Lateral masking, levels of processing and stimulus category: A comparative study between normal and dyslexic readers

Disrupted Spatial Organization of Cued Exogenous Attention Persists Into Adulthood in Developmental Dyslexia

Purpose: Abnormal exogenous attention orienting and diffused spatial distribution of attention have been associated with reading impairment in children with developmental dyslexia. However, studies in adults have failed to replicate such relationships. The goal of the present study was to address this issue by assessing exogenous visual attention and its peripheral spatial distribution in adults with developmental dyslexia.

Methods: We measured response times, accuracy and eye movements of 18 dyslexics and 19 typical readers in a cued discrimination paradigm, in which stimuli were presented at different peripheral eccentricities.

Results: Results showed that adults with developmental dyslexia were slower that controls in using their mechanisms of exogenous attention orienting. Moreover, we found that while controls became slower with the increase of eccentricity, dyslexics showed an abnormal inflection at 10° as well as similar response times at the most distant eccentricities. Finally, dyslexics show attentional facilitation deficits above 12° of eccentricity, suggesting an attentional engagement deficit at far periphery.

Conclusion: Taken together, our findings indicate that, in dyslexia, the temporal deficits in orientation of attention and its abnormal peripheral spatial distribution are not restricted to childhood and persist into adulthood. Our results are, therefore, consistent with the hypothesis that the neural network underlying selective spatial attention is disrupted in dyslexia.

Too little or too much? Parafoveal preview benefits and parafoveal load costs in dyslexic adults

Two different forms of parafoveal dysfunction have been hypothesized as core deficits of dyslexic individuals: reduced parafoveal preview benefits ("too little parafovea") and increased costs of parafoveal load ("too much parafovea"). We tested both hypotheses in a single eye-tracking experiment using a modified serial rapid automatized naming (RAN) task. Comparisons between dyslexic and non-dyslexic adults showed reduced parafoveal preview benefits in dyslexics, without increased costs of parafoveal load. Reduced parafoveal preview benefits were observed in a naming task, but not in a silent letter-finding task, indicating that the parafoveal dysfunction may be consequent to the overload with extracting phonological information from orthographic input. Our results suggest that dyslexics' parafoveal dysfunction is not based on strict visuo-attentional factors, but nevertheless they stress the importance of extra-phonological processing. Furthermore, evidence of reduced parafoveal preview benefits in dyslexia may help understand why serial RAN is an important reading predictor in adulthood.

DOES DYSLEXIA DEVELOP FROM LEFT-EYE DOMINANCE?

The purpose of this theoretical analysis and synthesis is to indicate how left-eye sighting dominance may lead to reading failure through dysfunctional right hemisphere letter encoding. Differing compensatory strategies are postulated to lead to outcomes that include the development of the phonologically impaired and phonologically proficient subtypes of dyslexia as well as specific spelling disability. Evidence is presented indicating that these disorders might be prevented by delaying the introduction of letter writing until the age of 8 years. Early childhood speech categorization in children genetically at-risk of developing dyslexia is also considered from this perspective. Convergent support for this premature writing hypothesis is provided by a comparison with the development of the left-hand inverted writing posture.

The contribution of discrete-trial naming and visual recognition to rapid automatized naming deficits of dyslexic children with and without a history of language delay

Children with Developmental Dyslexia (DD) are impaired in Rapid Automatized Naming (RAN) tasks, where subjects are asked to name arrays of high frequency items as quickly as possible. However the reasons why RAN speed discriminates DD from typical readers are not yet fully understood. Our study was aimed to identify some of the cognitive mechanisms underlying RAN-reading relationship by comparing one group of 32 children with DD with an age-matched control group of typical readers on a naming and a visual recognition task both using a discrete-trial methodology, in addition to a serial RAN task, all using the same stimuli (digits and colors). Results showed a significant slowness of DD children in both serial and discrete-trial naming (DN) tasks regardless of type of stimulus, but no difference between the two groups on the discrete-trial recognition task. Significant differences between DD and control participants in the RAN task disappeared when performance in the DN task was partialled out by covariance analysis for colors, but not for digits. The same pattern held in a subgroup of DD subjects with a history of early language delay (LD). By contrast, in a subsample of DD children without LD the RAN deficit was specific for digits and disappeared after slowness in DN was partialled out. Slowness in DN was more evident for LD than for noLD DD children. Overall, our results confirm previous evidence indicating a name-retrieval deficit as a cognitive impairment underlying RAN slowness in DD children. This deficit seems to be more marked in DD children with previous LD. Moreover, additional cognitive deficits specifically associated with serial RAN tasks have to be taken into account when explaining deficient RAN speed of these latter children. We suggest that partially different cognitive dysfunctions underpin superficially similar RAN impairments in different subgroups of DD subjects.

