Sylviane Valdois

Developmental dyslexia: The visual attention span deficit hypothesis

The visual attention (VA) span is defined as the amount of distinct visual elements which can be processed in parallel in a multi-element array. Both recent empirical data and theoretical accounts suggest that a VA span deficit might contribute to developmental dyslexia, independently of a phonological disorder. In this study, this hypothesis was assessed in two large samples of French and British dyslexic children whose performance was compared to that of chronological-age matched control children. Results of the French study show that the VA span capacities account for a substantial amount of unique variance in reading, as do phonological skills. The British study replicates this finding and further reveals that the contribution of the VA span to reading performance remains even after controlling IQ, verbal fluency, vocabulary and single letter identification skills, in addition to phoneme awareness. In both studies, most dyslexic children exhibit a selective phonological or VA span disorder. Overall, these findings support a multi-factorial view of developmental dyslexia. In many cases, developmental reading disorders do not seem to be due to phonological disorders. We propose that a VA span deficit is a likely alternative underlying cognitive deficit in dyslexia.

A connectionist multiple-trace memory model for polysyllabic word reading.

A connectionist feedforward network implementing a mapping from orthography to phonology is described. The model develops a view of the reading system that accounts for both irregular word and pseudoword reading without relying on any system of explicit or implicit conversion rules. The model assumes, however, that reading is supported by 2 procedures that work successively: a global procedure using knowledge about entire words and an analytic procedure based on the activation of word syllabic segments. The model provides an account of the basic effects that characterize human skilled reading performance including a frequency by consistency interaction and a position-of-irregularity effect. Furthermore, early in training, the network shows a performance similar to that of less skilled readers. It also offers a plausible account of the patterns of acquired phonological and surface dyslexia when lesioned in different ways.

Phonological and visual processing deficits can dissociate in developmental dyslexia : Evidence from two case studies

The present study describes two Frenchteenagers with developmental reading andwriting impairments whose performance wascompared to that of chronological age andreading age matched non-dyslexic participants.Laurent conforms to the pattern of phonologicaldyslexia: he exhibits a poor performance inpseudo-word reading and spelling, producesphonologically inaccurate misspellings butreads most exception words accurately. Nicolas,in contrast, is poor in reading and spelling ofexception words but is quite good atpseudo-word spelling, suggesting that hesuffers from surface dyslexia and dysgraphia.The two participants were submitted to anextensive battery of metaphonological tasks andto two visual attentional tasks. Laurentdemonstrated poor phonemic awareness skills butgood visual processing abilities, while Nicolasshowed the reverse pattern with severedifficulties in the visual attentional tasksbut good phonemic awareness. The presentresults suggest that a visual attentionaldisorder might be found to be associated withthe pattern of developmental surface dyslexia.The present findings further show thatphonological and visual processing deficits candissociate in developmental dyslexia.

Predictors of developmental dyslexia in European orthographies with varying complexity

BACKGROUND  The relationship between phoneme awareness, rapid automatized naming (RAN), verbal short-term/working memory (ST/WM) and diagnostic category is investigated in control and dyslexic children, and the extent to which this depends on orthographic complexity. METHODS General cognitive, phonological and literacy skills were tested in 1,138 control and 1,114 dyslexic children speaking six different languages spanning a large range of orthographic complexity (Finnish, Hungarian, German, Dutch, French, English). RESULTS Phoneme deletion and RAN were strong concurrent predictors of developmental dyslexia, while verbal ST/WM and general verbal abilities played a comparatively minor role. In logistic regression models, more participants were classified correctly when orthography was more complex. The impact of phoneme deletion and RAN-digits was stronger in complex than in less complex orthographies. CONCLUSIONS Findings are largely consistent with the literature on predictors of dyslexia and literacy skills, while uniquely demonstrating how orthographic complexity exacerbates some symptoms of dyslexia.

The eye movements of dyslexic children during reading and visual search : Impact of the visual attention span

The eye movements of 14 French dyslexic children having a VA span reduction and 14 normal readers were compared in two tasks of visual search and text reading. The dyslexic participants made a higher number of rightward fixations in reading only. They simultaneously processed the same low number of letters in both tasks whereas normal readers processed far more letters in reading. Importantly, the childrens VA span abilities related to the number of letters simultaneously processed in reading. The atypical eye movements of some dyslexic readers in reading thus appear to reflect difficulties to increase their VA span according to the task request.

The visual attention span deficit in dyslexia is visual and not verbal

The visual attention (VA) span deficit hypothesis of dyslexia posits that letter string deficits are a consequence of impaired visual processing. Alternatively, some have interpreted this deficit as resulting from a visual-to-phonology code mapping impairment. This study aims to disambiguate between the two interpretations by investigating performance in a non-verbal character string visual categorization task with verbal and non-verbal stimuli. Results show that VA span ability predicts performance for the non-verbal visual processing task in normal reading children. Furthermore, VA span impaired dyslexic children are also impaired for the categorization task independently of stimuli type. This supports the hypothesis that the underlying impairment responsible for the VA span deficit is visual, not verbal.

