The caudal infrasylvian surface in dyslexia: novel magnetic resonance imaging-based findings

Neuroanatomy of developmental dyslexia: Pitfalls and promise

Investigations into the neuroanatomical bases of developmental dyslexia have now spanned more than 40 years, starting with the post-mortem examination of a few individual brains in the 60s and 70s, and exploding in the 90s with the widespread use of MRI. The time is now ripe to reappraise the considerable amount of data gathered with MRI using different types of sequences (T1, diffusion, spectroscopy) and analysed using different methods (manual, voxel-based or surface-based morphometry, fractional anisotropy and tractography, multivariate analyses…). While selective reviews of mostly small-scale studies seem to provide a coherent view of the brain disruptions that are typical of dyslexia, involving left perisylvian and occipito-temporal regions, we argue that this view may be deceptive and that meta-analyses and large-scale studies rather highlight many inconsistencies and limitations. We discuss problems inherent to small sample size as well as methodological difficulties that still undermine the discovery of reliable neuroanatomical bases of dyslexia, and we outline some recommendations to further improve this research area.

Visuomotor processing and hand force coordination in dyslexic children during a visually guided manipulation task

Developmental Dyslexia negatively affects children's reading and writing ability and, in most cases, performance in sensorimotor tasks. These deficits have been associated with structural and functional alterations in the cerebellum and the posterior parietal cortex (PPC). Both neural structures are active during visually guided force control and in the coordination of load force (LF) and grip force (GF) during manipulation tasks. Surprisingly, both phenomena have not been investigated in dyslexic children. Therefore, the aim of this study was to compare dyslexic and non-dyslexic children regarding their visuomotor processing ability and GF-LF coordination during a static manipulation task. Thirteen dyslexic (8-14 YO) and 13 age- and sex-matched non-dyslexic (control) children participated in the study. They were asked to grasp a fixed instrumented handle using the tip of all digits and pull the handle upward exerting isometric force to match a ramp-and-hold force profile displayed in a computer monitor. Task performance (i.e., visuomotor coordination) was assessed by RMSE calculated in both ramp and hold phases. GF-LF coordination was assessed by the ratio between GF and LF (GF/LF) calculated at both phases and the maximum value of a cross-correlation function (rmax) and its respective time lag calculated at ramp phase. The results revealed that the RMSE at both phases was larger in dyslexic than in control children. However, we found that GF/LF, rmax, and time lags were similar between groups. Those findings indicate that dyslexic children have a mild deficit in visuomotor processing but preserved GF-LF coordination. Altogether, these findings suggested that dyslexic children could present mild structural and functional alterations in specific PPC or cerebellum areas that are directly related to visuomotor processing.

Planum temporale asymmetry in developmental dyslexia: Revisiting an old question

Among the various asymmetrical structures of the human brain, the planum temporale, an anatomical region associated with a variety of auditory and language-related processes, has received particular attention. While its surface area has been shown to be greater in the left hemisphere compared to the right in about two-thirds of the general population, altered patterns of asymmetry were revealed by post mortem analyses in individuals with developmental dyslexia. These findings have been inconsistently replicated in magnetic resonance imaging studies of this disorder. In this report, we attempt to resolve past inconsistencies by analyzing the T1-weighted MR images of 81 children (mean age: 11 years, sd: 17 months), including 46 control (25 boys) and 35 dyslexic children (20 boys). We manually outlined Heschl's gyri, the planum temporale and the posterior rami of the Sylvian fissure on participants' brain images, using the same anatomical criteria as in post mortem studies. Results revealed an altered pattern of asymmetry of the planum temporale surface area in dyslexic boys only, with a greater proportion of rightward asymmetrical cases among dyslexic boys compared to control boys. Additionally, analyses of cortical thickness showed no asymmetry differences between groups for any of the regions of interest. Finally, a greater number of Heschl's gyrus full duplications emerged for the right hemisphere of dyslexic boys compared to controls. The present findings confirm and extend early post mortem observations. They also stress the importance of taking gender into account in studies of developmental dyslexia.