Adults with dyslexia demonstrate large effects of crowding and detrimental effects of distractors in a visual tilt discrimination task

Previous research has shown that adults with dyslexia (AwD) are disproportionately impacted by close spacing of stimuli and increased numbers of distractors in a visual search task compared to controls [1]. Using an orientation discrimination task, the present study extended these findings to show that even in conditions where target search was not required: (i) AwD had detrimental effects of both crowding and increased numbers of distractors; (ii) AwD had more pronounced difficulty with distractor exclusion in the left visual field and (iii) measures of crowding and distractor exclusion correlated significantly with literacy measures. Furthermore, such difficulties were not accounted for by the presence of covarying symptoms of ADHD in the participant groups. These findings provide further evidence to suggest that the ability to exclude distracting stimuli likely contributes to the reported visual attention difficulties in AwD and to the aetiology of literacy difficulties. The pattern of results is consistent with weaker and asymmetric attention in AwD.

Naming Fluency in Dyslexic and Nondyslexic Readers: Differential Effects of Visual Crowding in Foveal, Parafoveal, and Peripheral Vision

Reading fluency is often indexed by performance on rapid automatized naming (RAN) tasks, which are known to reflect speed of access to lexical codes. We used eye tracking to investigate visual influences on naming fluency. Specifically, we examined how visual crowding affects fluency in a RAN-letters task on an item-by-item basis, by systematically manipulating the interletter spacing of items, such that upcoming letters in the array were viewed in the fovea, parafovea, or periphery relative to a given fixated letter. All lexical information was kept constant. Nondyslexic readers’ gaze durations were longer in foveal than in parafoveal and peripheral trials, indicating that visual crowding slows processing even for fluent readers. Dyslexics’ gaze durations were longer in foveal and parafoveal trials than in peripheral trials. Our results suggest that for dyslexic readers, influences of crowding on naming speed extend to a broader visual span (to parafoveal vision) than that for nondyslexic readers, but do not extend as far as peripheral vision. The findings extend previous research by elucidating the different visual spans within which crowding operates for dyslexic and nondyslexic readers in an online fluency task.

No Deficiency in Left-to-Right Processing of Words in Dyslexia but Evidence for Enhanced Visual Crowding

Whitney and Cornelissen hypothesized that dyslexia may be the result of problems with the left-to-right processing of words, particularly in the part of the word between the word beginning and the reader's fixation position. To test this hypothesis, we tachistoscopically presented consonant trigrams in the left and the right visual field (LVF, RVF) to 20 undergraduate students with dyslexia and 20 matched controls. The trigrams were presented at different locations (from –2.5° to + 2.5°) in both visual half fields. Participants were asked to identify the letters, and accuracy rates were compared. In line with the predictions of the SERIOL (sequential encoding regulated by inputs to oscillations within letter units) model of visual word recognition, a typical U-shaped pattern was found at all retinal locations. Accuracy also decreased the further away the stimulus was from the fixation location, with a steeper decrease in the LVF than in the RVF. Contrary to the hypothesis, the students with dyslexia showed the same pattern of results as did the control participants, also in the LVF, apart from a slightly lower accuracy rate, particularly for the central letter. The latter is in line with the possibility of enhanced crowding in dyslexia. In addition, in the dyslexia group but not in the control group the degree of crowding correlated significantly with the students’ word reading scores. These findings suggest that lateral inhibition between letters is associated with word reading performance in students with dyslexia.

Effect of orthographic processes on letter identity and letter-position encoding in dyslexic children

The ability to identify letters and encode their position is a crucial step of the word recognition process. However and despite their word identification problem, the ability of dyslexic children to encode letter identity and letter-position within strings was not systematically investigated. This study aimed at filling this gap and further explored how letter identity and letter-position encoding is modulated by letter context in developmental dyslexia. For this purpose, a letter-string comparison task was administered to French dyslexic children and two chronological age (CA) and reading age (RA)-matched control groups. Children had to judge whether two successively and briefly presented four-letter strings were identical or different. Letter-position and letter identity were manipulated through the transposition (e.g., RTGM vs. RMGT) or substitution of two letters (e.g., TSHF vs. TGHD). Non-words, pseudo-words, and words were used as stimuli to investigate sub-lexical and lexical effects on letter encoding. Dyslexic children showed both substitution and transposition detection problems relative to CA-controls. A substitution advantage over transpositions was only found for words in dyslexic children whereas it extended to pseudo-words in RA-controls and to all type of items in CA-controls. Letters were better identified in the dyslexic group when belonging to orthographically familiar strings. Letter-position encoding was very impaired in dyslexic children who did not show any word context effect in contrast to CA-controls. Overall, the current findings point to a strong letter identity and letter-position encoding disorder in developmental dyslexia.

Impaired letter-string processing in developmental dyslexia: what visual-to-phonology code mapping disorder?

