Procedural learning and dyslexia

Three major 'neural systems', specialized for different types of information processing, are the sensory, declarative, and procedural systems. It has been proposed (Trends Neurosci., 30(4), 135-141) that dyslexia may be attributable to impaired function in the procedural system together with intact declarative function. We provide a brief overview of the increasing evidence relating to the hypothesis, noting that the framework involves two main claims: first that 'neural systems' provides a productive level of description avoiding the underspecificity of cognitive descriptions and the overspecificity of brain structural accounts; and second that a distinctive feature of procedural learning is its extended time course, covering from minutes to months. In this article, we focus on the second claim. Three studies-speeded single word reading, long-term response learning, and overnight skill consolidation-are reviewed which together provide clear evidence of difficulties in procedural learning for individuals with dyslexia, even when the tasks are outside the literacy domain. The educational implications of the results are then discussed, and in particular the potential difficulties that impaired overnight procedural consolidation would entail. It is proposed that response to intervention could be better predicted if diagnostic tests on the different forms of learning were first undertaken.

Behavioural and neurophysiological correlates of dyslexia in the continuous performance task

OBJECTIVE

To investigate whether attentional difficulties are a "core" feature of developmental Dyslexia.

METHODS

Behavioural indices and event related potentials (ERPs) were recorded from 10 dyslexic participants (ages 15.5-17.4) and 10 control participants (ages 14.4-18.3) in the Continuous Performance Task (CPT), an established test of attentional performance. Participants were screened to ensure that none was diagnosable as attention deficit (ADHD).

RESULTS

There were no significant differences in mean reaction time, error rate or sustained attention between the groups. By contrast, the P3 amplitude was significantly smaller and its latency significantly longer for the dyslexic group. This component was significantly lateralised in controls, whereas in dyslexics it was symmetrical.

CONCLUSIONS

Under the relatively light workload conditions of the CPT, "pure" dyslexic participants showed no behavioural signs of attentional difficulties. The attenuated, delayed and symmetrical ERPs in our dyslexic group may reflect abnormal information processing in the right parietal lobe and abnormal interhemispheric asymmetry in Dyslexia.

SIGNIFICANCE

The behavioural data suggest that abnormal attentional performance is not a "core" feature of developmental Dyslexia, and highlight the importance of distinguishing between dyslexic participants with and without ADHD symptoms. The presence of electrophysiological markers of Dyslexia in CPT revealed the atypical brain organisation that characterises "pure" Dyslexia.

Developmental dyslexia, learning and the cerebellum

Theoretical frameworks for dyslexia must explain how the well-established phonological deficits and the literacy deficits arise. Our longstanding research programme has led to a distinctive 'twin level' framework that proposes, first, that the core deficits are well described in terms of poor skill automaticity. Second, these 'cognitive level' symptoms are attributed to abnormal cerebellar function--a 'brain-level' analysis. The evidence includes data from behavioural, imaging, neuroanatomical and learning studies. The frame-work leads to an 'ontogenetic' analysis that links cerebellar deficit at birth, via problems in articulation and working memory, to the known phonological, speed and literacy difficulties. Differences in locus of cerebellar impairment, experience and/or links to other brain regions may account for subtypes of dyslexia and possibly other developmental disorders. The automaticity/ cerebellar deficit framework provides an explicit demonstration that it is possible to explain motor, speed and phonological deficits within a unified account, integrating previously opposed approaches.

Developmental dyslexia: the cerebellar deficit hypothesis

Surprisingly, the problems faced by many dyslexic children are by no means confined to reading and spelling. There appears to be a general impairment in the ability to perform skills automatically, an ability thought to be dependent upon the cerebellum. Specific behavioural and neuroimaging tests reviewed here indicate that dyslexia is indeed associated with cerebellar impairment in about 80% of cases. We propose that disorders of cerebellar development can in fact cause the impairments in reading and writing characteristic of dyslexia, a view consistent with the recently appreciated role of the cerebellum in language-related skills. This proposal has implications for early remedial treatment.