An interaction-dominant perspective on reading fluency and dyslexia

The background noise of response times is often overlooked in scientific inquiries of cognitive performances. However, it is becoming widely acknowledged in psychology, medicine, physiology, physics, and beyond that temporal patterns of variability constitute a rich source of information. Here, we introduce two complexity measures (1/f scaling and recurrence quantification analysis) that employ background noise as metrics of reading fluency. These measures gauge the extent of interdependence across, rather than within, cognitive components. In this study, we investigated dyslexic and non-dyslexic word-naming performance in beginning readers and observed that these complexity metrics differentiate reliably between dyslexic and average response times and correlate strongly with the severity of the reading impairment. The direction of change in the introduced metrics suggests that developmental dyslexia resides from dynamical instabilities in the coordination among the many components necessary to read, which could explain why dyslexic readers score below average on so many distinct tasks and modalities.

Surface area accounts for the relation of gray matter volume to reading-related skills and history of dyslexia

It is unknown whether the abnormalities in brain structure and function observed in dyslexic readers are congenital or arise later in development. Analyzing the 2 components of gray matter volume separately may help in differentiating these possibilities. Gray matter volume is the product of cortical surface area, determined during prenatal brain development, and cortical thickness, determined during postnatal development. For this study, 16 adults with a history of phonological dyslexia and 16 age- and gender-matched controls underwent magnetic resonance imaging and an extensive battery of tests of reading-related skills. Cortical surface area and gray matter volume measures of the whole brain, the inferior frontal gyrus, and the fusiform gyrus were similarly related to phonological skills and a history of dyslexia. There was no relationship between cortical thickness and phonological skills or history of dyslexia. Because cortical surface area reflects cortical folding patterns determined prenatally, this suggests that brain differences in dyslexia are rooted in early cortical development and are not due to compensatory changes that occur during postnatal development and would be expected to influence cortical thickness. This study demonstrates the importance of examining the separate components of gray matter volume when studying developmental abnormalities.

Hemispheric asymmetry of sulcus-function correspondence: quantization and developmental implications

Spatial covariances between the geometric centers of human occipital sulci and visual functional areas were calculated to reduce the spatial variance of functional-area locations between subjects. Seven visual areas in each occipital hemisphere were retinotopically mapped, using horizontal- and vertical-meridian stimuli and (15)O PET in 11 subjects. Sulcal locations were determined using anatomic brain models derived from high-resolution MRI images. Location variability for sulci and functional areas was similar in magnitude, with average standard deviations of (2.7x, 5.3y, 5.7z) mm and (4.3x, 5.4y, 5.3z) mm, respectively. Sulcal locations were predictive of functional-area locations (i.e., significant spatial covariance) in the minority of structure-function pairings tested (25 of 168). Location variability was reduced by an average of 27% for functional areas showing significant covariation with sulcal features. Early-developing sulci were stronger predictors of functional-area location than late-developing sulci. Sulcus-function covariance was stronger in the left occipital lobe than in the right occipital lobe. Notably, the left calcarine fissure demonstrated powerful covariances with functional areas in both hemispheres, suggesting that it serves as a developmental "anchor" for functional areas in the occipital cortex. These findings support the hypothesis that hemispheric lateralization of function is reflected in the strength of correspondence between cortical surface anatomy and function.

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.

Minicolumnar pathology in dyslexia

The minicolumn is an anatomical and functional unit of the brain whose genesis accrues from germinal cell divisions in the ventricular zone of the brain. Disturbances in the morphometry of minicolumns have been demonstrated recently for both autism and Down's syndrome. We report minicolumnar abnormalities in the brain of a dyslexic patient. The corresponding developmental disturbance (ie, large minicolumns) could account for the perceptual errors observed in dyslexia.

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.

Structural imaging in dyslexia: the planum temporale

The search for a neurobiological substrate for dyslexia has focused on anomalous planum symmetry. The results of imaging studies of the planum have been inconsistent, perhaps due to diagnostic uncertainty, technical differences in measurement criteria, and inadequate control of handedness, sex, and cognitive ability. Although structural imaging studies have not clarified the neurobiology of reading disability, converging evidence suggests that variation in asymmetry of the planum temporale does have functional significance. Studies in a variety of populations have shown a significant association between planar asymmetry, the strength of hand preference, and general verbal skills such as vocabulary and comprehension. Future structural imaging studies of dyslexia should match participants on hand preference and general verbal ability in order to determine the relationship between brain structure and written and oral language.