The magnocellular theory of developmental dyslexia

A double dissociation between sensorimotor impairments and reading disability: A comparison of autistic and dyslexic children

Does sensorimotor dysfunction underlie reading impairment? To investigate this question, a battery of literacy, phonology, auditory, visual, and motor tests were administered to age- and ability-matched groups of dyslexic, autistic, and control children. As in previous studies, only a subset of the dyslexic children had sensory and/or motor impairments, whilst some dyslexics were entirely spared, suggesting that sensorimotor impairments are not necessary to cause reading disability. A subset of autistic children was also found to have sensorimotor impairments; however, some of these children did not have reading problems, suggesting that sensorimotor impairments are not sufficient to cause reading disability. We conclude that sensorimotor and reading impairments are doubly dissociable. Sensorimotor impairments do not seem to be the cause of reading disability, but can be seen as nonspecific markers for developmental disorder.

Is coherent motion an appropriate test for magnocellular sensitivity?

The suggestion that coherent motion may serve as a test of magnocellular sensitivity is problematic. However, the nature of the problems depends on how the "magnocellular system" is defined. If this term is limited to subcortical entities, the problems are that subcortical neurons are not directionally selective, and that their receptive fields are too small to account for the spatial summation of coherent motion. If "magnocellular system" is defined to include cortical entities, such as area MT, one is faced with the fact that this definition itself is problematic as well as the problem that area MT is known to receive parvocellular and koniocellular inputs.

Dynamic auditory processing, musical experience and language development

Children with language-learning impairments (LLI) form a heterogeneous population with the majority having both spoken and written language deficits as well as sensorimotor deficits, specifically those related to dynamic processing. Research has focused on whether or not sensorimotor deficits, specifically auditory spectrotemporal processing deficits, cause phonological deficit, leading to language and reading impairments. New trends aimed at resolving this question include prospective longitudinal studies of genetically at-risk infants, electrophysiological and neuroimaging studies, and studies aimed at evaluating the effects of auditory training (including musical training) on brain organization for language. Better understanding of the origins of developmental LLI will advance our understanding of the neurobiological mechanisms underlying individual differences in language development and lead to more effective educational and intervention strategies. This review is part of the INMED/TINS special issue "Nature and nurture in brain development and neurological disorders", based on presentations at the annual INMED/TINS symposium (http://inmednet.com/).

Scotopic sensitivity in schizophrenia

Among participants with schizophrenia there is evidence for early-stage visual processing deficits, which may arise in the rod pathways. Input to the earliest level of this pathway, however, has not been tested in this population. It has been widely hypothesized that schizophrenia participants have magnocellular deficits that occur at the pre-cortical level. To address this hypothesis, we studied absolute scotopic (dark-adapted) sensitivity in fifteen schizophrenia and fifteen matched control participants. Scotopic thresholds were assessed using a 1.85-deg, 510-nm circular test stimulus located at 10 degrees eccentricity in the left visual field and presented in Maxwellian-view. Thresholds were obtained using a two-alternative forced-choice paradigm (an average of 200 trials per participant was obtained). Threshold estimates were derived using probit analysis. In this procedure the transformed binomial data (the inverse of the normal probability integral) is fit with a weighted linear regression. Noise was defined as the average deviation from this line. Lens optical density was also assessed by comparing absolute scotopic thresholds to the extinction spectrum of rhodopsin. Scotopic thresholds and lens density values of the two groups were evaluated using independent samples t-tests. The scotopic thresholds, and associated noise, did not differ between the schizophrenia and control participants. Lens density was also nearly identical between groups. These results suggest that magnocellular deficits in schizophrenia may not be due to problems at the level of the rods but are more likely to occur later in the visual pathway.

The role of sensorimotor impairments in dyslexia: a multiple case study of dyslexic children

This study attempts to investigate the role of sensorimotor impairments in the reading disability that characterizes dyslexia. Twenty-three children with dyslexia were compared to 22 control children, matched for age and non-verbal intelligence, on tasks assessing literacy as well as phonological, visual, auditory and motor abilities. The dyslexic group as a whole were significantly impaired on phonological, but not sensorimotor, tasks. Analysis of individual data suggests that the most common impairments were on phonological and visual stress tasks and the vast majority of dyslexics had one of these two impairments. Furthermore, phonological skill was able to account for variation in literacy skill, to the exclusion of all sensorimotor factors, while neither auditory nor motor skill predicted any variance in phonological skill. Visual stress seems to account for a small proportion of dyslexics, independently of the commonly reported phonological deficit. However, there is little evidence for a causal role of auditory, motor or other visual impairments.

