Inducing letter-by-letter dyslexia in normal readers

Letter-by-letter (LBL) dyslexia is an acquired reading disorder characterized by very slow reading and a large linear word length effect. This suggests the use of a sequential LBL strategy, in sharp contrast with the parallel letter processing used by normal subjects. Recently, we have proposed that the reading difficulty of LBL dyslexics is due to a deficit in discriminating visually similar letters based on parallel letter processing [Arguin, M., Fiset, S., & Bub, D. Sequential and parallel letter processing in letter-by-letter dyslexia. Cognitive Neuropsychology, 19, 535-555, 2002]. The visual mechanisms underlying this deficit and the LBL strategy, however, are still unknown. In this article, we propose that LBL dyslexic patients have lost the ability to use, for parallel letter processing, the optimal spatial frequency band for letter and word recognition. We claim that, instead, they rely on lower spatial frequencies for parallel processing, that these lower spatial frequencies produce confusions between visually similar letters, and that the LBL compensatory strategy allows them to extract higher spatial frequencies. The LBL strategy would thus increase the spatial resolution of the visual system, effectively resolving the issue pertaining to between-letter similarity. In Experiments 1 and 2, we succeeded in replicating the main features characterizing LBL dyslexia by having normal individuals read low-contrast, high-pass-filtered words. Experiment 3, conducted in LBL dyslexic L.H., shows that, indeed, the letter confusability effect is based on low spatial frequencies, whereas this effect was not supported by high spatial frequencies.     

Visual evoked potential abnormalities in dyslexic children.

Developmental reading disability (dyslexia) has traditionally been attributed to impaired linguistic skills. Recent psychophysical data suggest that dyslexia may be related to a visual perceptual deficit. A few visual evoked potential (VEP) studies have addressed this hypothesis, but their results are far from consistent. We submitted 9 dyslexic subjects and 9 age- and sex-matched normal controls to checkerboard pattern reversal VEPs. The main experimental variables were: large (0.5 cycles per degree; cpd) and small (2 cpd) checks and two reversal frequencies (2.1 Hz and 8 Hz); mean luminance and contrast (60 cd/m2 and 50%, respectively) were kept constant in all four conditions. Transient VEP (2.1 Hz) parameters did not differ between controls and dyslexics at 2 cpd. At 0.5 cpd, N70 amplitude was significantly smaller and N70 latency significantly shorter in dyslexics. Amplitudes for the fundamental frequency (8 Hz), as well as for the second and third harmonics of the steady-state VEPs were smaller in dyslexics for both stimulus sizes. A discriminant analysis correctly classified each subject. Our data confirm the hypothesis of a perceptual deficit in dyslexic subjects. The abnormalities are related to spatial and temporal stimulus frequencies: they appear when large stimuli are presented, or when the stimulation frequency is high. These data support the hypothesis of selective magnocellular dysfunction in dyslexia.     

Luminance and chromatic contrast sensitivity in dyslexia: the magnocellular deficit hypothesis revisited

The hypothesis of a magnocellular channel deficit in dyslexia was tested. Subjects were 10-year-old dyslexics and normal readers. Psychophysical thresholds for luminance and chromatic contrasts were estimated using black and white and red and green sinusoidal gratings of various spatial frequencies, presented in static and dynamic conditions (drift and reversal). Significant group differences were found for luminance contrast, with a higher sensitivity in dyslexics. No group differences were obtained for chromatic contrast. High luminance contrast sensitivity correlated with low reading and writing skills. The typical finding of an increase contrast sensitivity to low spatial frequency gratings, due to their dynamic presentations, was absent in dyslexics. The results provide support for the magnocellular deficit hypothesis. The pattern of this deficit, however, is much more complex than that emerging from previous research.     

Evaluating the effects of spatial frequency on migraines by using pattern-reversal visual evoked potentials.

