Implicit learning deficit in children with developmental dyslexia

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Abstract

Several neuropsychological deficits have been reported as characteristic of the cognitive profile of dyslexic children. Phonological and visual processing are often impaired as well as auditory processing, attention and information processing speed. We investigated whether implicit learning, is impaired in dyslexic children and adolescents.

Tests of implicit and declarative learning were administered to 18 clinically defined dyslexics and 18 similar age controls.

Dyslexics showed a reduced learning rate in the implicit but not in the declarative task, suggesting a specific deficit of implicit learning.

Although alternative hypothesis cannot be ruled out, considering that implicit learning is a cognitive function primarily processed by the cerebellum and that recent neurological and physiological data suggest a cerebellar dysfunction in dyslexia, the present results suggest an impairment of cerebellar system in reading disabilities.

Introduction

Developmental dyslexia is commonly defined as a reading disability occurring in children with normal intelligence, no sensory or neurological impairment and conventional instruction and socio-economic opportunity (DSM IV). Dyslexia is the most common of the intellectual disabilities; its prevalence varies from 3 to 8% across countries and languages. Converging behavioural evidence suggests that a core problem in dyslexia is a deficit in phonological processing and, specifically, in phonological awareness, the ability to identify and manipulate the sound structure of words [1], [7], [30], [28]. Many studies have also shown other cognitive abnormalities in individuals with developmental dyslexia. Namely, visual processing linked to the transient or magnocellular visual subsystem is often impaired in these subjects [3], [15] as well as auditory processing [31], attention [5] and information processing speed [19].

Despite increasing evidence supporting a neurobiological basis for dyslexia, its underlying causes are still unknown. A number of brain imaging studies have attempted to locate brain regions with abnormal activation in dyslexics. Functional brain imaging research revealed that cortical regions involved in phonological processing are critically affected. In particular, dyslexic readers only partially activate posterior regions (Wernicke’s area, the angular gyrus and striate cortex) and over-activate the anterior region (inferior frontal gyrus). This picture has been interpreted as reflecting a functional disruption or a “disconnection” in the phonological system (for a review see [3]). Although the involvement of brain regions that process phonological abilities is universally accepted, other cortical areas also seem to be affected. In fact, functional magnetic resonance imaging studies devoted to exploring visual processing abilities in subjects with dyslexia revealed little activation in portions of the magnocellular visual system in response to the perception of subtle changes in motion [4]. Finally, recent observations made with magnetic resonance spectroscopy [20] and positron emission tomography [18] reported low cerebellar activation in dyslexic adults. This suggests that the cerebellum may be a key structure affecting developmental dyslexia, albeit this is still matter of debate [11], [17], [38].

Traditionally, the cerebellum has been associated with motor control and co-ordination, but recent studies have extended its contribution to cognitive functions, such as language, abstract reasoning, emotions and the ability to process logical sequences [12], [27]. Moreover, Molinari et al. [16] demonstrated a specific impairment in the procedural learning of adults with cerebellar lesions, suggesting a cerebellar role in detecting and recognising event sequences and in acquiring and automatising new cognitive procedures. It is interesting to note that precisely these capabilities appear to play a decisive role in reading which requires the acquisition and automation of several competencies, such as elementary articulatory and auditory skills, processing of eye movements and letter recognition.

On the basis of these findings, it seemed worthwhile to investigate implicit and explicit learning abilities in a selected group of individuals with developmental dyslexia and to compare them with those of normally reading children. Two different experiments were carried out. In the first one, we employed a serial-reaction time task used in previous studies on children with learning disabilities [34], [35]. This test is retained to analyse prevalently implicit learning, even if some explicit competence cannot be a priori excluded. In the second experiment, the effect of declarative knowledge of the sequence on the performance of the same serial-reaction time task was analysed. Even if inescapably encompasses some knowledge implicitly learned, this second paradigm emphasises the explicit competences. If the dyslexics had difficulty only in implicit learning, then their performances should have been different from those of the controls in the implicit (experiment 1) but not in the declarative (experiment 2) condition.

Section snippets

Participants

The study included 18 children and adolescents (6 female, 12 male) with developmental dyslexia. All the subjects came from the Children Hospital Bambino Gesù of Santa Marinella, Rome, where they received the clinic diagnosis of dyslexia. The diagnosis was based on the standard exclusionary criterion of children with normal or above normal intelligence (IQ of 90 or more), without neurobehavioral, sensorial or socio-economic problems, whose reading abilities were at least two standard deviations

Discussion

The main result of the present study indicates the following: individuals with developmental dyslexia are impaired in the detection of a sequence and in the acquisition of implicit knowledge about it; that is, implicit learning of a visuo-motor task is impaired in dyslexia. We emphasise that differences between performance profiles of dyslexics and controls were observed only in the first experiment, that is, in the prevalently implicit condition, but not in the second experiment, when the

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