Mental rotation of letters, pictures, and three-dimensional objects in German dyslexic children

This study examines mental rotation ability in children with developmental dyslexia. Prior investigations have yielded equivocal results that might be due to differences in stimulus material and testing formats employed. Whereas some investigators found dyslexic readers to be impaired in mental rotation, others did not report any performance differences or even superior spatial performance for dyslexia. Here, we report a comparison of mental rotation for letters, three-dimensional figures sensu Shepard and Metzler, and colored pictures of animals or humans in second-grade German dyslexic readers. Findings indicate that dyslexic readers are impaired in mental rotation for all three kinds of stimuli. Effects of general intelligence were controlled. Furthermore, dyslexic children were deficient in other spatial abilities like identifying letters or forms among distracters. These results are discussed with respect to the hypotheses of a developmental dysfunction of the parietal cortex or a subtle anomaly in cerebellar function in dyslexic readers.

Temporal processing deficits in Hebrew speaking children with reading disabilities

The purpose of this study was to assess to what extent specific reading disabilities and poor phonologic processing in children who read Hebrew, a primarily consonant orthography, are related to central auditory temporal processing deficits (TPDs).Twenty-four Hebrew-speaking children (ages 10-13) with and without reading disabilities were asked to discriminate auditorily pairs of syllables (/ba/ vs. /pa/) that differ by voice onset time (VOT) only. Two paradigms were used, 1 with a short interstimulus interval (ISI) (50 ms) and 1 with a long ISI (500 ms). Event-related potentials (ERPs) were measured in response to the two syllables in an auditory oddball task. Results showed significantly lowered accuracy, longer reaction times, and prolonged P3 latency among the group with reading disabilities compared with the control group. No significant differences were found between the short ISI task and the long ISI task. However, significant correlations were found between the phonologic processing tasks and the short ISI task. These findings in the Hebrew language are consistent with findings from other languages and add support to the central TPD hypothesis of reading disabilities. The discussion highlights how investigating different orthographic systems can deepen our understanding of the role TPD plays in reading.

Coherent motion detection in preschool children at family risk for dyslexia

We tested sensitivity to coherent motion (CM) in random dot kinematograms in a group of 5-year-old preschool children genetically at risk for dyslexia, compared to a group of well-matched control children. No significant differences were observed, either in a group analysis or in an individual deviance analysis. Nonetheless, CM-thresholds were significantly related to emerging orthographic skills. In a previous study on the same subjects (Boets, Wouters, van Wieringen, & Ghesquière, in press), we demonstrated that both risk groups already differed on measures of phonological awareness and letter knowledge. Moreover, auditory spectral processing (especially 2 Hz FM detection) was significantly related to phonological ability. In sum, the actual visual and previous auditory data combined, seem to suggest an exclusive relation between CM sensitivity and orthographic skills on the one hand, and FM sensitivity and phonological skills on the other.

Latencies of stimulus-driven eye movements are shorter in dyslexic subjects

Eye movements latencies toward peripherally presented stimuli were measured in 10-year-old dyslexic and control children. Dyslexic subjects, previously found to be oversensitive to stimulation of the magnocellular channel, showed reduced latencies as compared to normally reading controls. An attention shifting task was also used which showed no group differences in latencies of eye movements. The data are discussed in terms of the hypothesis of magnocellular system alteration and attention dysfunction in dyslexia. Additionally, sex differences in eye movement latencies were found and are discussed.

Deficits in achromatic phantom contour perception in poor readers

In a previous study [Sperling, A. J., Lu, Z. L., Manis, F. R., & Seidenberg, M. S. (2003). Selective deficits in magnocellular processing: A "phantom contour" study. Neuropsychologia, 41, 1422-1429] we found that dyslexic children were relatively slower in processing achromatic phantom contours. The maximum temporal frequency at which they could identify achromatic phantom contours was correlated with reading ability and orthographic skill in particular. Here we investigated whether similar deficits could be identified in adults. Poor readers were chosen who scored below the 25th percentile on either a standardized test of word identification or nonword pronunciation. Good readers were chosen who scored above the 40th percentile on both reading tasks. We replicated the findings of the child study: poor readers had slower processing in the achromatic version of the task, but not in the chromatic version. Achromatic performance correlated with several measures of reading and reading-related skills, including exception word reading and phonological awareness. We discuss the possibility that the deficits may indicate impairment in noise exclusion that is more readily apparent at higher temporal frequencies.

Serum lipid fatty acids, phonological processing, and reading in children with oral clefts

Reading skill is suggested to be related to phonological processing ability and polyunsaturated fatty acids (PUFAs). Here we investigated whether fatty acids (FAs) are related to phonological processing, whether the relations between PUFAs and reading generalize to other FAs, whether these relations are mediated by phonological processing, and whether relations of FAs are specific for language-related functions. Blood samples of 49 ten-year-old children with oral clefts were collected for FA proportion analysis in serum cholesteryl esters and phospholipids. On the same day, they performed tasks of phonological processing, reading, and both verbal and nonverbal intelligence. Sequential regression analyses (adjusted for age, gender, and cleft type) showed that phonological processing was inversely related to myristic acid in phospholipids and positively related to eicosapentaenoic acid in cholesteryl esters. Reading was inversely related to palmitoleic and gammalinolenic acids in phospholipids. The relations between FAs and reading were not mediated by phonological processing and FAs related only to language-related functions.