OBJECTIVE: To clarify the effects of contrast and spatial frequency in patients with migraine by means of pattern-reversal visual evoked potentials (PVEPs). METHODS: PVEPs were obtained from 14 patients who had migraine without aura (MO), 11 patients who had migraine with aura (MA), and 25 age-matched, healthy controls (CO). PVEPs were binocularly recorded with a reversal rate of 1Hz (2 reversal/s) at 3 spatial frequencies (0.5, 1.0 and 4.0 cpd) at high (98%), medium (83%) and low (29%) contrast. N75, P100 and N135 latency and the amplitudes of P50-N75, N75-P100 and P100-N135 were analyzed. RESULTS: Increased amplitude of PVEPs in patients with migraines were revealed at 3 different spatial frequencies in all components. The MO and the MA showed increased amplitudes mostly in high contrasts (98%). These findings were detected more at a high spatial frequency (4.0 cpd) than at a low spatial frequency (0.5 cpd). Increased amplitude with prolonged latency of N135 were found both in MO and MA at 4.0 cpd. CONCLUSIONS: We conclude that pattern stimuli of high contrasts may be particularly effective in uncovering abnormal cortical reactivity which may be modified in the primary and secondary visual cortex in the interictal state of migraine. SIGNIFICANCE: These findings indicate that there is abnormal visual cortex processing in patients with migraine.     

Auditory event related potentials and source current density estimation in phonologic/auditory dyslexics.

OBJECTIVE: To determine the generality of auditory processing impairment in phonologic dyslexics by studying their auditory Event-Related Potentials (ERPs) and the spatio-temporal distribution of their brain activity to auditory linguistic and non-linguistic stimuli with temporal and spectral discriminating cues. METHODS: Fourteen adult phonologic dyslexics and 14 normal reading students, all with high academic achievements, were compared. ERP waveform analysis and current density source estimation (Low resolution Electromagnetic Tomographic Analysis-LORETA) were conducted on 21-channel records from subjects who passively listened or actively discriminated 4 types of auditory stimuli: linguistic and non-linguistic stimuli that differed in spectral or temporal characteristics. RESULTS: Significant differences were found for all ERP latencies (N1, P2, N2, P3) in response to all stimuli, with dyslexics presenting longer latencies compared to normal readers. Current density distributions and their time courses also differed significantly, regardless of stimulus type or attention allocation. Among normal readers, early activity (around N1) was characterized by a rapid change of maximum activity from right to left temporal lobe. Later activity (around P3) was characterized by a stable temporal activity with bilaterally synchronous peak activity. Among the dyslexics, the early N1 activity was stable with left hemisphere prominence, with no alternation between the hemispheres, while the later P3 activity peaked earlier in the right hemisphere than in the left. CONCLUSIONS: Dyslexics were different from controls in processing all auditory stimuli: verbal and non-verbal stimuli with temporal as well as with spectral discriminating cues. The differences mainly consisted of latency and time courses of current density distributions, beginning as early as N1 and extending to the late P3. SIGNIFICANCE: Differences in processing auditory stimuli by phonologic dyslexics are not restricted to linguistic (phonological) stimuli, supporting a general auditory processing impairment in phonologic dyslexia.     

Altered patterns of directed connectivity within the reading network of dyslexic children and their relation to reading dysfluency

Reading is a complex cognitive skill subserved by a distributed network of visual and language-related regions. Disruptions of connectivity within this network have been associated with developmental dyslexia but their relation to individual differences in the severity of reading problems remains unclear. Here we investigate whether dysfunctional connectivity scales with the level of reading dysfluency by examining EEG recordings during visual word and false font processing in 9-year-old typically reading children (TR) and two groups of dyslexic children: severely dysfluent (SDD) and moderately dysfluent (MDD) dyslexics. Results indicated weaker occipital to inferior-temporal connectivity for words in both dyslexic groups relative to TRs. Furthermore, SDDs exhibited stronger connectivity from left central to right inferior-temporal and occipital sites for words relative to TRs, and for false fonts relative to both MDDs and TRs. Importantly, reading fluency was positively related with forward and negatively with backward connectivity. Our results suggest disrupted visual processing of words in both dyslexic groups, together with a compensatory recruitment of right posterior brain regions especially in the SDDs during word and false font processing. Functional connectivity in the brain’s reading network may thus depend on the level of reading dysfluency beyond group differences between dyslexic and typical readers.     

