Neuronal asymmetries in primary visual cortex of dyslexic and nondyslexic brains

Failure of resting-state frontal-occipital connectivity in linking visual perception with reading fluency in Chinese children with developmental dyslexia

It is widely accepted that impairment in visual perception impedes children's reading development, and further studies have demonstrated significant enhancement in reading fluency after visual perceptual training. However, the mechanism of the neural linkage between visual perception and reading is unclear. The purpose of this study was to examine the intrinsic functional relationship between visual perception (indexed by the texture discrimination task,TDT) and reading ability (character reading and reading fluency) in Chinese children with developmental dyslexia (DD) and those with typical development (TD). The resting-state functional connectivity (RSFC) between the primary visual cortex (V1, BA17) and the entire brain was analyzed. In addition, how RSFC maps are associated with TDT performance and reading ability in the DD and TD groups was examined. The results demonstrated that the strength of the RSFC between V1 and the left middle frontal gyrus (LMFG, BA9/BA46) was significantly correlated with both the threshold (SOA) of the TDT and reading fluency in TD children but not in DD children. Moreover, LMFG-V1 resting-state connectivity played a mediating role in the association of visual texture discrimination and reading fluency, but not in character reading, in TD children. In contrast, this mediation was absent in DD children, albeit their strengths of RSFC between V1 and the left middle frontal gyrus (LMFG) were comparable to those for the TD group. These findings indicate that typically developing children use the linkage of the RSFC between the V1 and LMFG for visual perception skills, which in turn promote fluent reading; in contrast, children with dyslexia, who had higher TDT thresholds than TD children, could not take advantage of their frontal-occipital connectivity to improve reading fluency abilities. These findings suggest that visual perception plays an important role in reading skills and that children with developmental dyslexia lack the ability to use their frontal-occipital connectivity to link visual perception with reading fluency.

Localization of Early-Stage Visual Processing Deficits at Schizophrenia Spectrum Illness Onset Using Magnetoencephalography

Impairments in early-stage visual processing are observed in chronic psychosis. However, their presence, localization within the brain, and contribution to cognitive symptoms remain less well established early in disease course. The present study utilized magnetoencephalography (MEG) to examine sensory responses within primary visual cortex (V1). MEG was recorded from 38 individuals diagnosed with a schizophrenia spectrum illness at first psychotic episode (FESz) and 38 matched healthy controls (HC) during visual search tasks. The inverse solution for cortical activity contributing to the M100 visual evoked field was derived. Task performance and V1 activation were compared between groups. FESz exhibited a reduced V1 response relative to HC. This group deficit, however, was selective for the left hemisphere (LH). A similar interaction was observed for response time with FESz exhibiting slower responses to right visual field targets, a difference not observed among HC. Among FESz, larger LH V1 activity was associated with larger hallucination subscale scores on the Scale for the Assessment of Positive Symptoms. Early-stage visual processing deficits localized to V1 are present at disease onset in the schizophrenia spectrum. This impairment appears to be restricted to the LH, consistent with previous reports detailing a predominantly LH disease process in early psychosis, and activity within this region was associated with an increased experience of hallucinations. These findings detail the cortical responses contributing to visual processing impairments and their relationship with symptoms at disease onset, advancing our understanding of their developmental trajectory over the course of psychotic illness.

Hemispheric asymmetries in the orientation and location of the lateral geniculate nucleus in dyslexia

Human brain asymmetry reflects normal specialization of functional roles and may derive from evolutionary, hereditary, developmental, experiential, and pathological factors (Toga & Thompson, 2003). Geschwind and Galaburda (1985) suggested that processing difficulties in dyslexia are due to structural differences between hemispheres. Because of its potential significance to the controversial magnocellular theory of dyslexia, we investigated hemispheric differences in the human lateral geniculate nucleus (LGN), the primary visual relay and control nucleus in the thalamus, in subjects with dyslexia compared to normal readers. We acquired and averaged multiple high-resolution proton density (PD) weighted structural magnetic resonance imaging (MRI) volumes to measure in detail the anatomical boundaries of the LGN in each hemisphere. We observed hemispheric asymmetries in the orientation of the nucleus in subjects with dyslexia that were absent in controls. We also found differences in the location of the LGN between hemispheres in controls but not in subjects with dyslexia. Neither the precise anatomical differences in the LGN nor their functional consequences are known, nor is it clear whether the differences might be causes or effects of dyslexia.