Sequential or simultaneous visual processing deficit in developmental dyslexia

The ability of dyslexic children with or without phonological problems to process simultaneous and sequential visual information was assessed using two tasks requiring the oral report of simultaneously or sequentially displayed letter-strings. The two groups were found to exhibit a simultaneous visual processing deficit but preserved serial processing skills. However, the impairment in simultaneous processing was larger in the dyslexic group with no phonological disorder. Although sequential and simultaneous processing skills both related to reading performance, simultaneous processing alone significantly contributed to reading speed and accuracy. These findings suggest that a simultaneous processing disorder might contribute to developmental dyslexia.

Polysyllabic pseudo-word processing in reading and lexical decision: Converging evidence from behavioral data, connectionist simulations and functional MRI

The cognitive mechanisms involved in polysyllabic pseudo-word processing -- and their neurobiological correlates -- were studied through the analysis of length effects on French words and pseudo-words in reading and lexical decision. Connectionist simulations conducted on the ACV98 network paralleled the behavioral data in showing a strong length effect on naming latencies for pseudo-words only and the absence of length effect for both words and pseudo-words in lexical decision. Length effects in reading were characterized at the neurobiological level by a significant and specific activity increase for pseudo-words as compared to words in the right lingual gyrus (BA 19), the left superior parietal lobule and precuneus (BA7), the left middle temporal gyrus (BA21) and the left cerebellum. The behavioral results suggest that polysyllabic pseudo-word reading mainly relies on an analytic procedure. At the biological level, additional activations in visual and visual attentional brain areas during long pseudo-word reading emphasize the role of visual and visual attentional processes in pseudo-word reading. The present findings place important constraints on theories of reading in suggesting the involvement of a serial mechanism based on visual attentional processing in pseudo-word reading.

Superior parietal lobule dysfunction in a homogeneous group of dyslexic children with a visual attention span disorder

A visual attention (VA) span disorder has been reported in dyslexic children as potentially responsible for their poor reading outcome. The purpose of the current paper was to identify the cerebral correlates of this VA span disorder. For this purpose, 12 French dyslexic children with severe reading and VA span disorders and 12 age-matched control children were engaged in a categorisation task under fMRI. Two flanked and isolated conditions were designed which both involved multiple-element simultaneous visual processing but taxed visual attention differently. For skilled readers, flanked stimuli processing activated a large bilateral cortical network comprising the superior and inferior parietal cortex, the inferior temporal cortex, the striate and extrastriate visual cortex, the middle frontal cortex and the anterior cingulate cortex while the less attention-demanding task of isolated stimuli only activated the inferior occipito-temporal cortex bilaterally. With respect to controls, the dyslexic children showed significantly reduced activation within bilateral parietal and temporal areas during flanked processing, but no difference during the isolated condition. The neural correlates of the processes involved in attention-demanding multi-element processing tasks were more specifically addressed by contrasting the flanked and the isolated conditions. This contrast elicited activation of the left precuneus/superior parietal lobule in the controls, but not in the dyslexic children. These findings provide new insights on the role of parietal regions, in particular the left superior parietal lobule, in the visual attention span and in developmental dyslexia.

A case study of developmental phonological dyslexia: Is the attentional deficit in the perception of rapid stimuli sequences amodal?

The attentional blink (AB) refers to a decrease in accuracy that occurs when participants are required to detect the second of two rapidly sequential targets displayed randomly in a stream of distracters. Dyslexic individuals have been shown to exhibit a prolonged AB in the visual modality, interpreted as evidence of sluggish attentional shifting (SAS). However, the amodal SAS theory predicts that the disorder should further extend to the auditory modality, then resulting in a phonological disorder as typically found in developmental dyslexia. Otherwise, it has been demonstrated that a visual attention (VA) span deficit contributes to the poor reading outcome of dyslexic individuals, independently of their phonological skills. The present study assesses the amodality assumption of the SAS theory together with questioning its relation with the VA span deficit. For this purpose, visual and auditory ABs were explored in a well compensated young adult, LL, who exhibits a pure phonological dyslexia characterised by poor pseudo-word processing and poor phonological skills but preserved VA span. The investigation revealed two different kinds of deficits in LL. Her AB was prolonged and marginally deeper in the visual modality whereas a primarily deeper in amplitude and a subtle prolonged AB was found in the auditory modality. The atypical performance patterns of LL in both modalities suggest that her perceptual attention disorder is amodal as predicted by the SAS theory. This amodal disorder was here reported in a dyslexic participant with a phonological disorder, well in accordance with the hypothesis that sluggish auditory attention shifting contributes to difficulties in phoneme awareness and literacy acquisition. Furthermore, prolonged VA blink was observed in the absence of VA span disorder, thus suggesting that visual attentional shifting and VA span might be distinct mechanisms, contributing independently to reading acquisition and developmental dyslexia.