Neurobiologische Grundlagen der Lese-Rechtschreib-Schwäche

Zusammenfassung: Diese Überblicksarbeit stellt die wichtigsten neurobiologischen Theorien zu Ursachen der entwicklungsbedingten Lese-Rechtschreib-Schwäche und daraus abgeleitete Therapieansätze vor. Es wird zunächst ein kurzer Überblick über die Symptomatik und die vermuteten genetischen Grundlagen der Störung gegeben. Die phonologische Defizithypothese als eine allgemein akzeptierte, auf der kognitiven Ebene angesiedelte Theorie wird vorgestellt. Neurobiologische Modelle versuchen zu erklären, auf welchen neuroanatomischen/physiologischen Ursachen das phonologische Defizit beruht. Die magnozelluläre Defizithypothese, die Hypothese eines grundlegenden Verarbeitungsdefizits für sich zeitlich schnell verändernde Reize, die cerebelläre Defizithypothese sowie neuroanatomische Unterschiede zwischen normalen und legasthenen Lesern werden vorgestellt. Neurobiologisch fundierte Therapieansätze der Lese-Rechtschreib-Schwäche versuchen, in Ergänzung zu traditionellen, eher linguistisch orientierten Therapien die betroffenen neuronalen Systeme in ihrer Funktionsfähigkeit zu trainieren und dadurch die Voraussetzungen für die betroffenen Kinder zu schaffen, vom normalen Lese- und Schreibunterricht zu profitieren. Einige dieser Ansätze werden dargestellt. Abschließend wird darauf hingewiesen, dass eine differenzielle Zuweisung von Patienten zu bestimmten Therapieprogrammen auf der Basis einer fundierten (Ursachen-)Diagnostik in der Praxis stärker als bislang üblich durchgeführt werden sollte.

"A Riddle Wrapped in a Mystery Inside an Enigma"

A. T. Cacace and D. J. McFarland (2005) define central auditory processing disorder (CAPD) as a " modality-specific perceptual dysfunction that is not due to peripheral hearing loss" and that "should be distinguishable from cognitive, language-based, and/or supramodal attentional problems" (p. 113). Although agreeing with the general thrust of their attempts to exclude supramodal causes of impaired auditory performance as being labeled CAPD, I argue that this definition suffers from a number of serious deficiencies. It is both too loose and too restrictive, excluding what might be low-level deficits that occur in more than one modality, at the same time including at least one form of modality-specific linguistic processing. I argue that any useful definition of CAPD must not only exclude supramodal causes of auditory deficits, but must be based on the notion of impaired brain function demonstrable for nonspeech sounds.

Phonological processing skills as predictors of literacy amongst Arabic speaking Bahraini children

This paper reports a study of the reading and spelling skills of grades 1-3 Arabic-speaking children in Bahrain. Children were tested on their literacy skills (single word reading and spelling), their ability to decode letter strings (non-word reading) and measures of phonological awareness, short-term memory, speed of processing and non-verbal ability. These tests were included to identify the best predictors of literacy skills amongst Arabic young readers. The results were consistent with the literature based on tests of English-speaking children in that measures of phonological skills (decoding and awareness) were the best predictors of variability in reading and spelling among the Bahraini children. The results are discussed in terms of the literacy experiences of the children and the use of short vowels in Arabic writing.

Dorsal stream deficits suggest hidden dyslexia among deaf poor readers: correlated evidence from reduced perceptual speed and elevated coherent motion detection thresholds

Prelingual deafness and developmental dyslexia have confounding developmental effects on reading acquisition. Therefore, standard reading assessment methods for diagnosing dyslexia in hearing people are ineffective for use with deaf people. Recently, Samar, Parasnis, and Berent (2002) reported visual evoked potential evidence that deaf poor readers, compared to deaf good readers, have dorsal stream visual system deficits like those previously found for hearing dyslexics. Here, we report new psychometric and psychophysical evidence that deficits in dorsal stream function, likely involving extrastriate area MT, are associated with relatively poor reading comprehension in deaf adults. Poorer reading comprehension within a group of 23 prelingually deaf adults was associated with lower scores on the Symbol Digit Modality Test, a perceptual speed test commonly used to help identify dyslexia in hearing people. Furthermore, coherent dot motion detection thresholds, which reflect the functional status of area MT, correlated negatively with reading scores in each visual quadrant. Elevated motion thresholds for deaf poor readers were not due to general cognitive differences in IQ but were specifically correlated with poor perceptual speed. With IQ controlled, a highly reliable right visual field advantage for coherent motion detection was found. Additional analyses suggested that the functional status of dorsal stream motion detection mechanisms in deaf people is related to reading comprehension, but the direction and strength of lateralization of those mechanisms is independent of reading comprehension. Our results generally imply that dyslexia is a hidden contributor to relatively poor reading skill within the deaf population and that assessment of dorsal stream function may provide a diagnostic biological marker for dyslexia in deaf people.

