The cognitive and neurological basis of developmental dyslexia: a theoretical framework and review.

It is argued that developmental dyslexics have difficulty in accessing the meaning of written words via phonological recoding, although they can successfully access meaning by a direct visual route. This difficulty with phonological recording is explained in terms of a short-term memory deficit. It is suggested that developmental dyslexia is a genetically-based dysfunction of the inferior parietal lobule and evidence is reviewed that this region is important in both reading and short-term memory. Implications for remedial instruction are discussed.     

Cross language learning disabilities and verbal versus spatial memory

The research reported in this talk involves comparisons of verbal and spatial memory tasks across groups of children (and adults) with different types of learning difficulties. The research focuses on children with literacy acquisition problems and investigates whether such problems are related to specific areas of deficit. In the first piece of research, children with dyslexia (literacy learning problems) and dyspraxia (motor deficits) were contrasted on measures of memory (for example, tasks that required the retention of sequences of verbal material or spatial movements) and additional measures of literacy (reading and spelling), phonological (awareness of sounds within words) and motor (fine and gross motor tasks) functioning. The data were consistent with a dissociation between tasks/groups such that dyslexics showed weak phonological processing but intact visuo-spatial processing, whereas children with dyspraxia showed weaknesses on task involving visuo-spatial information, but average levels of performance on tasks that required phonological processing. Similar results were identified amongst adult groups, consistent with a deviant level of functioning rather than a developmental delay. A second line of research contrasted children with or without literacy problems across language backgrounds (English, Arabic, Chinese and bilingual children). Consistent with the dyslexia data, children with poor English literacy skills showed weaknesses in verbal/phonological memory tasks but not in visuo-spatial memory. However, for Chinese-language children, visuo-spatial memory differed between good and poor literacy learners, but there was little evidence for verbal memory differences. In contrast, the Arabic and bilingual children showed differences in both verbal and visuo-spatial areas, although the evidence was consistent with enhanced visual/spatial skills amongst the good literacy groups, rather than poor literacy children showing weaknesses in those tasks. These data suggest that the influence of memory skills on learning may vary with the language of instruction. A final line of enquiry considers whether teaching strategies to children with learning difficulties may overcome some of the identified memory deficits and lead to better levels of learning. English language children with learning difficulties were taught visual and verbal strategies to support retention of materials in short-term memory tasks. In the majority of cases, learning was improved when it focused on visuo-spatial strategies but not when verbal strategies were used. These data support the relationship between learning difficulties and different aspects of short-term memory that may lead to poor levels of learning. It also presents evidence that memory (particularly those related to visuo-spatial) processes are influenced by the context within which learning is taking place, both in terms of the language of instruction and the strategies used to support learning. For some children with educational difficulties based around language-related deficits, visuo-spatial strategies may support acquisition.     

Phonological and visual working memory in mental addition

The goal of the present research was to examine the role of working memory in mental arithmetic. Adults (n = 96) solved multidigit arithmetic problems (e.g., 52 + 3; 3 + 52) alone and in combination with either a phonological memory load (i.e., nonwords, such as gup) or a visual memory load (i.e., random pattern of asterisks). The participants solved problems presented in a vertical format significantly faster than problems presented in a horizontal format. They also solved double digit first problems (e.g., 52 + 3) more quickly than the reverse (e.g., 3 + 52), but only when the problems were presented horizontally. Performance was worse in the phonological load condition than in the visual load condition for the participants who solved problems presented horizontally, whereas performance was worse in the visual load condition than in the phonological load condition when problems were presented vertically. The present research provides evidence that both phonological and visual aspects of working memory are involved in mental arithmetic but that the role of each working memory component will depend on such factors as presentation format.     

Deficient implicit phonological representations in children with dyslexia

This study tested the segmentation hypothesis of dyslexia by measuring implicit phonological representations in reading-disabled 11- to 13-year-olds. Implicit measures included lexical gating, priming, and syllable similarity tasks designed to reduce metalinguistic demands. Children with dyslexia performed consistently worse than CA and RA controls when more segmental representations were required across all three tasks. Implicit phonological representations were correlated with measures of speech perception, phoneme awareness, and phonological short-term memory, but not rapid automatized naming, and accounted for unique variance in predicting reading ability. Results provide strong support for less mature implicit phonological representations in children with dyslexia.     