Cerebellar tests differentiate between groups of poor readers with and without IQ discrepancy

A comprehensive test battery, including phonological, speed, motor and cerebellar tasks, was administered to the entire cohort of two schools for children with learning disabilities. Testing was undertaken blind without accessing the psychometric data on the children. Children were then allocated to a discrepancy group on the basis of their IQ, with the majority (n = 29) classified as nondiscrepant (IQ < 90) and a smaller set (n = 7), with IQ of at least 90, classified as discrepant (with dyslexia). Both groups showed significant deficits relative to age-matched controls on almost all the tests. On phonological, speed, and motor tasks, the nondiscrepant group were at least as severely impaired as the discrepant group. By contrast, on the cerebellar tests of postural stability and muscle tone, the nondiscrepant group performed significantly better than the children with dyslexia and close to the level of the controls. The findings indicate that cerebellar tests may prove a valuable method of differentiating between poor readers with and without IQ discrepancy. The findings are interpreted in terms of the cerebellar deficit hypothesis for dyslexia.

Evaluating frequency proximity in stream segregation

Consecutive sounds of similar structure that are close in frequency or pitch are more likely to be perceived as part of the same sequence than those at greater frequency separations. The principle of grouping into such perceptual sequences, or auditory streams, is known as frequency proximity. However, the metric by which one frequency difference is judged to be greater or less than another in complex auditory scenes is not yet known. Two experiments explored the metric for frequency proximity. We presented repeating three-tone stimulus patterns at a rate where they are normally heard as two streams, one containing the highest tone and one containing the lowest. The middle tone joined one stream or the other depending on its frequency. Subjects reported the perceived allocation of the variable tone by responding on a 5-point scale. The frequency at which either of these two percepts was equally probable was found to be lower than a logarithmic midpoint or the midpoints on a cochlear map or the Mel scale; that is, it was unlike metrics arrived at by direct comparisons of tones. Further, the midpoint for high and low tones presented synchronously was lower than that for the same tones presented sequentially, demonstrating that in addition to a proximity factor, some additional factor or factors must operate differently when the lower and upper fixed tones are, or are not, presented simultaneously.

Slowing of reaction time in Parkinson's disease: the involvement of the frontal lobes

This study investigated the possibility that the previously mixed findings relating to cognitive deficits in Parkinson's disease might be attributable to inhomogeneity within the patients sampled, with attentional deficits occurring only for those Parkinson's patients who also have additional frontal lobe impairment. Twenty-five patients with idiopathic Parkinson's disease were classified as showing frontal dysfunction, or not, on the basis of their performance on the Wisconsin Card Sorting Test and the picture arrangement subtest of the WAIS. The two groups, and a control group of normal elderly subjects matched for age and IQ, undertook tests of visual attention designed to dissociate baseline response speed from central information processing speed. Error rates did not differ between the groups. Performance of the non-frontally impaired Parkinson's group was indistinguishable from that of the controls. By contrast, the 'frontally impaired' Parkinson's group responded significantly more slowly than the controls. Further analyses indicated that for the frontally-impaired Parkinson's group, information processing and automatic functions were unimpaired but there was a generalised slowing (as reflected by increased baseline response time) which may represent a non-specific global cognitive impairment. These findings suggest that the frontal lobes may be implicated in slowed response speed in Parkinson's disease.

Association of abnormal cerebellar activation with motor learning difficulties in dyslexic adults

BACKGROUND

In addition to their impairments in literacy-related skills, dyslexic children show characteristic difficulties in phonological skill, motor skill, and balance. There is behavioural and biochemical evidence that these difficulties may be attributable to mild cerebellar dysfunction. We wanted to find out whether there was abnormal brain activation when dyslexic adults undertook tasks known normally to involve cerebellar activation.

METHODS

Brain activation was monitored by positron emission tomography in matched groups of six dyslexic adults and six control adults as they carried out either a prelearned sequence or learned a novel sequence of finger movements.

FINDINGS

Brain activation was significantly lower (p<0.01) for the dyslexic adults than for the controls in the right cerebellar cortex and the left cingulate gyrus when executing the prelearned sequence, and in the right cerebellar cortex when learning the new sequence.

INTERPRETATION

The results provided direct evidence that, for this group of dyslexic adults, the behavioural signs of cerebellar abnormality reflect underlying abnormalities in cerebellar activation.