Perhaps correlational but not causal: no effect of dyslexic readers' magnocellular system on their eye movements during reading

During reading, dyslexic readers exhibit more and longer fixations and a higher percentage of regressions than normal readers. It is still a matter of debate, whether these divergent eye movement patterns of dyslexic readers reflect an underlying problem in word processing or whether they are - as the proponents of the magnocellular deficit hypothesis claim - associated with deficient visual perception that is causal for dyslexia. To overcome problems in the empirical linkage of the magnocellular theory with reading, a string processing task is presented that poses similar demands on visual perception (in terms of letter identification) and oculomotor control as reading does. Two experiments revealed no differences in the eye movement patterns of dyslexic and control readers performing this task. Furthermore, no relationship between the functionality of the participants' magnocellular system assessed by the coherent motion task and string processing were found. The perceptual and oculomotor demands required during string processing were functionally equivalent to those during reading and the presented consonant strings had similar visual characteristics as reading material. Thus, a strong inference can be drawn: Dyslexic readers do not seem to have difficulties with the accurate perception of letters and the control of their eye movements during reading - their reading difficulties therefore cannot be explained in terms of oculomotor and visuo-perceptual problems.

Motor coordination difficulties in a municipality group and in a clinical sample of poor readers

The purpose of the study was to investigate incidence, severity and types of motor problems in two groups of poor readers compared to good reading controls. A group of children with severe dyslexia referred to specialist evaluation, a teacher selected municipality sample comprising the 5% poorest readers, and a control group consisting of the 5% best readers were all assessed applying a norm-based, standardized measure by Henderson and Sugden 1992; (The Movement Assessment Battery for Children. Kent: The Psychological Corporation). The three groups were compared with regard to total motor impairment scores as well as motor function within the areas of manual dexterity, ball-skills and balance. More than 50% of the children in both groups of poor readers showed definite motor coordination difficulties at or below the 5th centile, for which motor intervention is recommended. Children in both groups showed difficulties within the sub-area of manual dexterity in particular and also performed significantly worse than controls within the sub-area of balance, but not in ball-skills. The high incidence of motor coordination problems in the two groups of poor readers indicates that all children with reading difficulties should be screened for possible motor difficulties.

Altered temporal profile of visual-auditory multisensory interactions in dyslexia

Recent studies have demonstrated that dyslexia is associated with deficits in the temporal encoding of sensory information. While most previous studies have focused on information processing within a single sensory modality, it is clear that the deficits seen in dyslexia span multiple sensory systems. Surprisingly, although the development of linguistic proficiency involves the rapid and accurate integration of auditory and visual cues, the capacity of dyslexic individuals to integrate information between the different senses has not been systematically examined. To test this, we studied the effects of task-irrelevant auditory information on the performance of a visual temporal-order-judgment (TOJ) task. Dyslexic subjects' performance differed significantly from that of control subjects, specifically in that they integrated the auditory and visual information over longer temporal intervals. Such a result suggests an extended temporal "window" for binding visual and auditory cues in dyslexic individuals. The potential deleterious effects of this finding for rapid multisensory processes such as reading are discussed.

Auditory perceptual grouping and attention in dyslexia

Despite dyslexia affecting a large number of people, the mechanisms underlying the disorder remain undetermined. There are numerous theories about the origins of dyslexia. Many of these relate dyslexia to low-level, sensory temporal processing deficits. Another group of theories attributes dyslexia to language-specific impairments. Here, we show that dyslexics perform worse than controls on an auditory perceptual grouping task. The results show differences in performance between the groups that depend on sound frequency and not solely on parameters related to temporal processing. Performance on this task suggests that dyslexics' deficits may result from impaired attentional control mechanisms. Such deficits are neither modality nor language-specific and may help to reconcile differences between theories of dyslexia.

Does a visual-orthographic deficit contribute to reading disability?

In this study, visual-orthographic skills were defined as the ability to recognize whether letters and numerals are correctly oriented. Aims were to investigate whether visual-orthographic skills would contribute independent variance to reading, and whether children with a visual-orthographic deficit would be more impaired readers than similar children without this deficit. Participants were 207 children, aged 8 to 10 years, who attended school in a small suburban community. Because of the evidence that phonological awareness and naming speed are strongly related to reading, visual-orthographic skills were entered into hierarchical regression analyses following these variables. With age, verbal IQ, and verbal short-term memory also controlled, visual-orthographic skills accounted for significant independent variance in all reading measures. When children with a visual- orthographic deficit (29% of the sample) were compared with those without this deficit, they were significantly lower on all reading variables. At 8 to 10 years of age, reading progress of some children continues to be hampered by a problem in orthographic memory for the orientation of letters and numerals. Such children will require special attention, but their problems may be overlooked. As recommended by Willows and Terepocki (1993), there is need for further research on the phenomenon of letter reversals when they occur among children beyond first grade.

