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.     

Non-verbal IQ is correlated with visual field advantages for short duration coherent motion detection in deaf signers with varied ASL exposure and etiologies of deafness

Studies have reported a right visual field (RVF) advantage for coherent motion detection by deaf and hearing signers but not non-signers. Yet two studies [Bosworth R. G., & Dobkins, K. R. (2002). Visual field asymmetries for motion processing in deaf and hearing signers. Brain and Cognition, 49, 170-181; Samar, V. J., & Parasnis, I. (2005). Dorsal stream deficits suggest hidden dyslexia among deaf poor readers: Correlated evidence from reduced perceptual speed and elevated coherent motion detection thresholds. Brain and Cognition, 58, 300-311.] reported a small, non-significant RVF advantage for deaf signers when short duration motion stimuli were used (200-250 ms). Samar and Parasnis (2005) reported that this small RVF advantage became significant when non-verbal IQ was statistically controlled. This paper presents extended analyses of the correlation between non-verbal IQ and visual field asymmetries in the data set of Samar and Parasnis (2005). We speculate that this correlation might plausibly be driven by individual differences either in age of acquisition of American Sign Language (ASL) or in the degree of neurodevelopmental insult associated with various etiologies of deafness. Limited additional analyses are presented that indicate a need for further research on the cause of this apparent IQ-laterality relationship. Some potential implications of this relationship for lateralization studies of deaf signers are discussed. Controlling non-verbal IQ may improve the reliability of short duration coherent motion tasks to detect adaptive dorsal stream lateralization due to exposure to ASL in deaf research participants.     

Deaf poor readers' pattern reversal visual evoked potentials suggest magnocellular system deficits: implications for diagnostic neuroimaging of dyslexia in deaf individuals

Deafness and developmental dyslexia in the same individual may jointly limit the acquisition of reading skills for different underlying reasons. A diagnostic marker for dyslexia in deaf individuals must therefore detect the presence of a neurobiologically based dyslexia but be insensitive to the ordinary developmental influences of deafness on reading skill development. We propose that the functional status of the magnocellular visual system in deaf individuals is potentially such a marker. We present visual evoked potential (VEP) evidence that adult deaf poor readers as a group display magnocellular system deficits not observed in deaf good readers. We recorded pattern-reversal VEPs to high- and low-contrast checkerboard stimuli, which primarily activate the parvocellular and magnocellular pathways, respectively. Principal components analysis of these VEPs produced a time-ordered sequence of three early components that displayed interactions between reading skill and stimulus contrast across multiple scalp recording sites. Deaf poor readers displayed an abnormal absence of contrast-sensitive VEP responses at occipital sites during early visual processing (75 ms poststimulus), whereas deaf good readers showed the expected early contrast-sensitive occipital VEP responses. Over the subsequent 225 ms, the occipital VEP behavior of deaf poor readers closely approximated that of deaf good readers. The VEPs of deaf poor readers were apparently characterized by delayed responses to low-contrast stimuli compared with deaf good readers. Our results provide the first neurobiological evidence that developmental dyslexia exists within the deaf population and is associated with the same underlying magnocellular system deficit that has been observed in hearing dyslexics. Direct neural imaging of the status of the magnocellular visual system in deaf individuals may eventually provide differential diagnosis of developmental dyslexia in the deaf population.     

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.     

Motion-perception deficits and reading impairment: it's the noise, not the motion

We tested the hypothesis that deficits on sensory-processing tasks frequently associated with poor reading and dyslexia are the result of impairments in external-noise exclusion, rather than motion perception or magnocellular processing. We compared the motion-direction discrimination thresholds of adults and children with good or poor reading performance, using coherent-motion displays embedded in external noise. Both adults and children who were poor readers had higher thresholds than their respective peers in the presence of high external noise, but not in the presence of low external noise or when the signal was clearly demarcated. Adults' performance in high external noise correlated with their general reading ability, whereas children's performance correlated with their language and verbal abilities. The results support the hypothesis that noise-exclusion deficits impair reading and language development and suggest that the impact of such deficits on the development of reading skills changes with age.     