Individual differences in phonological learning and verbal STM span

A relationship between phonological short-term memory tasks (e.g., nonword repetition, digit span) and vocabulary learning in both experimental and real-life conditions has been reported in numerous studies. A mechanism that would explain this correlation is, however, not known. The present study explores the possibility that it is the quality of phonological representations that affects both short-term recall and long-term learning of novel wordlike items. In Experiment 1, groups with relatively good and poor span for pseudowords were established. The good group was found to perform better at explicit memory tasks tapping the incidental learning of a limited stimulus pool used in an auditory immediate serial pseudoword recall task. In Experiment 2, the results of Experiment 1 were replicated when experience of correct recall was controlled. In Experiment 3, the immediate recall performance of the good group was found to benefit more than that of the poor group from syllable repetition within stimulus pools. It is concluded that the efficiency of a process that creates phonological representations is related both to short-term capacity for verbal items, and to long-term phonological learning of the structure of novel phonological items.     

Book Reviews

There is long-standing evidence for verbal working memory impairments in both children and adults with dyslexia. By contrast, spatial memory appears largely to be unimpaired. In an attempt to distinguish between phonological and central executive accounts of the impairments in working memory, a set of phonological and spatial working memory tasks was designed to investigate the key issues in working memory, task type, task demands (static, dynamic, and updating), and task complexity. Significant differences emerged between the dyslexic and nondyslexic participants on the verbal working memory tasks employed in Experiment 1, thereby providing further evidence for continuing dyslexic impairments of working memory into adulthood. The nature of the deficits suggested a problem with the phonological loop, with there being little evidence to implicate an impairment of the central executive. Due to the difficulties associated with separating verbal working memory and phonological processing, however, performance was investigated in Experiment 2 using visuospatial measures of working memory. The results of the visuospatial tasks indicated no between-group differences in static spatial memory, which requires the short-term storage of simultaneously presented information. In almost all conditions there were no between-group differences in dynamic spatial memory that demands the recall of both location and order of stimuli presented sequentially. However, a significant impairment occurred on the dynamic task under high memory updating load, on which dyslexic adults showed nonphonological working memory deficits. In the absence of an explanation involving verbal recoding, this finding is interpreted in terms of a central executive or automaticity impairment in dyslexia.     

The role of inferior frontal cortex in phonological processing

Recent neuroimaging studies of language processing are examining the neural substrate of phonology because of its critical role in mapping sound information onto higher levels of language processing (e.g., words) as well as providing codes in which verbal information can be temporarily stored in working memory. However, the precise role of the inferior frontal cortex in spoken and written phonological tasks has remained elusive. Although lesion studies have indicated the presence of selective deficits in phonological processing, the location of lesions underlying these impairments has not revealed a consistent pattern. Despite efforts to refine methods and tasks, functional neuroimaging studies have also revealed variability in activation patterns. Reanalysis of evidence from these neuroimaging studies suggests that there are functional subregions within the inferior frontal gyrus that correspond to specific components of phonological processing (e.g., orthographic to phonological conversion in reading, and segmentation in speech).     

Orthography, phonology, short-term memory and the effects of concurrent articulation on rhyme and homophony judgements

The role of phonological short-term memory (pSTM) in phonological judgement tasks of print has been widely explored using concurrent articulation (CA). A number of studies have examined the effects of CA on written word/nonword rhyme and homophone judgements but the findings have been mixed and few studies have examined both tasks within subjects. Also important is the influence of orthographic similarity on such tasks (i.e., items that share phonology often strongly overlap on orthography). Although there are reports of orthographic similarity effects (e.g., LOAD-TOAD vs. DIAL-MILE) on rhyme judgements, it is unknown whether (a) similar orthographic effects are present with homophone judgements, (b) the degree to which such orthographic effects interact with CA, and (c) the degree to which such orthographic effects interact with lexical status (words vs. nonwords). The present work re-examines these three issues in a within subject design. CA and orthographic similarity yielded subtle differences across tasks. CA impaired accuracy for both homophone and rhyme judgement, but only slowed RTs on the rhyme judgement task, and then only for words. Orthographic similarity yielded an increase in false positives for similar items and vice versa for dissimilar items, suggesting a general impact of an orthographically based ‘bias’ in choosing similar or dissimilar sounding items. This pattern was amplified under CA but only on the homophone judgement task. These results highlight important interactions between phonological and orthographic representations in phonological judgement tasks, and the findings are considered both with reference to earlier studies and several models of pSTM.     