Children's reading performance is correlated with white matter structure measured by diffusion tensor imaging

We investigated the white matter structure in children (n = 14) with a wide range of reading performance levels using diffusion tensor imaging (DTI), a form of magnetic resonance imaging. White matter structure in a left temporo-parietal region that had been previously described as covarying with reading skill in adult readers also differs between children who are normal and poor readers. Specifically, the white matter structure measured using fractional anisotropy (FA) and coherence index (CI) significantly correlated with behavioral measurements of reading, spelling, and rapid naming performance. In general, lower anisotropy and lower coherence were associated with lower performance scores. Although the magnitude of the differences in children are smaller than those in adults, the results support the hypothesis that the structure of left temporoparietal neural pathways is a significant component of the neural system needed to develop fluent reading.

Language disorder in schizophrenia as a developmental learning disorder

Receptive language disorder in schizophrenia is hypothesized to represent a learning disorder that involves a neurodevelopmental etiology. It is argued that a preexisting developmental language disorder may characterize a subset of schizophrenia patients. A primary deficit in the temporal dynamics of brain function is assumed to cause receptive language disorder in schizophrenia. This hypothesized core deficit includes both disturbance in the processing of rapid, sequential information and disruptions to patterns of brain activation and synchronization. These timing deficits may alter the way associative connections are formed and/or accessed in semantic memory. It is suggested that abnormalities in second-messenger pathways of subcortical-cortical circuitry offer an etiological nexus for language dysfunction in schizophrenia and developmental dyslexia.

Atypical brainstem representation of onset and formant structure of speech sounds in children with language-based learning problems

This study investigated how the human auditory brainstem represents constituent elements of speech sounds differently in children with language-based learning problems (LP, n = 9) compared to normal children (NL, n = 11), especially under stress of rapid stimulation. Children were chosen for this study based on performance on measures of reading and spelling and measures of syllable discrimination. In response to the onset of the speech sound /da/, wave V-V(n) of the auditory brainstem response (ABR) had a significantly shallower slope in LP children, suggesting longer duration and/or smaller amplitude. The amplitude of the frequency following response (FFR) was diminished in LP subjects over the 229-686 Hz range, which corresponds to the first formant of the/da/ stimulus, while activity at 114 Hz, representing the fundamental frequency of /da/, was no different between groups. Normal indicators of auditory peripheral integrity suggest a central, neural origin of these differences. These data suggest that poor representation of crucial components of speech sounds could contribute to difficulties with higher-level language processes.

The cognitive deficits responsible for developmental dyslexia: review of evidence for a selective visual attentional disorder

There is strong converging evidence suggesting that developmental dyslexia stems from a phonological processing deficit. However, this hypothesis has been challenged by the widely admitted heterogeneity of the dyslexic population, and by several reports of dyslexic individuals with no apparent phonological deficit. In this paper, we discuss the hypothesis that a phonological deficit may not be the only core deficit in developmental dyslexia and critically examine several alternative proposals. To establish that a given cognitive deficit is causally related to dyslexia, at least two conditions need to be fulfilled. First, the hypothesized deficit needs to be associated with developmental dyslexia independently of additional phonological deficits. Second, the hypothesized deficit must predict reading ability, on both empirical and theoretical grounds. While most current hypotheses fail to fulfil these criteria, we argue that the visual attentional deficit hypothesis does. Recent studies providing evidence for the independence of phonological and visual attentional deficits in developmental dyslexia are reviewed together with empirical data showing that phonological and visual attentional processing skills contribute independently to reading performance. A theoretical model of reading is outlined in support of a causal link between a visual attentional disorder and a failure in reading acquisition.

Retinotopic organization and functional subdivisions of the human lateral geniculate nucleus: a high-resolution functional magnetic resonance imaging study

Functional magnetic resonance imaging (fMRI) has provided intriguing insights into the topography and functional organization of visual cortical areas in the human brain. However, little is known about the functional anatomy of subcortical nuclei. Here, we used high-resolution fMRI (1.5 x 1.5 x 2 mm3) at 3 tesla to investigate the retinotopic organization of the human lateral geniculate nucleus (LGN). The central 15 degrees of the visual field were mapped using periodic flickering checkerboard stimuli that evoked a traveling wave of activity. The contralateral visual hemifield was represented with the lower field in the medial-superior portion and the upper field in the lateral-inferior portion of each LGN. The horizontal meridian was significantly overrepresented relative to the vertical meridian. The fovea was represented in posterior and superior portions, with increasing eccentricities represented more anteriorly. The magnification of the fovea relative to the periphery was similar to that described for human primary visual cortex. The magnocellular regions of the LGN were distinguished based on their sensitivity to low stimulus contrast and tended to be located in its inferior and medial portions. Our results demonstrate striking similarities in the topographic organization of the macaque and human LGN and support accounts of a constant magnification from the retina through the cortex in both species.

Magnocellular mediated visual-spatial attention and reading ability

This study explores the relationship between attentional processing mediated by visual magnocellular (MC) processing and reading ability. Reading ability in a group of primary school children was compared to performance on a visual cued coherent motion detection task. The results showed that a brief spatial cue was more effective in drawing attention either away or towards a visual target in the group of readers ranked in the upper 25% of the sample compared to lower ranked readers. Regression analysis showed a significant relationship between attentional processing and reading when the effects of age and intellectual ability were removed. Results suggested a stronger relationship between visual attentional and non-word reading compared to irregular word reading.