动态视觉加工与儿童汉字阅读

使用视觉阈限测验、图片命名、字形相似性判断实验和语音意识等测验 ,考察了小学五年级儿童视觉加工技能与汉字阅读之间的关系。结果发现 :动态视觉加工与图片命名错误率、字形判断反应时和错误率、语音意识均有显著相关 ,静态视觉加工只与图片命名错误率相关显著 ;控制识字量后的偏相关分析显示 ,动态视觉加工与其他变量的相关关系不变 ,静态视觉加工与图片命名错误率的相关不再显著 ;回归分析发现动态视觉加工在识字量和语音意识的影响控制后 ,能够分别解释阅读流畅性、字形判断反应时和图片命名错误率 7%、2 5 %和 5 6 %的变化 ;语音意识能够解释识字量和阅读流畅性 9%和 10 %的变化 ;对差读者的动态视觉加工和语音意识分析发现 ,儿童在这两种测验上的个体差异很大。上述结果表明 ,阅读过程受基本知觉技能影响 ,动态视觉加工作用于汉字阅读的特定过程。     

On the use of metacontrast to assess magnocellular function in dyslexic readers

It has been proposed that dyslexia is the result of a deficit in the magnocellular system. Reduced metacontrast masking in dyslexic readers has been taken as support for this view. In metacontrast, a masking stimulus reduces the visibility of a spatially adjacent target stimulus when the target stimulus precedes the masking stimulus by about 30–100 msec. Recent evidence indicates that the latency difference between the magnocellular and parvocellular subcortical pathways is at most 20 msec and may be as small as only 5 msec, or even less. This makes it difficult to attribute the latency in metacontrast to the latency differences between the magnocellular and parvocellular systems. It is therefore problematic to attribute reduced metacontrast masking to a deficit in the magnocellular system.     

On the use of metacontrast to assess magnocellular function in dyslexic readers.

It has been proposed that dyslexia is the result of a deficit in the magnocellular system. Reduced metacontrast masking in dyslexic readers has been taken as support for this view. In metacontrast, a masking stimulus reduces the visibility of a spatially adjacent target stimulus when the target stimulus precedes the masking stimulus by about 30-100 msec. Recent evidence indicates that the latency difference between the magnocellular and parvocellular subcortical pathways is at most 20 msec and may be as small as only 5 msec, or even less. This makes it difficult to attribute the latency in metacontrast to the latency differences between the magnocellular and parvocellular systems. It is therefore problematic to attribute reduced metacontrast masking to a deficit in the magnocellular system.     

On the prevalence of magnocellular deficits in the visual system of non-dyslexic individuals

Numerous studies have found visual deficits associated with dyslexia. This has made it important to understand how these deficits may be related to reading difficulties. A widely held theory is that dyslexia is the result of a deficit in the magnocellular part of the visual system (earlier called the transient system). In support for this theory, the prevalence of magnocellular deficits has been reported to be high among dyslexic readers and very low among non-dyslexic ones. This creates the impression that in the population as a whole dyslexic individuals have magnocellular deficits and non-dyslexic individuals do not. However, we show that because the prevalence of dyslexia itself is low this need not be the case. On the basis of previously published data we have estimated the number of non-dyslexic and dyslexic individuals with magnocellular deficits. Our estimates indicate that there should be a large number of non-dyslexic individuals with magnocellular deficits. Paradoxically more individuals without dyslexia have magnocellular deficits than individuals with dyslexia. This poses a challenge to the view that dyslexia is the result of a magnocellular deficit.     

M-stream deficits and reading-related visual processes in developmental dyslexia

Some visual processing deficits in developmental dyslexia have been attributed to abnormalities in the subcortical M stream and/or the cortical dorsal stream of the visual pathways. The nature of the relationship between these visual deficits and reading is unknown. The purpose of the present article was to characterize reading-related perceptual processes that may link the visual deficits to reading problems. We identified contrast sensitivity, position encoding, oculomotor control, visual attention, parafoveal/foveal interactions, and saccadic suppression as potential reading-related dorsal stream processes. We then evaluated the role of each process in reading and the status of each process in dyslexia. In theory, a number of dorsal stream processes (e.g., oculomotor control and visual attention) might contribute to reading problems in developmental dyslexia. More work is needed to demonstrate the connection empirically.     

发展性阅读障碍与知觉加工

近年来许多行为实验和神经生理学实验都发现 ,发展性阅读障碍与基本知觉障碍有关。在视觉领域研究者提出了巨细胞障碍假设 ,这种假设认为阅读障碍者视觉神经系统中的巨细胞障碍导致他们对某种类型的视觉刺激加工存在困难 ,进而影响阅读。在听觉领域研究发现阅读障碍者加工快速、系列、短暂呈现的声音刺激存在障碍。研究者认为阅读障碍者加工快速刺激输入障碍反映了普遍的时间知觉障碍。这方面的研究发展非常迅速 ,理论观点非常明确 ,并且直接与内在的神经机制相联系 ,形成了与传统的“语言障碍”理论迥然不同的“知觉障碍”理论。“知觉障碍”理论综合了行为、认知和神经等多个层次的研究 ,反映了神经科学发展所带来的巨大影响和认知加工模块化理论的渐渐衰退。     

Visual temporal processing in dyslexia and the magnocellular deficit theory: the need for speed?