Multi-parameter machine learning approach to the neuroanatomical basis of developmental dyslexia

Despite decades of research, the anatomical abnormalities associated with developmental dyslexia are still not fully described. Studies have focused on between-group comparisons in which different neuroanatomical measures were generally explored in isolation, disregarding potential interactions between regions and measures. Here, for the first time a multivariate classification approach was used to investigate grey matter disruptions in children with dyslexia in a large (N = 236) multisite sample. A variety of cortical morphological features, including volumetric (volume, thickness and area) and geometric (folding index and mean curvature) measures were taken into account and generalizability of classification was assessed with both 10-fold and leave-one-out cross validation (LOOCV) techniques. Classification into control vs. dyslexic subjects achieved above chance accuracy (AUC = 0.66 and ACC = 0.65 in the case of 10-fold CV, and AUC = 0.65 and ACC = 0.64 using LOOCV) after principled feature selection. Features that discriminated between dyslexic and control children were exclusively situated in the left hemisphere including superior and middle temporal gyri, subparietal sulcus and prefrontal areas. They were related to geometric properties of the cortex, with generally higher mean curvature and a greater folding index characterizing the dyslexic group. Our results support the hypothesis that an atypical curvature pattern with extra folds in left hemispheric perisylvian regions characterizes dyslexia. Hum Brain Mapp 38:900-908, 2017. © 2016 Wiley Periodicals, Inc.

White matter integrity of cerebellar-cortical tracts in reading impaired children: A probabilistic tractography study

Little is known about the white matter integrity of cerebellar-cortical pathways in individuals with dyslexia. Building on previous findings of decreased volume in the anterior lobe of the cerebellum, we utilized novel cerebellar segmentation procedures and probabilistic tractography to examine tracts that connect the anterior lobe of the cerebellum and cortical regions typically associated with reading: the temporoparietal (TP), occipitotemporal (OT), and inferior frontal (IF) regions. The sample included 29 reading impaired children and 27 typical readers. We found greater fractional anisotropy (FA) for the poor readers in tracts connecting the cerebellum with TP and IF regions relative to typical readers. In the OT region, FA was greater for the older poor readers, but smaller for the younger ones. This study provides evidence for discrete, regionally-bound functions of the cerebellum and suggests that projections from the anterior cerebellum appear to have a regulatory effect on cortical pathways important for reading.

How reliable are gray matter disruptions in specific reading disability across multiple countries and languages? Insights from a large-scale voxel-based morphometry study

The neural basis of specific reading disability (SRD) remains only partly understood. A dozen studies have used voxel-based morphometry (VBM) to investigate gray matter volume (GMV) differences between SRD and control children, however, recent meta-analyses suggest that few regions are consistent across studies. We used data collected across three countries (France, Poland, and Germany) with the aim of both increasing sample size (236 SRD and controls) to obtain a clearer picture of group differences, and of further assessing the consistency of the findings across languages. VBM analysis reveals a significant group difference in a single cluster in the left thalamus. Furthermore, we observe correlations between reading accuracy and GMV in the left supramarginal gyrus and in the left cerebellum, in controls only. Most strikingly, we fail to replicate all the group differences in GMV reported in previous studies, despite the superior statistical power. The main limitation of this study is the heterogeneity of the sample drawn from different countries (i.e., speaking languages with varying orthographic transparencies) and selected based on different assessment batteries. Nevertheless, analyses within each country support the conclusions of the cross-linguistic analysis. Explanations for the discrepancy between the present and previous studies may include: (1) the limited suitability of VBM to reveal the subtle brain disruptions underlying SRD; (2) insufficient correction for multiple statistical tests and flexibility in data analysis, and (3) publication bias in favor of positive results. Thus the study echoes widespread concerns about the risk of false-positive results inherent to small-scale VBM studies.

Asymmetry of White Matter Pathways in Developing Human Brains

Little is known about the emergence of structural asymmetry of white matter tracts during early brain development. We examined whether and when asymmetry in diffusion parameters of limbic and association white matter pathways emerged in humans in 23 brains ranging from 15 gestational weeks (GW) up to 3 years of age (11 ex vivo and 12 in vivo cases) using high-angular resolution diffusion imaging tractography. Age-related development of laterality was not observed in a limbic connectional pathway (cingulum bundle or fornix). Among the studied cortico-cortical association pathways (inferior longitudinal fasciculus [ILF], inferior fronto-occipital fasciculus, and arcuate fasciculus), only the ILF showed development of age-related laterality emerging as early as the second trimester. Comparisons of ages older and younger than 40 GW revealed a leftward asymmetry in the cingulum bundle volume and a rightward asymmetry in apparent diffusion coefficient and leftward asymmetry in fractional anisotropy in the ILF in ages older than 40 GW. These results suggest that morphometric asymmetry in cortical areas precedes the emergence of white matter pathway asymmetry. Future correlative studies will investigate whether such asymmetry is anatomically/genetically driven or associated with functional stimulation.