Auditory and visual automatic attention deficits in developmental dyslexia

Several studies have provided evidence for a phonological deficit in developmental dyslexia. However, recent studies provide evidence for a multimodal temporal processing deficit in dyslexia. In fact, dyslexics show both auditory and visual abnormalities, which could result from a more general problem in the perceptual selection of stimuli. Here we report the results of a behavioral study showing that children with dyslexia have both auditory and visual deficits in the automatic orienting of spatial attention. These findings suggest that a deficit of selective spatial attention may distort the development of phonological and orthographic representations that is essential for learning to read.     

Abnormal spatial frequency processing in high-functioning children with pervasive developmental disorder (PDD).

OBJECTIVE: Basic abnormalities in visual information processing could be associated with the local visual bias often found in subjects with PDD. Therefore, the present study investigated the existence of deficits in spatial frequency processing at an early sensory level in children with PDD. METHODS: Visual evoked potentials (VEPs) and VEP dipole sources elicited by high and low spatial frequency gratings were analyzed in high-functioning children with PDD and matched controls. RESULTS: Around 80 ms (N80-latency) children with PDD did not show the same robust differences between high and low spatial frequencies in VEP amplitude and VEP brain sources as controls, because of atypical processing of high frequencies. Analyses at the P1-latency (130 ms) revealed that, although similar inferior-medial brain sources were activated for the processing of both spatial frequencies in the PDD and control group, source strength in response to both frequencies was weaker in the PDD compared to control group. Moreover, additional superior-lateral brain sources were activated during the processing of both frequencies in the PDD group. CONCLUSIONS: Decreased specialized processing of high and low spatial frequencies might be a robust characteristic of PDD. Early in processing abnormalities in high spatial frequency processing seem to occur in PDD. At a later phase in processing there seems to be both atypical high and low spatial frequency processing. Considering that the processing of specific spatial frequencies plays an important role in the processing of global and local aspects of hierarchical stimuli and faces and of emotions, present data suggest that peculiarities in PDD subjects with respect to these stimuli might be related to an abnormality in more fundamental visual processes. SIGNIFICANCE: A basic abnormality in visual frequency processing is established in children with PDD.     

Contribution of the anterior insula to temporal auditory processing deficits in developmental dyslexia

Developmental dyslexia has been assumed to arise from general auditory deficits, compromising rapid temporal integration both of linguistic and nonlinguistic acoustic stimuli. Because the effort of auditory temporal processing of speech and nonspeech test materials may depend on presentation rate, fMRI measurements were performed in dyslexics and controls during passive listening to series of syllable and click sounds, using a parametric approach. Controls showed a decrease of hemodynamic brain activation within the right and an increase within the left anterior insula as a function of the presentation rate both of click as well as syllable trains. By contrast, dyslexics exhibited this profile of hemodynamic responses under the nonspeech condition only. As concerns syllables, activation in dyslexics did not depend on presentation rate. Moreover, a subtraction analysis of hemodynamic main effects across conditions and groups revealed decreased activation both of the left and right anterior insula in dyslexics compared to controls during application both of click and syllables. These results indicate, in line with preceding studies, that the insula of both hemispheres is involved in auditory temporal processing of nonlinguistic auditory stimuli and demonstrate, furthermore, that these operations of intrasylvian cortex also extend to the linguistic domain. In addition, our data suggest that the anterior insula represents an important neural correlate of deficient temporal processing of speech and nonspeech sounds in dyslexia. Hum Brain Mapp 2009. © 2008 Wiley‐Liss, Inc.     

Auditory temporal processing deficit in dyslexia is associated with enhanced sensitivity in the visual modality

Developmental dyslexia has been associated with a deficit in temporal processing, but it is controversial whether the postulated deficit is pansensory or limited to the auditory modality. We present psychophysical assessment data of auditory and visual temporal processing abilities in children with dyslexia. While none of the dyslexic children displayed temporal processing abnormalities in the visual sensory modality, dyslexics with poor auditory temporal scores reached high-level visual performance. Our results do not confirm the hypothesis of a general temporal processing deficit for dyslexia but suggest that limitations in auditory temporal processing might be compensated for by a well-functioning visual sensory modality.     

