Dyslexia: the Role of Vision and Visual Attention

Abstract

Dyslexia is more than just difficulty with translating letters into sounds. Many dyslexics have problems with clearly seeing letters and their order. These difficulties may be caused by abnormal development of their visual “magnocellular” (M) nerve cells; these mediate the ability to rapidly identify letters and their order because they control visual guidance of attention and of eye fixations. Evidence for M cell impairment has been demonstrated at all levels of the visual system: in the retina, in the lateral geniculate nucleus, in the primary visual cortex and throughout the dorsal visuomotor “where” pathway forward from the visual cortex to the posterior parietal and prefrontal cortices. This abnormality destabilises visual perception; hence, its severity in individuals correlates with their reading deficit. Treatments that facilitate M function, such as viewing text through yellow or blue filters, can greatly increase reading progress in children with visual reading problems. M weakness may be caused by genetic vulnerability, which can disturb orderly migration of cortical neurones during development or possibly reduce uptake of omega-3 fatty acids, which are usually obtained from fish oils in the diet. For example, M cell membranes require replenishment of the omega-3 docosahexaenoic acid to maintain their rapid responses. Hence, supplementing some dyslexics’ diets with DHA can greatly improve their M function and their reading.

The Role of Visual-Spatial Abilities in Dyslexia: Age Differences in Children’s Reading?

“…A possible interpretation of the regression results concerning VMI measures and reading deficits is that during the first educational stage, more complex and extensive visual-spatial abilities could be required for reading. Neuroanatomically, when children are in the first educational stage, there is a strict connection between the dorsal and ventral stream, and the angular and supramarginal gyri seem to help the ventral regions to focus on individual letters in order to identify them and their order (Stein, 2014). …”

“…Indeed, as the children grow-up, they become more expert in reading and apply a whole recognition strategy to identify a word. Accordingly, the ventral word form area (VWFA), located in the fusiform gyrus, seemed to play an important role in whole word recognition and in the form analysis of the words (Stein, 2014), as identified by a number of neuroimaging studies (Cohen et al, 2000, 2002; Dehaene and Cohen, 2011). When the more expert-reader has improved the lexicon, the VWFA rapidly recognizes the whole strings and allocates to it the meaning (Stein, 2014).…”

“…More specifically, visual-spatial processes have been documented to play a crucial role in reading and a number of studies reported a relationship between visual-spatial deficits and DD (Felmingham and Jakobson, 1995; Talcott et al, 1998, 2000; Vidyasagar and Pammer, 2010; Stein, 2014; Gori and Facoetti, 2015). However, contrasting results have been found in investigating visual-spatial abilities in DD.…”

The need to differentiate the magnocellular system from the dorsal stream in connection with dyslexia

“…Also, Spear, Moore, Kim, Xue, and Tumosa (1994) did not find a statistically significant difference in spatial frequency between the magno-and parvocellular systems. For a discussion see Skottun & Skoyles, 2008. 2 It has been suggested that these results may be consistent with a magnocellular deficiency due to the brief duration of the stimuli which, it has been proposed, should have meant that these stimuli are associated with high temporal frequencies (Stein, 2014). Specifically, it was suggested (Stein, 2014) that stimuli of 17 ms and 34 ms duration respectively have amplitudes at 59 and 29 Hz and that these could drive the magnocellular system, The fact is that a stimulus of 17 ms duration has essentially zero amplitude at 59 Hz and a stimulus of 34 ms duration has essentially zero amplitude at 29 Hz.…”

“…A number of authors have written of a ''magnocellular-dorsal stream'' deficit (or ''magnocellular/dorsal stream'' deficit) in connection with dyslexia (Boden & Giaschi, 2007;Facoetti, Corradi, Ruffino, Gori, & Zorzi, 2010;Gori, Cecchini, Bigoni, Molteni, & Facoetti, 2014;Gori & Facoeatti, 2014;Jednoróg, Gawron, Marchewka, Heim, & Grabowska, 2014;Jednoróg, Marchewka, Tacikowski, Heim, & Grabowska, 2011;Laycock, Crewther, Fitzgerals, & Crewther, 2009;Pammer, 2014;Ruffino, Gori, Boccardi, Massimo Molteni, & Facoetti, 2014;Zhao, Qian, Bi, & Coltheart, 2014) and Stein (2014) wrote of a magnocellular impairment throughout the dorsal visuomotor ''where'' pathway forward from the visual cortex . Also, Goswami (2015) wrote that ''The dorsal pathway.…”