Spatial load factor in prediction of reading performance

Abstract This study investigated whether there is a relationship between reading age and clinical optometric tests that have varying degrees of spatial loading in their design. Spatial loading in this context is the demand on the visual system to process information about the relative position and orientation of stimuli. A total of 112 children aged 8-11 years were assessed using saccadic eye movement and rapid naming tasks with varying spatial loads. All were subtests of Garzia's Developmental Eye Movement test and Liubinas' SeeRite Reading Diagnostic Programme. Variability in load was achieved by comparing rapid naming of numerals vs the spatially loaded letters p, d, b, q; and by comparing the speed of reading numerals presented in increasingly complex arrays. Reading Age was assessed independently and results were analysed by multiple logistic regression. Spatially loaded naming tasks performed at speed exposed a Spatial Loading Factor which clearly differentiates children at risk with reading.

Sequential spatial frequency discrimination is consistently impaired among adult dyslexics

The degree and nature of dyslexics' difficulties in performing basic visual tasks have been debated for more than thirty years. We recently found that dyslexics' difficulties in detecting temporally modulated gratings are specific to conditions that require accurate comparisons between sequentially presented stimuli [Brain 124 (2001) 1381]. We now examine dyslexics' spatial frequency discrimination (rather than detection), under simultaneous (spatial forced choice) and sequential (temporal forced choice) presentations. Sequential presentation (at SOAs of 0.5, 0.75 and 2.25 s) yielded better discrimination thresholds among the majority of controls (around 0.5 c/ degrees reference), but not among dyslexics. Consequently, there was a (large and significant) group effect only for the sequential conditions. Within the same dyslexic group, performance on a sequential auditory task, two-tone frequency discrimination, was impaired in a smaller proportion of the participants. Taken together, our findings indicate that visual paradigms requiring sequential comparisons are difficult for the majority of dyslexic individuals, perhaps because deficits either in visual perception or in visual memory could both lead to difficulties on these paradigms.

Two visual motion processing deficits in developmental dyslexia associated with different reading skills deficits

Developmental dyslexia is associated with deficits in the processing of visual motion stimuli, and some evidence suggests that these motion processing deficits are related to various reading subskills deficits. However, little is known about the mechanisms underlying such associations. This study lays a richer groundwork for exploration of such mechanisms by more comprehensively and rigorously characterizing the relationship between motion processing deficits and reading subskills deficits. Thirty-six adult participants, 19 of whom had a history of developmental dyslexia, completed a battery of visual, cognitive, and reading tests. This battery combined motion processing and reading subskills measures used across previous studies and added carefully matched visual processing control tasks. Results suggest that there are in fact two distinct motion processing deficits in developmental dyslexia, rather than one as assumed by previous research, and that each of these deficits is associated with a different type of reading subskills deficit. A deficit in detecting coherent motion is selectively associated with low accuracy on reading subskills tests, and a deficit in discriminating velocities is selectively associated with slow performance on these same tests. In addition, evidence from visual processing control tasks as well as self-reports of ADHD symptoms suggests that these motion processing deficits are specific to the domain of visual motion, and result neither from a broader visual deficit, nor from the sort of generalized attention deficit commonly comorbid with developmental dyslexia. Finally, dissociation between these two motion processing deficits suggests that they may have distinct neural and functional underpinnings. The two distinct patterns of motion processing and reading deficits demonstrated by this study may reflect separable underlying neurocognitive mechanisms of developmental dyslexia.

Developmental dyslexia

Developmental dyslexia, or specific reading disability, is a disorder in which children with normal intelligence and sensory abilities show learning deficits for reading. Substantial evidence has established its biological origin and the preponderance of phonological disorders even though important phenotypic variability and comorbidity have been recorded. Diverse theories have been proposed to account for the cognitive and neurological aspects of dyslexia. Findings of genetic studies show that different loci affect specific reading disability although a direct relation has not been established between symptoms and a given genomic locus. In both children and adults with dyslexia, results of neuroimaging studies suggest defective activity and abnormal connectivity between regions crucial for language functions--eg, the left fusiform gyrus for reading--and changes in brain activity associated with performance improvement after various remedial interventions.

Magnocellular and parvocellular developmental course in infants during the first year of life

The visual system undergoes major modifications during the first year of life. We wanted to examine whether the magnocellular (M) and parvocellular (P) pathways mature at the same rate or if they follow a different developmental course. A previous study carried out in our laboratory had shown that the N1 and P1 components of pattern visual evoked potentials (PVEPs) were preferentially related to the activity of P and M pathways, respectively. In the present study, PVEPs were recorded at Oz in 33 infants aged between 0 and 52 weeks, in response to two spatial frequencies (0.5 and 2.5 c deg(-1)) presented at four contrast levels (4, 12, 28 and 95%). Results indicate that the P1 component appeared before the N1 component in the periods tested and was unambiguously present at birth. The P1 component showed a rapid gain in amplitude in the following months, to reach a ceiling around 4-6 months. Conversely, the N1 component always appeared later and then gained in amplitude until the end of the first year without reaching a plateau. Latencies were also computed but no developmental dissociation was revealed. Results obtained on amplitude are interpreted as demonstrating a developmental dissociation between the underlying M and P pathways, suggesting that the former is functional earlier and matures faster than the latter during the first year of life.