Sex-specific gray matter volume differences in females with developmental dyslexia

Developmental dyslexia, characterized by unexpected reading difficulty, is associated with anomalous brain anatomy and function. Previous structural neuroimaging studies have converged in reports of less gray matter volume (GMV) in dyslexics within left hemisphere regions known to subserve language. Due to the higher prevalence of dyslexia in males, these studies are heavily weighted towards males, raising the question whether studies of dyslexia in females only and using the same techniques, would generate the same findings. In a replication study of men, we obtained the same findings of less GMV in dyslexics in left middle/inferior temporal gyri and right postcentral/supramarginal gyri as reported in the literature. However, comparisons in women with and without dyslexia did not yield left hemisphere differences, and instead, we found less GMV in right precuneus and paracentral lobule/medial frontal gyrus. In boys, we found less GMV in left inferior parietal cortex (supramarginal/angular gyri), again consistent with previous work, while in girls differences were within right central sulcus, spanning adjacent gyri, and left primary visual cortex. Our investigation into anatomical variants in dyslexia replicates existing studies in males, but at the same time shows that dyslexia in females is not characterized by involvement of left hemisphere language regions but rather early sensory and motor cortices (i.e., motor and premotor cortex, primary visual cortex). Our findings suggest that models on the brain basis of dyslexia, primarily developed through the study of males, may not be appropriate for females and suggest a need for more sex-specific investigations into dyslexia.

Distinct neural signatures of cognitive subtypes of dyslexia with and without phonological deficits

Developmental dyslexia can be distinguished as different cognitive subtypes with and without phonological deficits. However, despite some general agreement on the neurobiological basis of dyslexia, the neurofunctional mechanisms underlying these cognitive subtypes remain to be identified. The present BOLD fMRI study thus aimed at investigating by which distinct and/or shared neural activation patterns dyslexia subtypes are characterized. German dyslexic fourth graders with and without deficits in phonological awareness and age-matched normal readers performed a phonological decision task: does the auditory word contain the phoneme/a/? Both dyslexic subtypes showed increased activation in the right cerebellum (Lobule IV) compared to controls. Subtype-specific increased activation was systematically found for the phonological dyslexics as compared to those without this deficit and controls in the left inferior frontal gyrus (area 44: phonological segmentation), the left SMA (area 6), the left precentral gyrus (area 6) and the right insula. Non-phonological dyslexics revealed subtype-specific increased activation in the left supramarginal gyrus (area PFcm; phonological storage) and angular gyrus (area PGp). The study thus provides the first direct evidence for the neurobiological grounding of dyslexia subtypes. Moreover, the data contribute to a better understanding of the frequently encountered heterogeneous neuroimaging results in the field of 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.

Neurobiological approaches on brains of children with dyslexia: review

Learning difficulties commonly comprise a heterogeneous group of disorders manifested by unexpected problems in some children's experiences in the academic performance arena. These problems especially comprise of a variety of disorders, which one of the most well-recognized learning difficulties is reading disability or dyslexia. The aim of this review is to explain the postmortem, structural or functional neuroimaging, and electrophysiological studies of human brains in children. The findings about these neuropathological and neurofunctional characteristics of developmental dyslexia, prospective studies beginning early in the life span and studies targeting remedial intervention will help to set the research agendas for future studies to follow.

A dual DTI approach to analyzing white matter in children with dyslexia

Using voxel-based (VBA) and region-of-interest (ROI) diffusion tensor imaging (DTI) analyses, we examined white matter (WM) organization in seven children with dyslexia and six age-matched controls. Both methods demonstrated reduced fractional anisotropy (FA) in the left superior longitudinal fasciculus (SLF) and abnormal orientation in the right SLF in dyslexics. Application of this complementary dual DTI approach to dyslexia, which included novel analyses of fiber orientation, demonstrates its usefulness for analyzing mild and complex WM abnormalities.