A controversial question in reading research is whether dyslexia is associated with impairments in the magnocellular system and, if so, how these low-level visual impairments might affect reading acquisition. This study used a novel chromatic flicker perception task to specifically explore temporal aspects of magnocellular functioning in 40 children with dyslexia and 42 age-matched controls (aged 7-11). The relationship between magnocellular temporal resolution and higher-level aspects of visual temporal processing including inspection time, single and dual-target (attentional blink) RSVP performance, go/no-go reaction time, and rapid naming was also assessed. The Dyslexia group exhibited significant deficits in magnocellular temporal resolution compared with controls, but the two groups did not differ in parvocellular temporal resolution. Despite the significant group differences, associations between magnocellular temporal resolution and reading ability were relatively weak, and links between low-level temporal resolution and reading ability did not appear specific to the magnocellular system. Factor analyses revealed that a collective Perceptual Speed factor, involving both low-level and higher-level visual temporal processing measures, accounted for unique variance in reading ability independently of phonological processing, rapid naming, and general ability.     

Visual versus phonological abilities in Spanish dyslexic boys and girls

Phonological and visual theories propose different primary deficits as part of the explanation for dyslexia. Both theories were put to test in a sample of Spanish dyslexic readers. Twenty-one dyslexic and 22 typically-developing children matched on chronological age were administered phonological discrimination and awareness tasks and coherent motion perception tasks. No differences were found between groups on the coherent motion tasks, whereas dyslexic readers were impaired relative to controls on phonological discrimination tasks. Gender differences followed the opposite pattern, with no differences on phonological tasks, and dyslexic girls performing significantly worse than dyslexic boys in coherent motion perception. These results point to the importance of phonological deficits related to speech perception in Spanish, and to possible gender differences in the neurobiological bases for dyslexia.     

阅读障碍的巨细胞系统缺陷理论之争

近几年诸多研究者认为阅读障碍是由于视觉系统巨细胞功能缺陷引起的。他们从对比度敏感性研究、巨细胞抑制功能研究与视觉运动知觉功能的研究结果中得到了支持证据。就这个问题的研究进行综述,表明不论是巨细胞功能缺陷理论的支持者还是反对者,都从巨细胞的对比度敏感性、视觉运动知觉与巨细胞抑制功能上得到了一些研究结果的支持。巨细胞功能缺陷理论的支持者还从阅读障碍的神经生理学研究和阅读的脑功能研究中得到了一些支持性的证据。新的研究结果揭示了巨细胞系统缺陷理论与阅读之间的关系以及巨细胞系统缺陷理论的应用价值。在巨细胞系统缺陷理论的进一步研究中,还需要统一实验方法与实验标准以获得更多的证据,同时需要有新的方法来有效区分小细胞系统与巨细胞系统     

The roles of family history of dyslexia,language, speech production and phonological processing in predicting literacy progress

It is well established that speech, language and phonological skills are closely associated with literacy, and that children with a family risk of dyslexia (FRD) tend to show deficits in each of these areas in the preschool years. This paper examines what the relationships are between FRD and these skills, and whether deficits in speech, language and phonological processing fully account for the increased risk of dyslexia in children with FRD. One hundred and fifty‐three 4–6‐year‐old children, 44 of whom had FRD, completed a battery of speech, language, phonology and literacy tasks. Word reading and spelling were retested 6 months later, and text reading accuracy and reading comprehension were tested 3 years later. The children with FRD were at increased risk of developing difficulties in reading accuracy, but not reading comprehension. Four groups were compared: good and poor readers with and without FRD. In most cases good readers outperformed poor readers regardless of family history, but there was an effect of family history on naming and nonword repetition regardless of literacy outcome, suggesting a role for speech production skills as an endophenotype of dyslexia. Phonological processing predicted spelling, while language predicted text reading accuracy and comprehension. FRD was a significant additional predictor of reading and spelling after controlling for speech production, language and phonological processing, suggesting that children with FRD show additional difficulties in literacy that cannot be fully explained in terms of their language and phonological skills.     

Neural correlates of coherent and biological motion perception in autism

Recent evidence suggests those with autism may be generally impaired in visual motion perception. To examine this, we investigated both coherent and biological motion processing in adolescents with autism employing both psychophysical and fMRI methods. Those with autism performed as well as matched controls during coherent motion perception but had significantly higher thresholds for biological motion perception. The autism group showed reduced posterior Superior Temporal Sulcus (pSTS), parietal and frontal activity during a biological motion task while showing similar levels of activity in MT+/V5 during both coherent and biological motion trials. Activity in MT+/V5 was predictive of individual coherent motion thresholds in both groups. Activity in dorsolateral prefrontal cortex (DLPFC) and pSTS was predictive of biological motion thresholds in control participants but not in those with autism. Notably, however, activity in DLPFC was negatively related to autism symptom severity. These results suggest that impairments in higher-order social or attentional networks may underlie visual motion deficits observed in autism.     