The relationship between membrane pathology and language disorder in schizophrenia

Receptive language disorder in schizophrenia has been hypothesized to involve a fundamental deficit in the temporal (time-based) dynamics of brain function that includes disruptions to patterns of activation and synchronization. In this paper, candidate mechanisms and pathways that could account for this basic deficit are discussed. Parallels are identified between the patterns of language dysfunction observed for schizophrenia and dyslexia, two separate clinical disorders that may share a common abnormality in cell membrane phospholipids. A heuristic is proposed which details a trajectory involving an interaction of brain fatty acids and second-messenger function that modulates synaptic efficacy, and, in turn, influences language processing in schizophrenia patients. It is additionally hypothesized that a primary deficit of functional excitation originating in the cerebellum, in combination with a compensatory decrease of functional inhibition in the prefrontal cortex, influences receptive language dysfunction in schizophrenia.

Consequences of comorbidity of developmental coordination disorders and learning disabilities for severity and pattern of perceptual-motor dysfunction

Children with developmental coordination disorder (DCD) have difficulty learning and performing age-appropriate perceptual-motor skills in the absence of diagnosable neurological disorders. Descriptive studies have shown that comorbidity of DCD exists with attention-deficit/hyperactivity disorder (ADHD) and learning disabilities (LD). This study examined the consequences of the comorbidity of DCD and LD for the severity and pattern of perceptual-motor dysfunction. Compared to children with DCD without LD, children with comorbid DCD and LD performed lower on a standardized assessment of perceptual-motor ability. Furthermore, it appeared that children with combined DCD and LD have particular difficulty performing manual dexterity and balance tasks but not ball-skill tasks. Implications for understanding the relationship between LD and perceptual-motor problems are discussed. We conclude that the comorbidity of DCD and LD not only affects the severity of perceptual-motor dysfunction but also is associated with a distinctive pattern of perceptual-motor dysfunction.

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.

Zeitliche Integration im auditiven System bei Erwachsenen mit konstitutioneller Dyslexie

Zusammenfassung: Unterschiede in der zeitlichen Integration bei präattentiver auditiver Reizverarbeitung wurden bei 12 erwachsenen dyslektischen Lesern und einer Kontrollgruppe untersucht. Dazu wurde die Mismatch-Negativität (MMN), eine Komponente des auditiven ereigniskorrelierten Hirnpotenzials, die die automatische Entdeckung von Veränderungen der akustischen Umgebung reflektiert, verwendet. Die von gelegentlich auftretenden Tonauslassungen in einer ansonsten regelmäßigen Tonfolge evozierte MMN wurde genutzt, um zeitliche Integrationsmechanismen im auditiven System zu untersuchen. In Experiment 1 wurden in einer ansonsten regelmäßigen Tonfolge gelegentlich Töne ausgelassen, wobei sich die präsentierten Blöcke in ihrem Interstimulus-Intervall unterschieden (120ms, 150ms, 180ms, 200ms, 220ms). In Experiment 2 wurde die Genauigkeit des auditiven Integrationsmechanismus untersucht. Es ergaben sich keine Gruppenunterschiede in Amplitude, Latenz oder Topographie für die durch Tonauslassungen oder Tonabweichungen evozierte MMN. Diese Ergebnisse werden im Hinblick auf theoretische Vorstellungen, die von einem zeitlichen Verarbeitungsdefizit bei konstitutioneller Dyslexie ausgehen, diskutiert.

The role of motion direction selective extrastriate regions in reading: a transcranial magnetic stimulation study

Why reading ability is correlated with motion processing ability is perplexing. Activity in motion direction processing regions (Area V5/MT+) was perturbed by means of repetitive transcranial magnetic stimulation (rTMS) to examine its effect on reading. A functional probe (significant shortening of the motion aftereffect) was used to identify Area V5/MT+. Right-handed participants (8 m, 8 f) received three 7.5 min blocks of rTMS, after which two phonological and one orthographic reading tasks were administered. Application of rTMS to Area V5/MT+ (as compared to a non-rTMS baseline) significantly decreased performance only during non-word naming. The pattern of naming errors and the absence of deficits on the second phonological task were not consistent with a role for Area V5/MT+ in phonological decoding. Instead, its role in reading may be limited to image stabilization and/or letter localization.

Developmental dyslexia: specific phonological deficit or general sensorimotor dysfunction?

Dyslexia research now faces an intriguing paradox. It is becoming increasingly clear that a significant proportion of dyslexics present sensory and/or motor deficits; however, as this 'sensorimotor syndrome' is studied in greater detail, it is also becoming increasingly clear that sensory and motor deficits will ultimately play only a limited role in a causal explanation of specific reading disability.