L/M Speed-Matching Ratio Predicts Reading in Children

PURPOSE

Many behavioral studies have found impaired perception of dynamic visual stimuli in dyslexia and several neuroimaging studies have found reduced activation of the human motion area MT+ in dyslexia. These results are often interpreted as a magnocellular (MC) deficit in dyslexia. It has also been claimed that colored filters can help dyslexics to read. One defining feature of the MC-pathway is a greater weight for L-cone input than M-cone input, and at most very weak S-cone input. We measured the subjective speed matches between L-, M-, and S-cone isolating stimuli in good and poor readers.

METHODS

Subjects performed a speed-matching task with drifting cone-isolating stimuli to find the point of subjective equality between two drifting patterns. Such a task is known to activate cortical area MT+, presumably via the MC-pathway.

RESULTS

L- to M-cone speed-match ratios were negatively correlated with single-word (r=-0.46) and irregular-word reading (r=-0.56) but not with non-word reading.

CONCLUSIONS

Results suggest that relative L-cone sensitivity within the MC-pathway may limit orthographic reading performance.

Mechanisms of hemispheric specialization: insights from analyses of connectivity

Traditionally, anatomical and physiological descriptions of hemispheric specialization have focused on hemispheric asymmetries of local brain structure or local functional properties, respectively. This article reviews the current state of an alternative approach that aims at unraveling the causes and functional principles of hemispheric specialization in terms of asymmetries in connectivity. Starting with an overview of the historical origins of the concept of lateralization, we briefly review recent evidence from anatomical and developmental studies that asymmetries in structural connectivity may be a critical factor shaping hemispheric specialization. These differences in anatomical connectivity, which are found both at the intra- and inter-regional level, are likely to form the structural substrate of different functional principles of information processing in the two hemispheres. The main goal of this article is to describe how these functional principles can be characterized using functional neuroimaging in combination with models of functional and effective connectivity. We discuss the methodology of established models of connectivity which are applicable to data from positron emission tomography and functional magnetic resonance imaging and review published studies that have applied these approaches to characterize asymmetries of connectivity during lateralized tasks. Adopting a model-based approach enables functional imaging to proceed from mere descriptions of asymmetric activation patterns to mechanistic accounts of how these asymmetries are caused.

Investigating the functional role of callosal connections with dynamic causal models

The anatomy of the corpus callosum has been described in considerable detail. Tracing studies in animals and human postmortem experiments are currently complemented by diffusion-weighted imaging, which enables noninvasive investigations of callosal connectivity to be conducted. In contrast to the wealth of anatomical data, little is known about the principles by which interhemispheric integration is mediated by callosal connections. Most importantly, we lack insights into the mechanisms that determine the functional role of callosal connections in a context-dependent fashion. These mechanisms can now be disclosed by models of effective connectivity that explain neuroimaging data from paradigms that manipulate interhemispheric interactions. In this article, we demonstrate that dynamic causal modeling (DCM), in conjunction with Bayesian model selection (BMS), is a powerful approach to disentangling the various factors that determine the functional role of callosal connections. We first review the theoretical foundations of DCM and BMS before demonstrating the application of these techniques to empirical data from a single subject.

Early Development of Neurophysiological Processes Involved in Normal Reading and Reading Disability: A Magnetic Source Imaging Study

This longitudinal study examined the development of the brain mechanism involved in phonological decoding in beginning readers using magnetic source imaging. Kindergarten students were assigned to 2 groups: those who showed mastery of skills that are important predictors of proficient reading (low-risk group) and those who initially did not show mastery but later benefited from systematic reading instruction and developed average-range reading skills at the end of Grade 1 (high-risk responders). Spatiotemporal profiles of brain activity were obtained during performance of letter-sound and pseudoword naming tasks before and after Grade 1 instruction. With few exceptions, low-risk children showed early development of brain activation profiles that are typical of older skilled readers. Provided that temporoparietal and visual association areas were recruited into the brain mechanism that supported reading, the majority of high-risk responder children benefited from systematic reading instruction and developed adequate reading abilities.

On the role of general system theory for functional neuroimaging

One of the most important goals of neuroscience is to establish precise structure-function relationships in the brain. Since the 19th century, a major scientific endeavour has been to associate structurally distinct cortical regions with specific cognitive functions. This was traditionally accomplished by correlating microstructurally defined areas with lesion sites found in patients with specific neuropsychological symptoms. Modern neuroimaging techniques with high spatial resolution have promised an alternative approach, enabling non-invasive measurements of regionally specific changes of brain activity that are correlated with certain components of a cognitive process. Reviewing classic approaches towards brain structure-function relationships that are based on correlational approaches, this article argues that these approaches are not sufficient to provide an understanding of the operational principles of a dynamic system such as the brain but must be complemented by models based on general system theory. These models reflect the connectional structure of the system under investigation and emphasize context-dependent couplings between the system elements in terms of effective connectivity. The usefulness of system models whose parameters are fitted to measured functional imaging data for testing hypotheses about structure-function relationships in the brain and their potential for clinical applications is demonstrated by several empirical examples.