Functional brain network abnormalities during verbal working memory performance in adolescents and young adults with dyslexia

Behavioral and functional neuroimaging studies indicate deficits in verbal working memory (WM) and frontoparietal dysfunction in individuals with dyslexia. Additionally, structural brain abnormalities in dyslexics suggest a dysconnectivity of brain regions associated with phonological processing. However, little is known about the functional neuroanatomy underlying cognitive dysfunction in dyslexia. In this study, functional magnetic resonance imaging and multivariate analytic techniques were used to investigate patterns of functional connectivity during a verbal WM task in individuals with dyslexia (n = 12) and control subjects (n = 13). Dyslexics were not significantly slower than controls; however, they were less accurate with increasing WM demand. Independent component analysis identified 18 independent components (ICs) among which two ICs were selected for further analyses. These ICs included functional networks which were positively correlated with the delay period of the activation task in both healthy controls and dyslexics. Connectivity abnormalities in dyslexics were detected within both networks of interest: within a “phonological” left-lateralized prefrontal network, increased functional connectivity was found in left prefrontal and inferior parietal regions. Within an “executive” bilateral frontoparietal network, dyslexics showed a decreased connectivity pattern comprising bilateral dorsolateral prefrontal and posterior parietal regions, while increased connectivity was found in the left angular gyrus, the left hippocampal cortex and the right thalamus. The functional connectivity strength in the latter regions was associated with WM task accuracy and with the numbers of errors during a spelling test. These data suggest functional connectivity abnormalities in two spatiotemporally dissociable brain networks underlying WM dysfunction in individuals with dyslexia.     

Luminance and spatial attention effects on early visual processing

Event-related potentials (ERPs) were recorded from healthy subjects in response to unilaterally flashed high and low luminance bar stimuli presented randomly to left and right field locations. Their task was to covertly and selectively attend to either the left or right stimulus locations (separate blocks) in order to detect infrequent shorter target bars of either luminance. Independent of attention, higher stimulus luminance resulted in higher ERP amplitudes for the posterior N95 (80–110 ms), occipital P1 (110–140 ms), and parietal N1 (130–180 ms). Brighter stimuli also resulted in shorter peak latency for the occipital N1 component (135–220 ms); this effect was not observed for the N1 components over parietal, central or frontal regions. Significant attention-related amplitude modulations were obtained for the occipital P1, occipital, parietal and central N1, the occipital and parietal P2, and the parietal N2 components; these components were larger to stimuli at the attended location. In contrast to the relatively short latencies of both spatial attention and luminance effects, the first interaction between luminance and spatial attention effects was observed for the P3 component to the target stimuli (350–750 ms). This suggests that interactions of spatial attention and stimulus luminance previously reported for reaction time measures may not reflect the earliest stages of sensory/perceptual processing. Differences in the way in which luminance and attention affected the occipital P1, occipital N1 and parietal N1 components suggest dissociations among these ERPs in the mechanisms of visual and attentional processing they reflect. Nonetheless, scalp current density mappings of the attention effects throughout the latency ranges of the P1 and N1 components show the most prominent attention-related activity to be in lateral occipital scalp areas. Such a pattern is consistent with the spatially selective filtering of information into the ventral stream of visual processing which is reponsible for complex feature analysis and object identification.     

Form and motion coherence processing in dyspraxia: evidence of a global spatial processing deficit

Form and motion coherence was tested in children with dyspraxia and matched controls to assess their global spatial and global motion processing abilities. Thresholds for detecting form coherence patterns were significantly higher in the dyspraxic group than in the control group. No corresponding difference was found on the motion coherence task. We tested eight children with dyspraxic disorder (mean age 8.2 years) and 50 verbal-mental-age matched controls (mean age 8.4 years) to test for a neural basis to the perceptual abnormalities observed in dyspraxia. The results provide evidence that children with dyspraxia have a specific impairment in the global processing of spatial information. This finding contrasts with other developmental disorders such as Williams syndrome, autism and dyslexia where deficits have been found in global motion processing and not global form processing. We conclude that children with dyspraxia may have a specific occipitotemporal deficit and we argue that testing form and motion coherence thresholds might be a useful diagnostic tool for the often coexistent disorders of dyspraxia and dyslexia.     

