Is Developmental Dyslexia Due to a Visual and Not a Phonological Impairment?

Abstract

It is a widely held belief that developmental dyslexia (DD) is a phonological disorder in which readers have difficulty associating graphemes with their corresponding phonemes. In contrast, the magnocellular theory of dyslexia assumes that DD is a visual disorder caused by dysfunctional magnocellular neural pathways. The review explores arguments for and against these theories. Recent results have shown that DD is caused by (1) a reduced ability to simultaneously recognize sequences of letters that make up words, (2) longer fixation times required to simultaneously recognize strings of letters, and (3) amplitudes of saccades that do not match the number of simultaneously recognized letters. It was shown that pseudowords that could not be recognized simultaneously were recognized almost without errors when the fixation time was extended. However, there is an individual maximum number of letters that each reader with DD can recognize simultaneously. Findings on the neurobiological basis of temporal summation have shown that a necessary prolongation of fixation times is due to impaired processing mechanisms of the visual system, presumably involving magnocells and parvocells. An area in the mid-fusiform gyrus also appears to play a significant role in the ability to simultaneously recognize words and pseudowords. The results also contradict the assumption that DD is due to a lack of eye movement control. The present research does not support the assumption that DD is caused by a phonological disorder but shows that DD is due to a visual processing dysfunction.

Topology of the lateral visual system: The fundus of the superior temporal sulcus and parietal area H connect nonvisual cerebrum to the lateral occipital lobe

“…Imaging studies on dyslexics have found abnormalities within the temporooccipital area, suggesting their importance in the complex cognitive process of reading (Bailey et al., 2018 ; Martin et al., 2016 ; Schurz et al., 2015 ; Ting et al., 2017 ). In fact, several studies have demonstrated that reading difficulties in dyslexia may be due to visual processing deficits, rather than phonological problems (Werth, 2021b ). Even pediatric patients with severe dyslexia demonstrate the ability to read pseudowords if given sufficient fixation time, suggesting they may require greater time for temporal summation, a process that occurs early in visual processing (Cheng et al., 2018 ; Werth, 2019 , 2021a , 2021b ).…”

“…In fact, several studies have demonstrated that reading difficulties in dyslexia may be due to visual processing deficits, rather than phonological problems (Werth, 2021b ). Even pediatric patients with severe dyslexia demonstrate the ability to read pseudowords if given sufficient fixation time, suggesting they may require greater time for temporal summation, a process that occurs early in visual processing (Cheng et al., 2018 ; Werth, 2019 , 2021a , 2021b ). Furthermore, it has been demonstrated that the fusiform gyrus plays a significant role in the ability to recognize words and pseudowords (Werth, 2021b ).…”

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. Other theories assume that DD results from an impaired ability to process auditory stimuli [ 24 , 25 , 26 , 27 ] or is caused by impaired control of reading eye movements [ 6 , 7 , 8 , 9 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 ].…”

Macular Anatomy Differs in Dyslexic Subjects

“…The pathophysiology of dyslexia is still controversial and has been attributed to phonological, auditory or visual alterations [ 3 , 4 ]. A number of neuroimaging investigations focused on the central nervous system (CNS) have been performed so far to study normal [ 5 ] and abnormal reading process [ 2 , 6 ], and also reading interventions [ 7 , 8 ].…”