The effects of the constancy of location and order in working memory visual–phonological binding of children with dyslexia

It has been suggested that children with dyslexia have difficulties in visual–phonological working memory (WM) binding, supporting the hypothesis that this ability is crucial in the formation of associations between written forms and phonological codes required by reading. However, research on this topic is currently scarce and has not clarified to what extent binding may be supported by spatial and temporal information. The present study examined visual–phonological WM binding performance in a group of children with dyslexia compared to a control group of typically developing children matched for age, gender, and grade. Children had to memorize ephemeral associations between meaningless shapes and nonwords, with stimuli presented in either fixed or variable spatial locations, and in either fixed or variable temporal order across trials; performance was assessed using a recognition task. Results showed that children with dyslexia have a deficit in visual–phonological WM binding in every presentation condition and that, unlike control children, they are not able to use fixed spatial locations as an aid to bind information. Crucially, however, children with dyslexia still benefit from the presentation of stimuli in a fixed temporal order. These findings support the hypothesis that a WM binding deficit is crucial in children with dyslexia, and have potential implications for treatment strategies.     

Written expression in boys with ADHD: The mediating roles of working memory and oral expression

The written expression difficulties experienced by children with ADHD are widely recognized; however, scant empirical evidence exists concerning the cognitive mechanisms and processes underlying these deficiencies. The current study investigated the independent and potentially interactive contributions of two developmentally antecedent cognitive processes – viz., working memory (WM) and oral expression – hypothesized to influence written expression ability in boys. Thirty-three boys with ADHD-Combined Presentation and 27 neurotypical (NT) boys 8–12 years of age were administered standardized measures of oral and written expression, and multiple counterbalanced tasks to assess WM central executive (CE) processes, WM phonological short-term memory (PH STM), and WM visuospatial short-term memory (VS STM). Bias-corrected bootstrapped mediation analyses revealed a significant mediation effect, wherein the independent and interactive effects of PH STM and oral expression collectively explained 76% of the diagnostic status to written expression relation. The implications of the obtained results for clinical practice suggest that children with ADHD may benefit by incorporating a blended approach that simultaneously strengthens PH STM capacity and oral expression abilities as antecedents to engaging in writing-related activities.     

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.     

Working memory deficit in children with mathematical difficulties: a general or specific deficit?

This study examined whether children with mathematical difficulties (MDs) or comorbid mathematical and reading difficulties have a working memory deficit and whether the hypothesized working memory deficit includes the whole working memory system or only specific components. In the study, 31 10-year-olds with MDs and 37 10-year-olds with both mathematical and reading difficulties were compared with 47 age-matched and 50 younger controls (9-year-olds) on a number of working memory tasks. Compared with the age-matched controls, both groups of children with MDs performed worse on tasks tapping the central executive (e.g., visual matrix span) and the phonological loop (e.g., word span). More important, the MD group performed worse than the younger controls on the counting span task, whereas the group with comorbid mathematical and reading difficulties performed worse on the counting span task and the visual matrix span task. These findings provide support for the assumption that children with MDs have a working memory deficit. More specifically, children with MDs have a central executive deficit connected to concurrent processing and storage of numerical and visual information.     

FMRI of two measures of phonological processing in visual word recognition: ecological validity matters

Previous functional magnetic resonance imaging (fMRI) studies have investigated the role of phonological processing by utilizing nonword rhyming decision tasks (e.g., Pugh et al., 1996). Although such tasks clearly engage phonological components of visual word recognition, it is clear that decision tasks are more cognitively involved than the simple overt naming tasks, which more closely map onto normal reading behavior. Our research aim for this study was to examine the advantages of overt naming tasks for fMRI studies of word recognition processes. Process models are presented to highlight the similarities and differences between two cognitive tasks that are used in the word recognition literature, pseudohomophone naming (e.g., pronounce BRANE) and rhyming decision (e.g., do LEAT and JEAT rhyme?). An fMRI study identified several differences in cortical activation associated with the differences observed in the process models. Specifically, the results show that the overt naming task involved the insular cortex and inferior frontal gyrus, whereas the rhyming decision task engaged the temporal-parietal regions. It is argued that future fMRI research examining the neuroanatomical components of basic visual word recognition utilize overt naming tasks.     