Visual implicit memory deficit and developmental surface dyslexia: a case of early occipital damage

This study reports the case of EBON, a fifteen-year-old right-handed female Swedish student, who suffered an early medial/dorsal occipital brain lesion and showed a clearly defined pattern of developmental surface dyslexia. EBON and 17 controls were examined with within and cross-modality (visual and auditory) word stem completion tasks together with tasks requiring free-recall and recognition for visually and auditory presented words. Compared to age-matched controls, EBON was found to show a significant deficit of visual priming following visual presentation, and a deficit approaching significance following auditory presentation. Explicit memory and visual and spatial abilities were not significantly different from controls. Therefore, EBON represents the first childhood case establishing the role of occipital regions in visual priming, as well as illustrating a profile of surface reading difficulty as a developmental consequence of this locus of lesion.     

Hemispheric processing of spatial frequencies in two commissurotomy patients

To test the hypothesis that in humans the left brain hemisphere is specialized for processing high spatial frequencies while the right hemisphere is specialized for processing low spatial frequencies, pairs of Gaussian windowed sinusoidal gratings were presented for 167 msec within the left and right visual fields of two commissurotomy patients. The gratings employed had spatial frequencies ranging from 1 to 8 cycles per degree, and horizontal or vertical orientations. The two gratings in each pair were identical in spatial frequency but could differ in orientation. Subjects reported if their orientations were the same or different. Twelve normal controls were also run. Accuracy data provides no indication of a relative advantage for high frequencies in the RVF or low frequencies in the LVF. One commissurotomy subject showed a trend in the reverse direction; the other was better with LVF presentations for all spatial frequencies. Control subjects failed to show a spatial frequency x visual field interaction. These outcomes suggest that at the processing stages required by the task, the hemispheres are not specialized for particular ranges of spatial frequencies.     

Understanding the time variant connectivity of the language network in developmental dyslexia: new insights using Granger causality

The reading process takes place in a neuronal network comprising the inferior frontal, posterior dorsal and posterior ventral brain areas. It is suggested that developmental dyslexia is caused by a disruption of the two posterior network areas. What remains debatable is whether these areas are affected in their functionality or whether the neuronal networking (connectivity) of these areas suffer from a disturbed information transfer. Thus, it is of major interest to investigate the time flow of the directed information transfer (time variant connectivity) within the neuronal reading network of dyslexic subjects. We investigated adolescents with dyslexia and normal-reading controls with functional magnetic resonance imaging and electroencephalography (EEG) with a paradigm addressing basic visual, orthographic and phonological processing. EEG data were analyzed with the time variant Granger causality index (tvGCI) to investigate the temporal order of the directed information transfer (time variant causal connectivity: which network node passes when information to which network node) during reading in dyslexic readers. Results show that the reading network of dyslexic readers comprises the same brain areas as identified in normal-reading subjects. The tvGCI analysis of the network profiles of dyslexic readers indicates that dyslexics show a difference in timing and localization of connectivity within this reading network compared to normal readers. Dyslexic readers use right hemisphere language areas to counterbalance posterior left hemisphere processing deficits. The compensatory involvement of homologue right hemisphere brain areas for the reading process may be the neurobiological background for the significantly longer reading times by dyslexics.     

Altered electroencephalographic networks in developmental dyslexia after remedial training: a prospective case-control study