Impaired performance of children with dyslexia on a range of cerebellar tasks

It is now thought that the cerebellum is involved in the acquisition of "language dexterity" in addition to its established role in motor skill acquisition and execution. Mild cerebellar impairment, therefore, provides a possible explanation of a range of problems shown by children with dyslexia. The authors have established suggestive evidence in support of this hypothesis in tests of balance and of time estimation. In a further test of the hypothesis, a battery of clinical tests for cerebellar impairment, including tests of muscle tone and of coordination, was administered to matched groups of children with dyslexia and control children aged 10, 14, and 18 years (55 subjects in all). The children with dyslexia showed highly significant impairments on all the cerebellar tests, and significant impairment compared even with reading age controls on 11 of the 14 tasks. Deficits on the majority of tests were among the largest found in our research program. The findings, therefore, provide further intriguing evidence of cerebellar impairment in dyslexia. We speculate that the well-established phonological deficits in dyslexia may arise initially from inefficient articulatory control attributable to cerebellar impairment.

Impaired recognition of traffic signs in adults with dyslexia

Ten adults with dyslexia (4 women and 6 men, mean age: 26.8 years, range: 19-43 years) and 11 controls (5 women and 6 men, mean age: 20.5 years, range: 18-29 years) were tested on their ability to differentiate between real and false traffic signs. The stimuli, computer-presented color pictures, were chosen to minimize the applicability of verbal or written linguistic skills to the task. The adults with dyslexia recognized the traffic signs significantly less well than did the controls. Furthermore, whereas for the controls there was a significant correlation between traffic sign recognition and driving experience, no such correlation was found for the adults with dyslexia. The results are interpreted in terms of a deficit in implicit learning.

Time estimation deficits in developmental dyslexia: evidence of cerebellar involvement

In addition to their language-related difficulties, dyslexic children suffer problems in motor skill, balance, automatization and speeded performance. Given the recent evidence for cerebellar involvement in the acquisition of language fluency, these problems suggest cerebellar deficit. To test the hypothesis of cerebellar dysfunction in dyslexia, a time estimation task considered to be a sensitive index of cerebellar function was administered to matched groups of dyslexic and control children. The dyslexic children showed the predicted deficit on time estimation (among the most severe obtained in our research programme) but not on a control, loudness estimation, task. Cerebellar dysfunction, therefore, provides a parsimonious account of otherwise disparate data on deficits in dyslexia.

Naming speed in children with dyslexia

A series of tests of naming speed in discrete reaction time format were undertaken by seven groups of children: three groups with dyslexia with mean ages 8, 13, and 17 years; three groups of normally achieving children matched for age and IQ with the dyslexic groups; and a group of 10-year-old children with mild learning difficulties (slow learners) matched for reading age with the youngest dyslexic group. The children with dyslexia were significantly slower than even their chronological age-matched controls, and equivalent to their reading age-matched controls, on naming colors, digits, and letters, and significantly slower than even their reading age-matched controls on naming pictures of common objects. Overall, performance of the 17-year-old children with dyslexia was closest to that of the 8-year-old controls. Performance of the slow learners was equivalent to that of the youngest children with dyslexia. The results show that children with dyslexia have persistent-and unexpectedly severe-problems in naming speed for all stimuli, regardless of whether or not the stimulus requires grapheme-phoneme decoding.

Reaction times and dyslexia

Five groups of children, including two groups of dyslexics (aged 15 and 11 years), were tested on simple reaction, selective choice reaction, and lexical decision tasks. In simple reactions to a pure tone, the dyslexic children responded as quickly as their chronological age controls and significantly faster than their reading age controls. In selective choice reactions to pure tones, the dyslexic children were significantly impaired compared with their chronological age controls and no faster than their reading age controls. This speed impairment obtained even though a selective choice reaction task has only one positive response. In "by-item" analyses of lexical decisions to spoken words, the dyslexic children were significantly impaired compared even with their reading age controls. The pattern of results suggests that at least two factors contribute to slowness of dyslexic children: a general deficit reflected in slower stimulus classification speed and a linguistic deficit reflected in slower lexical access speed.