Working memory functioning in developmental dyslexia

Working memory impairments in dyslexia are well documented. However, research has mostly been limited to the phonological domain, a modality in which people with dyslexia have a range of problems. In this paper, 22 adult students with dyslexia and 22 age- and IQ-matched controls were presented with both verbal and visuospatial working memory tasks. Performance was compared on measures of simple span, complex span (requiring both storage and processing), and dynamic memory updating in the two domains. The dyslexic group had significantly lower spans than the controls on all the verbal tasks, both simple and complex, and also on the spatial complex span measure. Impairments remained on the complex span measures after controlling statistically for simple span performance, suggesting a central executive impairment in dyslexia. The novelty of task demands on the initial trials of the spatial updating task also proved more problematic for the dyslexic than control participants. The results are interpreted in terms of extant theories of dyslexia. The possibility of a supervisory attentional system deficit in dyslexia is also raised. It seems clear that working memory difficulties in dyslexia extend into adulthood, can affect performance in both the phonological and visuospatial modalities, and implicate central executive dysfunction, in addition to problems with storage.     

Phonological and semantic information in word and nonword reading in a deep dyslexic patient

Deep dyslexia is diagnosed when brain-injured, previously literate adults make reading errors that include hallmark semantic paralexias (e.g., reading HEART as BLOOD) and are also impaired at reading nonwords (e.g., FRIP). The diversity of these symptoms have led most researchers to conclude that there are multiple sources of impairment in this syndrome and that one of the most critical is a failure to process phonological information at a sublexical level. The patient (SD) reported in this study fits the deep dyslexia profile to the extent that she makes several semantically related reading errors. She also shows the classic frequency and image ability effects of the syndrome. However, as we report, she does read some nonwords correctly and she shows a strong advantage for naming when phonemic cues are presented. We discuss the performance of SD, on these preliminary tasks, in terms of a phonological selection impairment.     

Dyslexia and reasoning: The importance of visual processes

Recent research has suggested that individuals with dyslexia rely on explicit visuospatial representations for syllogistic reasoning while most non‐dyslexics opt for an abstract verbal strategy. This paper investigates the role of visual processes in relational reasoning amongst dyslexic reasoners. Expt 1 presents written and verbal protocol evidence to suggest that reasoners with dyslexia generate detailed representations of relational properties and use these to make a visual comparison of objects. Non‐dyslexics use a linear array of objects to make a simple transitive inference. Expt 2 examined evidence for the visual‐impedance effect which suggests that visual information detracts from reasoning leading to longer latencies and reduced accuracy. While non‐dyslexics showed the impedance effects predicted, dyslexics showed only reduced accuracy on problems designed specifically to elicit imagery. Expt 3 presented problems with less semantically and visually rich content. The non‐dyslexic group again showed impedance effects, but dyslexics did not. Furthermore, in both studies, visual memory predicted reasoning accuracy for dyslexic participants, but not for non‐dyslexics, particularly on problems with highly visual content. The findings are discussed in terms of the importance of visual and semantic processes in reasoning for individuals with dyslexia, and we argue that these processes play a compensatory role, offsetting phonological and verbal memory deficits.     

Developmental dyslexics do not fit in boxes: Evidence from six new case studies

Abstract

Six subjects with a reading problem were tested on a variety of reading and other tasks including tests of phonological awareness and processing, short-term memory, visual analysis and synthesis, memory for faces and pictures, and matching of letters and shapes. The subjects demonstrated differences in their reading strategies, but these showed no relation to any differences in the patterns of deficit on non-reading tasks. Five of them showed severe problems on tasks involving phonological processing and short-term memory but the seventy of these was unrelated to reading strategy. There were some indications of visual weaknesses, but these were severe in only one subject who had a pronounced defect in memory for faces; there was no evidence that the visual weaknesses were consistently related to reading ability or strategy.

It is concluded that all the subjects in this sample (with one 'possible exception) had a deficit in phonological processing but adopted different strategies to cope with this when faced with the task of converting print to sound. Differences in strategy are likely to be affected by the relative efficiency of other cognitive processes, personality and introdctional factors.     

Impairment in processing meaningless verbal material in several modalities: The relationship between short-term memory and phonological skills

Phonological processing abilities were studied in a patient who, following focal brain damage, showed selective impairment in non-word reading, writing, and repetition and also a severe short-term memory (STM) deficit specific for auditorily presented verbal material. The patient could execute tasks involving phonemic manipulation and awareness perfectly. Our data, in contrast with earlier observations in a case of developmental phonological dyslexia, show that acquired impairment in non-word reading, writing, repetition, and immediate memory may occur despite good phonological processing abilities. The role of STM in processing meaningless verbal material is discussed.