with disconjugation and abnormalities in their eye velocity profiles. We propose there may be a link between dyslexic increased creativity and their eye movement abnormalities. Similar to reading nonsense text, we propose that Magritte’s contradictory paintings exacerbate dyslexics’ eye movement abnormalities. These eye movement abnormalities while viewing these particular paintings might provide a physiological signature suggesting a contribution of their unusual eye control to their higher creativity scores.