Neuroanatomical markers for dyslexia: a review of dyslexia structural imaging studies

A neuroanatomical description of dyslexia has been elusive, due in part to the complex cognitive nature of dyslexia. People with dyslexia have varying degrees of impairment in reading skills that engage oral and written language (reading) neural networks. Although findings for the inferior parietal lobule, inferior frontal gyrus, and cerebellum have been relatively consistent across studies, these studies also demonstrate that anatomical patterns of results vary according to the reading skills that characterize dyslexia. The number and likelihood of atypical anatomical findings in oral and/or written language systems appears to be related to the pattern of impairments in measures of phonology, orthography, and fluency. A comprehensive neurobiological understanding of dyslexia will depend on studies of dyslexic individuals with homogeneous perceptual, cognitive, and genetic backgrounds.

Left hemisphere and male sex dominance of cerebral hemiatrophy (Dyke-Davidoff-Masson Syndrome)

Although radiological findings of cerebral hemiatrophy (Dyke-Davidoff-Masson Syndrome) are well known, there is no systematic study about the gender and the affected side in this syndrome. Brain images in 26 patients (mean aged 11) with cerebral hemiatrophy were retrospectively reviewed. Nineteen patients (73.5%) were male and seven patients (26.5%) were female. Left hemisphere involvement was seen in 18 patients (69.2%) and right hemisphere involvement was seen in eight patients (30.8%). We conclude that male gender and left side involvement are frequent in cerebral hemiatrophy disease.

Selective magnocellular deficits in dyslexia: a "phantom contour" study

A technique by Rogers-Ramachandran and Ramachandran [Vis. Res. 38 (1998) 71-77] was adapted to evaluate magnocellular (M) and parvocellular (P) visual processing efficiency, with identical task structure, in normal and dyslexic children. A battery of phonological, orthographic and cognitive tasks was administered to assess reading ability and component reading skills in both groups. For the visual processing experiment, children identified shapes created by patterns of dots flickering in counter-phase. The dots were black and white in the M condition, versus isoluminant red and green in the P condition. A staircase procedure determined the children's threshold flicker rate for shape identification. Dyslexics displayed selectively slower visual processing in the M condition but not in the P condition. Across all subjects, performance in the M condition was correlated with measures of orthographic skill, consistent with previous findings linking M processing and orthographic skill. Within the dyslexic group, processing in the M condition was negatively correlated with level of phonological awareness. The results are not consistent with the argument that dyslexics with phonological impairments suffer from deficits across all sensory modalities, as those children with the poorest phonological awareness displayed magnocellular processing well within the normal range.

Diminished auditory mismatch fields in dyslexic adults

Electroencephalographic studies have demonstrated smaller auditory responses to infrequent deviances of speech and nonspeech sounds in dyslexic than normal-reading subjects. We used a whole-scalp neuromagnetometer to study selectively reactivity of the auditory cortices to sound deviances in 8 dyslexic and 11 normal-reading adults. Within a monotonous sequence of 50-millisecond 1000 Hz binaural tones, tones of 920 and 1080 Hz occurred with 7% probability each. Magnetic mismatch fields, elicited by the stimulus deviances, were diminished in the left hemisphere of the dyslexic subjects. The results indicate deficient change detection in the left auditory cortex of right-handed dyslexic adults.

Auditory cortical responses to speech-like stimuli in dyslexic adults

Auditory cortical processing of speech-like sounds was studied in 9 dyslexic and 11 normal-reading adults. Noise/square-wave sequences, mimicking transitions from a fricative consonant to a vowel, were presented binaurally once every 1.1 sec and the cortical responses were recorded with a whole-scalp neuromagnetometer. The auditory cortices of both hemispheres were less reactive to acoustical changes in dyslexics than in controls, as was evident from the weaker responses to the noise/square-wave transitions. The results demonstrate that dyslexic adults are deficient in processing acoustic changes presented in rapid succession within tens to hundreds of milliseconds. The observed differences could be related to insufficient triggering of automatic auditory attention, resulting, for instance, from a general deficiency of the magnocellular system.