Electroencephalographic studies using graph theoretic analysis have found aberrations in functional connectivity in children with developmental dyslexia. However, how the training with visual tasks can change the functional connectivity of the semantic network in developmental dyslexia is still unclear. We looked for differences in local and global topological properties of functional networks between 21 healthy controls and 22 dyslexic children(8–9 years old) before and after training with visual tasks in this prospective case-control study. The minimum spanning tree method was used to construct the subjects' brain networks in multiple electroencephalographic frequency ranges during a visual word/pseudoword discrimination task. We found group differences in the theta, alpha, beta and gamma bands for four graph measures suggesting a more integrated network topology in dyslexics before the training compared to controls. After training, the network topology of dyslexic children had become more segregated and similar to that of the controls. In the θ, α and β1-frequency bands, compared to the controls, the pre-training dyslexics exhibited a reduced degree and betweenness centrality of the left anterior temporal and parietal regions. The simultaneous appearance in the left hemisphere of hubs in temporal and parietal(α, β1), temporal and superior frontal cortex(θ, α), parietal and occipitotemporal cortices(β1), identified in the networks of normally developing children was not present in the brain networks of dyslexics. After training, the hub distribution for dyslexics in the theta and beta1 bands had become similar to that of the controls. In summary, our findings point to a less efficient network configuration in dyslexics compared to a more optimal global organization in the controls. This is the first study to investigate the topological organization of functional brain networks of Bulgarian dyslexic children. Approval for the study was obtained from the Ethics Committee of the Institute of Neurobiology and the Institute for Population and Human Studies, Bulgarian Academy of Sciences(approval No. 02-41/12.07.2019) on March 28, 2017, and the State Logopedic Center and the Ministry of Education and Science(approval No. 09-69/14.03.2017) on July 12, 2019.     

Auditory temporal processing and dyslexia in an orthographically consistent language

We examined two hypotheses relating auditory processing to dyslexia in Greek, an orthographically consistent language. Study I examined the “P-center” or “beat detection” hypothesis (Goswami et al., 2002) in a sample of Grade 6 dyslexics, Grade 6 chronological age (CA) controls, and Grade 4 reading age (RA) controls. Study II examined the “temporal processing,” or “rapid auditory processing” hypothesis (Tallal, 1980) in a sample of Grade 7 dyslexics, CA controls, and in two groups of CA matched children with low frequency discrimination or low tone sequencing performance. Both studies indicate that (a) as a group, dyslexic children did not perform significantly worse on auditory processing measures than the control groups; (b) measures of auditory processing mostly did not account for unique amount of variance in phonological processing, reading, or spelling; and (c) at an individual level of analysis, some of the dyslexic children experienced auditory temporal processing deficits. Implications on the importance of auditory processing in reading in orthographically consistent languages are discussed.     

Neural mechanisms of global/local processing of bilateral visual inputs: an ERP study.

OBJECTIVE: Examine the neural mechanisms of global/local processing of multiple hierarchical stimuli. METHODS: Event-related brain potentials (ERPs) were recorded from adults who selectively attended to the global or local level of two compound letters that were simultaneously presented in the left and right visual fields, respectively. The compound stimuli were either broadband in spatial frequency (SF) spectrum or contrast balanced to remove low SFs. Subjects were asked to detect the presence of a global or local target that might appear in either the left or the right visual field in separate blocks of trials. RESULTS: Attention to the local level of broadband stimuli elicited a positivity over lateral occipital sites at 80-120 ms (P1) with larger amplitude than those in the global attention condition. However, global attention produced an enhanced positivity at 240-320 ms (P2) over lateral occipital sites relative to local attention. Both the P1 and P2 waves in the global condition were of larger amplitudes over the left than right hemispheres. Contrast balancing eliminated the P1 and P2 effects and modulated the hemispheric asymmetry of the long-latency occipital positivity. CONCLUSIONS: The results provide ERP evidence for modulations of neural activities in the visual cortex by global/local attention to concurrently presented multiple compound letters. Moreover, the modulation of brain activities by global/local attention depends upon the presence of low SFs in the compound stimuli. SIGNIFICANCE: The ERP results of this study contribute to the understanding of neural mechanisms of the processing of simultaneously-presented multiple compound stimuli.     

Patterns of task-related slow brain potentials in dyslexia

Six dyslexic youths who met the Hughes and Denckla criteria of "dyslexia pure" and 10 normal control subjects were investigated with DC recordings of task-related cortical negativity using 10 leads over the right and left hemispheres. Potentials were recorded during the following tasks: reading, orthographic error detection, letter series, word assembly, word fluency, finding of antonyms, and viewing of Necker cubes. Both groups exhibited greater right hemisphere negativity during the Necker cube task. Whereas controls revealed greater left hemisphere negativity during linguistic tasks, the reverse was found with dyslexics. The right shift was pronounced with the reading, orthographic error detection, and antonym conditions. Differences between the two groups were also found for the pattern of cortical distribution. We conclude that dyslexia is associated with changes in the lateral distribution of cortical activity during certain types of language processing.