Crossmodal temporal processing acuity impairment aggravates with age in developmental dyslexia

Temporal processing has been found to be impaired in developmental dyslexia. We investigated how aging affects crossmodal temporal processing impairment with 39 dyslexic and 40 fluent 20-59-year-old readers. Cognitive temporal acuity was measured at millisecond levels in six tasks. They consisted of order judgments of two brief non-speech stimulus pulses, the stimuli being audiotactile, visuotactile and audiovisual, and of simultaneity/nonsimultaneity detection of the pulses in two parallel three-pulse trains. Temporal acuity declined with age in both reading groups and its impairment was observed in developmental dyslexia. A new finding was that the crossmodal temporal impairment, directly relevant to reading, increased with age. The age-related exacerbation suggests a developmental neuronal deficit, possibly related to magnocells, which exists before dyslexia and is its ontogenetic cause.

Developmental dyslexics show altered allocation of attention in visual classification tasks

OBJECTIVES

Event-related brain potentials (ERPs) were recorded to investigate allocation of attention in adult developmental dyslexics.

SUBJECTS AND METHODS

Twelve adult developmental dyslexics and 12 matched normal controls performed three visual choice reaction tasks. In the passive oddball condition, subjects watched two different simple visual stimuli presented with 87.5 and 12.5% probability. In the active oddball condition, participants responded to the low-probability target stimulus. In the active 50/50-condition, both stimuli were presented with 50% probability and a response was required to the target stimulus only.

RESULTS

No group differences emerged for performance, P300 latency or laterality and for N200 amplitude, latency or laterality. An enhancement of P300 amplitude with a frontal distribution was found for NoGo (standard)-stimuli in both active conditions for the dyslexic sample.

CONCLUSION

Results are discussed in the context of deviances in allocation of attentional resources in dyslexic readers.

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.

The association of abnormal cerebellar function in children with developmental coordination disorder and reading difficulties

Children with developmental coordination disorder/dyspraxia (DCD) are at high risk of reading and writing delay. The difficulties with motor skills are heterogeneous and many children have features of poor cerebellar function, reflected in problems with posture, balance and fast accurate control of movement. This study confirmed a high level of parental reporting of reading and writing delay in a clinical group of 23 children with DCD, defined on the basis of both clinical examination and standardized testing of motor function. Direct measurement of reading delay, identified still further children in the group. Those children with reading delay had associated findings typical of phonological awareness difficulties. The children also underwent a standardized test of neurological function and although they all had difficulties with cerebellar function, no distinctive pattern emerged for those whose presentation was complicated by delayed reading and writing. Both the children with DCD and 136 typically developing children, completed the pilot parental questionnaire on gross motor skills. The three skills of catching a ball, jumping on a moving playground roundabout and handwriting, distinguished the children with DCD. This study therefore confirms that children with DCD should be assessed for difficulties in phonological awareness. Additionally, children aged between 7 and 12 years are on the whole, highly competent in a range of gross motor skills and further study might determine whether a simple parental questionnaire might detect children who would benefit from further assessment. The study also suggests that all the children with DCD have cerebellar dysfunction and further work with a larger group might determine particular patterns associated with reading delay.

Maternal antibody-mediated dyslexia? Evidence for a pathogenic serum factor in a mother of two dyslexic children shown by transfer to mice using behavioural studies and magnetic resonance spectroscopy

The causes of dyslexia are unknown, but previous studies have suggested an immunological basis in some cases. We hypothesised that maternal antibodies, which cross the placenta and bind to fetal antigens, could be responsible, particularly when the dyslexia recurs in consecutive pregnancies. We injected serum samples from five mothers of two or more children with dyslexia into pregnant mice, and tested the offspring for behavioural abnormalities and cerebellar metabolites by magnetic resonance spectroscopy (MRS). Mice exposed in utero to serum factors from one woman with two dyslexic children, who had also had three spontaneous fetal losses, showed deficits in motor tests which correlated with cerebellar choline (Cho) and creatine (Cr) levels. These preliminary results are consistent with a role for maternal serum factors, probably antibodies, in causing some of the features of dyslexia, and possibly in other neurodevelopmental disorders.

Developmentally impaired processing speed decreases more than normally with age

Several studies show that although function may recover after brain damage the insult can nevertheless cause accelerated deterioration in old age. This has been interpreted as indicating reduced neuronal capacity to counteract age-related decline with plastic changes. Psychosocial and compensatory factors obscure the neuronal explanation. Since the speed of processing sequential temporal information is impaired in developmental dyslexia, we investigated its dependence on age (20-59 years) in psychosocially comparable groups of dyslexic and fluent readers using six tasks. Processing speed was impaired in dyslexia and decreased with age. The decrement was faster in dyslexic than normal readers in processing periodic stimuli. No exacerbation occurred in reading and other experiential factors. Our results, therefore, support the neuronal explanation.