Theories about Developmental Dyslexia

Abstract

Despite proving its usefulness for over a century, the concept of developmental dyslexia (DD) is currently in severe disarray because of the recent introduction of the phonological theory of its causation. Since mastering the phonological principle is essential for all reading, failure to do so cannot be used to distinguish DD from the many other causes of such failure. To overcome this problem, many new psychological, signal detection, and neurological theories have been introduced recently. All these new theories converge on the idea that DD is fundamentally caused by impaired signalling of the timing of the visual and auditory cues that are essential for reading. These are provided by large ‘magnocellular’ neurones which respond rapidly to sensory transients. The evidence for this conclusion is overwhelming. Especially convincing are intervention studies that have shown that improving magnocellular function improves dyslexic children’s reading, together with cohort studies that have demonstrated that the magnocellular timing deficit is present in infants who later become dyslexic, long before they begin learning to read. The converse of the magnocellular deficit in dyslexics may be that they gain parvocellular abundance. This may often impart the exceptional ‘holistic’ talents that have been ascribed to them and that society needs to nurture.

Saccadic compression of time as a marker for Developmental Dyslexia

“…On a final note, why should the absence of saccadic transient distortions observed in dyslexic children involve only temporal information? Considering the extensive literature on Dyslexia that thrived in the last 40 years, here we speculate that such a domain-specificity reflects the magnocellular impairment characterizing the clinical picture of this specific learning disability 20,26–29,32,33 . Given their fast response properties, magnocellular neurons are indeed well-suited to capture rapid changes in the environment 24 , and impairment at the magnocellular level is often linked to reduced temporal sensitivity – such as in dyslexic individuals 36–40 .…”

Dyslexia: Causes and Concomitant Impairments

“…Since the German ophthalmologist Oswald Berkhan [ 1 ] first described the symptoms of dyslexia in 1881 and Rudolf Berlin introduced the term ”dyslexia“ [ 2 ], numerous theories have been proposed about its causes and treatments, [ 3 , 4 ] (for review). The magnocellular theory of dyslexia [ 5 , 6 , 7 , 8 , 9 ], the theory of unusual foveal and parafoveal processing of letters including an unusual crowding effect [ 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 ], and the temporal summation theory [ 20 , 21 , 22 , 23 ] regard developmental dyslexia (DD) as a visual perceptual disorder.…”

Does vision therapy for visual information processing improve academic performance? A randomised clinical trial

“…[1][2][3][4][5] However, while associations between abnormal visual function and learning are commonly reported, [6][7][8] a causative role for vision in learning issues remains contentious. [9][10][11][12][13] General learning difficulties may result from multiple intrinsic (health, cognitive, sensory, personality, emotional/behavioural, neurological and developmental) and/ or extrinsic (language, cultural, socio-economic, family and school) factors. 3 Developmental dyslexia is a specific learning disorder characterised by problems with accurate and/or fluent word recognition, poor decoding and poor spelling.…”

“…3 Developmental dyslexia is a specific learning disorder characterised by problems with accurate and/or fluent word recognition, poor decoding and poor spelling. 14 Developmental dyslexia should not be conflated with more general learning difficulties, but has its own set of causation debates, giving primacy to deficits in auditory-verbal, 15,16 cerebellar, 17 attention, 18 magnocellular 13,[19][20][21][22][23][24] or a mixture of functions. 25,26 The theoretical relevance of vision in general learning difficulties or developmental dyslexia depends on the factors and/or deficits considered critical.…”

“…Academic progress varies substantially between children for a myriad of complex and interactive reasons, with learning issues affecting up to 16% of children depending on the definitions used 1–5 . However, while associations between abnormal visual function and learning are commonly reported, 6–8 a causative role for vision in learning issues remains contentious 9–13 …”