Visual processing deficits in dyslexia: receptors or neural mechanisms?

Sequential processing deficit as a shared persisting biomarker in dyslexia and childhood apraxia of speech

The purpose of this study was to investigate the hypothesis that individuals with dyslexia and individuals with childhood apraxia of speech share an underlying persisting deficit in processing sequential information. Levels of impairment (sensory encoding, memory, retrieval, and motor planning/programming) were also investigated. Participants were 22 adults with dyslexia, 10 adults with a probable history of childhood apraxia of speech (phCAS), and 22 typical controls. All participants completed nonword repetition, multisyllabic real word repetition, and nonword decoding tasks. Using phonological process analysis, errors were classified as sequence or substitution errors. Adults with dyslexia and adults with phCAS showed evidence of persisting nonword repetition deficits. In all three tasks, the adults in the two disorder groups produced more errors of both classes than the controls, but disproportionally more sequencing than substitution errors during the nonword repetition task. During the real word repetition task, the phCAS produced the most sequencing errors, whereas during the nonword decoding task, the dyslexia group produced the most sequencing errors. Performance during multisyllabic motor speech tasks, relative to monosyllabic conditions, was correlated with the sequencing error component during nonword repetition. The results provide evidence for a shared persisting sequential processing deficit in the dyslexia and phCAS groups during linguistic and motor speech tasks. Evidence for impairments in sensory encoding, short-term memory, and motor planning/programming was found in both disorder groups. Future studies should investigate clinical applications regarding preventative and targeted interventions towards cross-modal treatment effects.

A magnetic stimulation examination of orthographic neighborhood effects in visual word recognition

The split-fovea theory proposes that visual word recognition is mediated by the splitting of the foveal image, with letters to the left of fixation projected to the right hemisphere (RH) and letters to the right of fixation projected to the left hemisphere (LH). We applied repetitive transcranial magnetic stimulation (rTMS) over the left and right occipital cortex during a lexical decision task to investigate the extent to which word recognition processes could be accounted for according to the split-fovea theory. Unilateral rTMS significantly impaired lexical decision latencies to centrally presented words, supporting the suggestion that foveal representation of words is split between the cerebral hemispheres rather than bilateral. Behaviorally, we showed that words that have many orthographic neighbors sharing the same initial letters ("lead neighbors") facilitated lexical decision more than words with few lead neighbors. This effect did not apply to end neighbors (orthographic neighbors sharing the same final letters). Crucially, rTMS over the RH impaired lead-, but not end-neighborhood facilitation. The results support the split-fovea theory, where the RH has primacy in representing lead neighbors of a written word.

Visual-perceptual and phonological factors in the acquisition of literacy among children with congenital developmental coordination disorder

Much research has shown that children with congenital developmental coordination disorder (CDCD) have marked impairments in the perception of visual-spatial information, a deficit which has been assumed to be causally related to difficulties that many CDCD children experience when learning to read and spell. However, current research in reading disability suggests that poor reading is mainly related to difficulties with the processing of phonological information or with metaphonological ability, not to visual-perceptual deficits. This study aimed to explore the relationship between reading achievement and the visual-perceptual, phonological and general cognitive ability of CDCD children. Twenty-eight children with a mean age of 8.3 years were tested on motor coordination, language, general ability, visual discrimination, visual-motor integration, phoneme awareness, phoneme discrimination, reading and spelling. Results showed that phoneme awareness was the most strongly related to reading and spelling performance, and that visual discrimination showed no relationship to reading ability, although it was related to spelling performance, phonological awareness, and motor coordination. These results suggest that CDCD children who have difficulty with the acquisition of literacy may suffer from visual-perceptual problems and metaphonological deficits, but these problems are differentially related to reading and spelling.

Dark adaptation in disabled readers screened for Scotopic Sensitivity Syndrome

A peripheral retina, photoreceptor, or transient visual-system deficit has been suggested as a basis for dyslexia. We performed dark adaptation using a Goldmann-Weekers adaptometer on 41 dyslexic readers subjected to the Irlen Differential Perceptual Schedule for the Scotopic Sensitivity Syndrome and on 23 volunteers of normal reading ability, all aged between 10 and 20 years. 12 of the 41 disabled readers examined had abnormally poor dark adaptation at peripheral retinal locations consistent with a rod processing-system deficit.

Visual evoked potentials in dyslexics and normals: failure to find a difference in transient or steady-state responses

We measured transient and steady-state checkerboard contrast-reversal visual evoked potentials (VEPs) in ten dyslexics, five patient controls, and 11 normals over a range of contrasts and luminances. Latency, amplitude, and phase measurements failed to distinguish the responses of dyslexics from those of normals or patient controls. Decreases in luminance or contrast resulted in an increased latency of the transient VEP in all groups, but these changes also did not distinguish the responses of dyslexics from those of the controls. Response variability was similar in dyslexics and normals, but was increased in subjects with attention deficit-hyperactivity disorder (ADHD). Performance on standardized psychometric testing did differentiate the dyslexics from controls, but did not correlate with VEP responses.

Visual perceptual differences in the dyslexic reader: evidence of greater visual peripheral sensitivity to color and letter stimuli

The present study replicated the 1987 work of Geiger and Lettvin and of Grosser and Spafford in 1989 in providing evidence for greater peripheral sensitivity to individual letter and color stimuli for 10 dyslexics as compared to 10 normal readers. Earlier studies have demonstrated that dyslexics tend to favor clearer images in the peripheral retina as compared to the foveal advantage of proficient readers. For letter and color stimuli individually presented there was a statistically significant difference between dyslexics and proficient readers based on the ability of dyslexics to recognize both letter and color stimuli at greater peripheral distances from the point of fixation. This study directly supports the consideration of visual perceptual factors in the identification and discussion of dyslexia.

Still no evidence for a photoreceptor-level abnormality in dyslexia

In reply to Grosser and Spafford's (1992) defence of their theory that there is a photoreceptor-level abnormality in dyslexia we argue that (1) their original experiments do not tap rod functions, (2) the theory fails at the quantitative level, and (3) the proposition that rod inputs generate transient responses is false.

Reply to Stuart and Lovegrove's question, "visual processing deficits in dyslexia: receptors or neural mechanisms?"

Recently Stuart and Lovegrove questioned the receptor hypothesis of Grosser and Spafford which these authors used to account for the findings that dyslexic individuals have superior peripheral color discrimination to normal readers but also have poorer peripheral brightness discrimination than normal readers. Stuart and Lovegrove hypothesized that dyslexics instead have an impaired transient visual system. The receptor hypothesis is an attempt by Grosser and Spafford to link the functioning of the rods and cones to transient and sustained visual system functioning in a more specific manner than has been tried heretofore by suggesting that, while the parvocellular system is almost entirely fed by cones, both kinds of receptors drive magnocellular cells (but with the rapid onset of early transient system responding being due to the highly light sensitive rods). The rods are proposed to be the receptors initiating the rapid onset of responding in the magnocellular, transient pathway. In dyslexic individuals, they maintain, there are relatively fewer rods to provide for the rapid onset of transient system responses, resulting in a diminished capacity of the transient system to inhibit sustained system activity (as occurs with normal readers). Their receptor hypothesis supplements the concept of transient-vs-sustained system differences.