Dyslexia, dysnomia, and lexical retrieval: a longitudinal investigation

Results from a 3-year longitudinal investigation of confrontation naming in 98 children (75 average readers, 14 severely impaired readers, 9 bilingual readers) indicate: kindergarten performance on confrontation naming predicts Grade 2 reading performance, particularly reading comprehension; confrontation naming differentiates average from severely impaired readers; and lexical retrieval, not vocabulary knowledge, is the major source of difference between reading groups. Findings on lexical retrieval are integrated with research on naming access speed. The implications of these results for specification of dyslexia subgroups are discussed.     

Is a "Phoenician" reading style superior to a "Chinese" reading style? Evidence from fourth graders

This study compared normally achieving fourth-grade "Phoenician" readers, who identify nonwords significantly more accurately than they do exception words, with "Chinese" readers, who show the reverse pattern. Phoenician readers scored lower than Chinese readers on word identification, exception word reading, orthographic choice, spelling, reading comprehension, and verbal ability. When compared with normally achieving children who read nonwords and exception words equally well, Chinese readers scored as well as these children on word identification, regular word reading, orthographic choice, spelling, reading comprehension, phonological sensitivity, and verbal ability and scored better on exception word reading. Chinese readers also used rhyme-based analogies to read nonwords derived from high-frequency exception words just as often as did these children. As predicted, Phoenician and Chinese readers adopted somewhat different strategies in reading ambiguous nonwords constructed by analogy to high-frequency exception words. Phoenician readers were more likely than Chinese readers to read ambiguous monosyllabic nonwords via context-free grapheme-phoneme correspondences and were less likely to read disyllabic nonwords by analogy to high-frequency analogues. Although the Chinese reading style was more common than the Phoenician style in normally achieving fourth graders, there were similar numbers of poor readers with phonological dyslexia (identifying nonwords significantly more accurately than exception words) and surface dyslexia (showing the reverse pattern), although surface dyslexia was more common in the severely disabled readers. However, few of the poor readers showed pure patterns of phonological or surface dyslexia.     

Neural dynamics of motion perception: Direction fields,apertures, and resonant grouping

A neural network model of global motion segmentation by visual cortex is described. Called the motion boundary contour system (BCS), the model clarifies how ambiguous local movements on a complex moving shape are actively reorganized into a coherent global motion signal, Unlike many previous researchers, we analyze how a coherent motion signal is imparted to all regions of a moving figure, not only to regions at which unambiguous motion signals exist. The model hereby suggests a solution to the global aperture problem. The motion BCS describes how preprocessing of motion signals by a motion oriented contrast (MOC) filter is joined to long-range cooperative grouping mechanisms in a motion cooperative-competitive (MOCC) loop to control phenomena such as motion capture. The motion BCS is computed in parallel with the static BCS of Grossberg and Mingolla (1985a, 1985b, 1987). Homologous properties of the motion BCS and the static BCS, specialized to process motion directions and static orientations, respectively, support a unified explanation of many data about static form perception and motion form perception that have heretofore been unexplained or treated separately. Predictions about microscopic computational differences of the parallel cortical streams V1 → MT and V1 → V2 → MT are made— notably, the magnocellular thick stripe and parvocellular interstripe streams. It is shown how the motion BCS can compute motion directions that may be synthesized from multiple orientations with opposite directions of contrast. Interactions of model simple cells, complex cells, hyper-complex cells, and bipole cells are described, with special emphasis given to new functional roles in direction disambiguation for endstopping at multiple processing stages and to the dynamic interplay of spatially short-range and long-range interactions.     

Phonological Awareness and Reading Proficiency in Adults with Profound Deafness

This study investigated differences in the phonological knowledge and reading skill of deaf adults using three experimental conditions that tested sensitivity to syllables, rhyme, and phonemes. Analysis of response latencies and accuracy in the three awareness tasks demonstrated that skilled deaf readers had superior phonological awareness skill to that of less-skilled readers and showed less reliance on orthography when making their phonological judgments. Rhyme had greater independent predictive strength than syllable or phoneme awareness for reading comprehension. Phoneme awareness did not contribute independently to either reading comprehension or word reading, but it did independently predict application of grapheme-phoneme correspondences.