Poor parallel letter-string processing in developmental dyslexia was taken as evidence of poor visual attention (VA) span, that is, a limitation of visual attentional resources that affects multi-character processing. However, the use of letter stimuli in oral report tasks was challenged on its capacity to highlight a VA span disorder. In particular, report of poor letter/digit-string processing but preserved symbol-string processing was viewed as evidence of poor visual-to-phonology code mapping, in line with the phonological theory of developmental dyslexia. We assessed here the visual-to-phonological-code mapping disorder hypothesis. In Experiment 1, letter-string, digit-string and colour-string processing was assessed to disentangle a phonological versus visual familiarity account of the letter/digit versus symbol dissociation. Against a visual-to-phonological-code mapping disorder but in support of a familiarity account, results showed poor letter/digit-string processing but preserved colour-string processing in dyslexic children. In Experiment 2, two tasks of letter-string report were used, one of which was performed simultaneously to a high-taxing phonological task. Results show that dyslexic children are similarly impaired in letter-string report whether a concurrent phonological task is simultaneously performed or not. Taken together, these results provide strong evidence against a phonological account of poor letter-string processing in developmental dyslexia.

Neural dissociation of phonological and visual attention span disorders in developmental dyslexia: FMRI evidence from two case reports

A dissociation between phonological and visual attention (VA) span disorders has been reported in dyslexic children. This study investigates whether this cognitively-based dissociation has a neurobiological counterpart through the investigation of two cases of developmental dyslexia. LL showed a phonological disorder but preserved VA span whereas FG exhibited the reverse pattern. During a phonological rhyme judgement task, LL showed decreased activation of the left inferior frontal gyrus whereas this region was activated at the level of the controls in FG. Conversely, during a visual categorization task, FG demonstrated decreased activation of the parietal lobules whereas these regions were activated in LL as in the controls. These contrasted patterns of brain activation thus mirror the cognitive disorders' dissociation. These findings provide the first evidence for an association between distinct brain mechanisms and distinct cognitive deficits in developmental dyslexia, emphasizing the importance of taking into account the heterogeneity of the reading disorder.

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.

When all hypotheses are right: a multifocal account of dyslexia

Many hypotheses have been proposed about the brain underpinnings of developmental dyslexia, but none of them accommodates the variable deficits observed. To address the issue of anatomical deficits in dyslexia; total and partial volumes, lateralization indices (LI), and local gray matter volumes (LGMV) were measured. Analyses were performed in large samples of control and dyslexic subjects, and in correlation with their performance on phonological, reading, and spelling tests. Results indicate an absence of net differences in terms of volumes but significant continuities and discontinuities between groups in their correlations between LI, LGMV, and performances. Structural connectivity also highlighted correlations between areas showing (dis)continuities between control and dyslexic subjects. Overall, our data put forward the idea of a multifocal brain abnormalities in dyslexia with a major implication of the left superior temporal gyrus, occipital-temporal cortices, and lateral/medial cerebellum, which could account for the diverse deficits predicted by the different theories.

Dyslexic participants show intact spontaneous categorization processes

We examine the performance of dyslexic participants on an unsupervised categorization task against that of matched non-dyslexic control participants. Unsupervised categorization is a cognitive process critical for conceptual development. Existing research in dyslexia has emphasized perceptual tasks and supervised categorization tasks (for which intact attentional processes are paramount), but there have been no studies on unsupervised categorization. Our investigation was based on Pothos and Chater's (Cognit. Sci., 2002; 26: 303-343) model of unsupervised categorization and the corresponding methodology for analysing results. Across all performance indices and various data-processing options, we could identify no difference between dyslexic and non-dyslexic participants.

Foveal crowding in posterior cortical atrophy: a specific early-visual-processing deficit affecting word reading

Visual crowding is a form of masking in which single-letter identification is compromised by the presence of additional letters or other simple visual forms in close proximity. This behavioural phenomenon has been studied most frequently in the context of amblyopic and normal peripheral vision. In the current study, we investigate this phenomenon in the context of two patients with peripheral dyslexia and a third with visual disorientation consequent to bilateral posterior cortical atrophy. In one case, reading showed the effects of word length typical of letter-by-letter reading, whereas the second case was unable to read any whole words. In a series of letter identification tasks, recognition accuracy was shown to decrease significantly in the presence of a range of flanking stimuli (e.g., letters, digits, letter fragments). Compatible with previous reports of the crowding phenomenon, the flanking effect was strengthened by increasing flanker proximity but was unaffected by target or flank size, flank contrast, target-flank lexicality, or flank category. One patient also showed amelioration of the flanking effect when the target and flankers were of opposite contrast polarity. To the best of our knowledge, this is the first demonstration of visual crowding in individuals with posterior cortical atrophy. We consider the relevance of these empirical findings to accounts of the letter-by-letter reading form of peripheral dyslexia. In particular, we suggest that crowding constitutes one specific form of early-visual-processing deficit, which impairs the reading process.