Adults with dyslexia have a deficit in voice recognition

A group of 7 dyslexic students and 8 nondyslexic students matched for age and IQ were tested on recognition of computer-presented voices and faces. Although face recognition showed a ceiling effect which prevented any solid conclusions being drawn from this task, the dyslexic group were significantly impaired on the recognition of voices.

Comparison of deficits in cognitive and motor skills among children with dyslexia

There is a growing body of evidence that children with dyslexia have problems not just in reading but in a range of skills including several unrelated to reading. In an attempt to compare the severity and incidence of deficits across these varied domains, children with dyslexia (mean ages 8, 12, and 16 years), and control groups of normally achieving children matched for IQ and for age or reading age, were tested on a range of primitive (basic) skills. The children with dyslexia performed significantly worse than the same-age controls on most tasks, and significantly worse even than the reading-age controls on phoneme segmentation, picture naming speed, word tachistoscopic word recognition, speeded bead threading and some balance tasks. The overall performance of the children with dyslexia is interpreted as showing less complete automatization than normal.

Automatisation deficits in balance for dyslexic children

Traditional theories of dyslexia have focused on components of the reading process. The Dyslexic Automatisation Deficit hypothesis takes a broader view, attributing deficits to an inability to become completely fluent in cognitive and motor skills. A series of experiments compared the balance of 15-yr.-old and 11-yr.-old groups of dyslexic children and normal children matched for age and IQ under single-task and dual-task conditions. There were no group differences in the single-task conditions. However, introduction of a concurrent secondary task led to a dissociation in that, whereas the balance of normal children was unaffected, the dyslexic children's balance was significantly impaired. It was concluded that the normal children balanced automatically whereas the dyslexic children did not. These results directly support the proposed framework.

Automaticity: a new framework for dyslexia research?

The performance of a group of 23 13-year-old dyslexic children was compared with that of same-age controls on a battery of tests of motor balance. A dual-task paradigm was used--subjects performed each test twice, once as a single task, and once as a dual task concurrently with a secondary task. Two alternative secondary tasks were used, the classic counting-backwards task and an auditory choice reaction task. Both secondary tasks were calibrated for each subject to ensure that their performance on the secondary task alone fell between pre-specified performance criteria. In all single-task conditions there was no difference between the performance of the two groups. By contrast, in 19 out of the 20 tests performed under dual-task conditions, the dyslexic group were significantly impaired, whereas the controls showed no impairment, thus resulting in significantly better performance by the control group than the dyslexic group. The sole exception was that the dyslexic children were not impaired on the easiest balance condition with the choice reaction task. Under the dual-task conditions the dyslexic children also performed worse than the controls on the secondary task. It is very hard to accommodate the findings within the traditional framework of a deficit specific to lexical skills. One plausible explanation of the results is that, unlike the controls, the dyslexic children need to invest significant conscious resources for monitoring balance, and thus their performance is adversely affected by any secondary task which serves to distract attention from the primary task. This need for "conscious compensation" suggests that for dyslexic children the skill of motor balance is poorly automatized. It is possible, therefore, that many of the reading deficits of dyslexic children are merely symptoms of a more general learning deficit--the failure to fully automatize skills.

Cognitive Factors in Simple Reactions

In an exploration of factors underlying the developmental increase in the speed of simple reaction to an auditory stimulus, two experiments were undertaken, with subjects aged 4, 10, and 20. The first experiment demonstrated that provision of visual feedback caused improvement for younger subjects but not for adults, whereas neither practice nor variable feedback caused any differential change. The second experiment was a simulated game where visual feedback was contingent on a "hit" on the previous trial. Following a hit the target moved faster, following a miss, slower. Practice caused a considerable improvement for the 4-year old subjects, but not for the older subjects. The nature it or miss feedback on the previous trial had powerful effects on the simple reaction times. For all subjects, a miss resulted in a subsequent substantial increase in speed on the next trial. Following a hit, the adults were unaffected, but the youngest subjects were substantially slower. The results are interpreted in terms of inappropriate relaxation following a hit for the 4-year old subjects, with active strategic behavior by the adults, following a miss. It is concluded that one variable (incentive provided by a miss) affects simple reaction for all ages, that two (disincentive provided by a hit, and visual feedback) differentially affect subjects of different ages, and that a fourth (use of a "trade-off" strategy) is not available to the youngest subjects.