Specific reading disability: a multiplanar view

In the past three decades a revolution has altered the way society approaches people with disabilities. Social changes resulted in a significant increase in fundamental and applied research that seeks to improve the lives of people with disabilities by facilitating better understanding of the mechanisms, manifestations, prevention, and treatment of functional impairment. Specific Reading Disability (SRD) has benefited from this revolution. This review focuses on the evolution of SRD, new information in its neurobiology and management, and the challenges that remain. Evidence from a wide spectrum of research provides strong support for the role of phonology in Specific Reading Disability. Despite the mounting evidence, the case is far from completely established. Adults with compensated SRD read but still demonstrate disordered phonology (Felton et al. [1990] Brain Language 39:485-497). Whether poor phonology is causal or a covariant remains to be demonstrated. Of children with poor phonology, it is not known how many are poor readers. While phonology is associated with SRD, other studies have questioned the uniqueness of SRD. Challenges have been made to the method of classification, the uniqueness of phonological dysfunction as a mechanism in SRD and the response to treatment. In the final analysis all poor readers may have a common core of dysphonology, independent of whether their reading is discrepant from their IQ.

The development of face expertise

Recent neuroimaging studies in adults indicate that visual areas selective for recognition of faces can be recruited through expertise for nonface objects. This reflects a new emphasis on experience in theories of visual specialization. In addition, novel work infers differences between categories of nonface objects, allowing a re-interpretation of differences seen between recognition of faces and objects. Whether there are experience-independent precursors of face expertise remains unclear; indeed, parallels between literature for infants and adults suggest that methodological issues need to be addressed before strong conclusions can be drawn regarding the origins of face recognition.

Visual motion sensitivity in dyslexia: evidence for temporal and energy integration deficits

In addition to poor literacy skills, developmental dyslexia has been associated with multisensory deficits for dynamic stimulus detection. In vision these deficits have been suggested to result from impaired sensitivity of cells within the retino-cortical magnocellular pathway and extrastriate areas in the dorsal stream to which they project. One consequence of such selectively reduced sensitivity is a difficulty in extracting motion coherence from dynamic noise, a deficit associated with both developmental dyslexia and persons with extrastriate, dorsal stream lesions. However the precise nature of the mechanism(s) underlying these perceptual deficits in dyslexia remain unknown. In this study, we obtained motion detection thresholds for 10 dyslexic and 10 control adults while varying the spatial and temporal parameters of the random dot kinematogram (RDK) stimuli. In Experiment 1 stimulus duration was manipulated to test whether dyslexics are specifically impaired for detecting short duration, rather than longer stimuli. Dot density was varied in Experiment 2 to examine whether dyslexics' reduced motion sensitivity was affected by the amount of motion energy present in the RDKs. Dyslexics were consistently less sensitive to coherent motion than controls in both experiments. Increasing stimulus duration did not improve dyslexics' performance, whereas increasing dot density did. Thus increasing motion energy assisted the dyslexics, suggesting that their motion detectors have a lower signal to noise ratio, perhaps due to spatial undersampling.

Microstructure of temporo-parietal white matter as a basis for reading ability: evidence from diffusion tensor magnetic resonance imaging

Diffusion tensor magnetic resonance imaging (MRI) was used to study the microstructural integrity of white matter in adults with poor or normal reading ability. Subjects with reading difficulty exhibited decreased diffusion anisotropy bilaterally in temporoparietal white matter. Axons in these regions were predominantly anterior-posterior in direction. No differences in T1-weighted MRI signal were found between poor readers and control subjects, demonstrating specificity of the group difference to the microstructural characteristics measured by diffusion tensor imaging (DTI). White matter diffusion anisotropy in the temporo-parietal region of the left hemisphere was significantly correlated with reading scores within the reading-impaired adults and within the control group. The anisotropy reflects microstructure of white matter tracts, which may contribute to reading ability by determining the strength of communication between cortical areas involved in visual, auditory, and language processing.

The neural basis of developmental dyslexia

Until recently, many thought developmental dyslexia was a behavioral disorder that primarily affected reading. In fact, it is a partly heritable condition, the clinical manifestations of which are extremely complex including deficits in reading, working memory, sensorimotor coordination, and early sensory processing. Even though extensive research has characterized these behavioral abnormalities carefully, the biological mechanisms of the clinical manifestations still are poorly understood. Recent research into both the nature of the structural and functional abnormalities in developmental dyslexia and the functional neuroanatomy of reading have rapidly advanced our understanding of the localization of the processes responsible for the signs and symptoms of dyslexia. This paper reviews recent evidence supporting a biological basis for developmental dyslexia.