Implicit learning deficits in dyslexic adults: An fMRI study

Intact first- and second-order implicit sequence learning in secondary-school-aged children with developmental dyslexia

We examined the influence of task complexity on implicit sequence learning in secondary-school-aged children with developmental dyslexia (DD). This was done to determine whether automatization problems in reading extend to the automatization of all skill and depend on the complexity of the to-be-learned skill. A total of 28 dyslexic children between 12 and 15 years and 28 matched control children carried out two serial reaction time tasks using a first-order conditional (FOC) and second-order conditional (SOC) sequence. In both tasks, children incidentally learned a sequence of hidden target positions, but whereas FOC sequence learning could be based on knowledge about the immediate preceding position, SOC sequence learning required more complex knowledge about the previous two positions. The results demonstrated that sequence learning was highly comparable in dyslexic and control children, regardless of the sequence complexity. This shows that implicit sequence learning, as manifested in the present study, is maintained in DD and is unrelated to task complexity. We suggest that previous reports of sequence-learning deficits in DD can be accounted for by attenuated explicit sequence learning, possibly related to malfunctions in prefrontal processing. The present findings indicate that deficits in skill learning and automatization in DD are not general in nature, but task dependent.

Implicit learning and dyslexia

Several studies have reported an association between dyslexia and implicit learning deficits. It has been suggested that the weakness in implicit learning observed in dyslexic individuals may be related to sequential processing and implicit sequence learning. In the present article, we review the current literature on implicit learning and dyslexia. We describe a novel, forced-choice structural "mere exposure" artificial grammar learning paradigm and characterize this paradigm in normal readers in relation to the standard grammaticality classification paradigm. We argue that preference classification is a more optimal measure of the outcome of implicit acquisition since in the preference version participants are kept completely unaware of the underlying generative mechanism, while in the grammaticality version, the subjects have, at least in principle, been informed about the existence of an underlying complex set of rules at the point of classification (but not during acquisition). On the basis of the "mere exposure effect," we tested the prediction that the development of preference will correlate with the grammaticality status of the classification items. In addition, we examined the effects of grammaticality (grammatical/nongrammatical) and associative chunk strength (ACS; high/low) on the classification tasks (preference/grammaticality). Using a balanced ACS design in which the factors of grammaticality (grammatical/nongrammatical) and ACS (high/low) were independently controlled in a 2 x 2 factorial design, we confirmed our predictions. We discuss the suitability of this task for further investigation of the implicit learning characteristics in dyslexia.

Motor sequence learning and developmental dyslexia

Beyond the reading-related deficits typical of developmental dyslexia (DD), recent evidence suggests that individuals afflicted with this condition also show difficulties in motor sequence learning. To date, however, little is known with respect to the characteristics of the learning impairments, nor to the neural correlates associated with this type of procedural deficit in DD patients. Here, we first summarize the results of the few behavioral and brain imaging studies that have investigated the effects of DD on motor sequence learning. To help guide research in this field, we then discuss relevant psychophysical and neuroimaging work conducted in healthy volunteers in relation to three different conceptual perspectives: when, how, and what. More specifically, we examine the cognitive boundaries that affect performance across the different stages of learning (i.e., "when"), the different cognitive processes (i.e., "how") under which learning occurs, and the mental representations (i.e., "what") that are elicited when acquiring this type of skilled behavior. It is hoped that this conceptual framework will be useful to researchers interested in further studying the nature of the motor learning impairment reported in DD.

Two forms of implicit learning in young adults with dyslexia

Implicit learning is thought to underlie the acquisition of many skills including reading. Previous research has shown that some forms of implicit learning are reduced in individuals with dyslexia (e.g., sequence learning), whereas other forms are spared (e.g., spatial context learning). However, it has been proposed that dyslexia-related motor dysfunction may have contributed to the implicit sequence learning deficits reported earlier. To assess implicit sequence learning in the absence of a motor sequence, 16 young adults diagnosed with dyslexia (20.6 +/- 1.5 years) and 18 healthy controls (20.8 +/- 2.0 years) completed a triplet frequency learning task (TRIP) that involved learning a sequential regularity in which the location of certain events followed a repeating pattern, but motor responses did not. Participants also completed the spatial contextual cueing task (SCCT), which involved learning a spatial regularity in which the location of distractors in some visual arrays predicted the target location. In addition, neuropsychological tests of real-word and pseudo-word reading were administered. TRIP task analyses revealed no between-group differences in pattern learning, but a positive correlation between individual learning scores and reading ability indicated that poor readers learned less well than did good readers. Thus, earlier reports of reduced implicit sequence learning in dyslexics cannot be entirely accounted for by motor sequencing deficits. No significant correlations or group differences in learning were found for SCCT. These findings offer additional evidence for a link between poor reading and impaired implicit sequence learning.

Neurocognitive basis of implicit learning of sequential structure and its relation to language processing

The ability to learn and exploit environmental regularities is important for many aspects of skill learning, of which language may be a prime example. Much of such learning proceeds in an implicit fashion, that is, it occurs unintentionally and automatically and results in knowledge that is difficult to verbalize explicitly. An important research goal is to ascertain the underlying neurocognitive mechanisms of implicit learning abilities and understand its contribution to perception, language, and cognition more generally. In this article, we review recent work that investigates the extent to which implicit learning of sequential structure is mediated by stimulus-specific versus domain-general learning mechanisms. Although much of previous implicit learning research has emphasized its domain-general aspect, here we highlight behavioral work suggesting a modality-specific locus. Even so, our data also reveal that individual variability in implicit sequence learning skill correlates with performance on a task requiring sensitivity to the sequential context of spoken language, suggesting that implicit sequence learning to some extent is domain-general. Taking into consideration this behavioral work, in conjunction with recent imaging studies, we argue that implicit sequence learning and language processing are both complex, dynamic processes that partially share the same underlying neurocognitive mechanisms, specifically those that rely on the encoding and representation of phonological sequences.

"Do what I do" and "do how I do": different components of imitative learning are mediated by different neural structures

The involvement of different neural structures in imitative learning was studied in two paradigms. In an experimental paradigm, rats observed actor rats learning spatial procedures in a water maze. After the observational training, the observers underwent a cerebellar lesion, preventing further procedural acquisitions, and then were tested in the water maze previously observed. The cerebellar networks appear to be indispensable for acquiring by imitation the spatial procedures. The procedural sequence was then dissected into the single behavioral units, demonstrating that such units do exist and can be independently acquired. By using repetitive transcranial magnetic stimulation (rTMS), the role of the cerebellum and prefrontal cortex in imitative learning was investigated in humans. Subjects observed an actor detecting a hidden sequence in a matrix and then performed the task detecting either the previously observed sequence or a new one. Cerebellar rTMS applied before the observational training interfered with performance of the new sequence, whereas prefrontal rTMS interfered also with performance of the previously observed one. rTMS delivered after the observational training did not influence task execution. These findings indicate that the cerebellum and prefrontal cortex interact in planning actions, the former by permitting the acquisition by imitation of procedural competencies and the latter by providing flexibility among already acquired solutions.

Procedural learning difficulties: reuniting the developmental disorders?

During the past 30 years, research into developmental disorders has fragmented, emphasizing differences rather than commonalities. We propose that reunification might be achieved by using a "neural-systems" approach. Deficits in dyslexia are attributed to an intact declarative learning system combined with an impaired procedural learning system--a network that includes prefrontal language systems and basal ganglia, parietal and cerebellar structures. A typology is provided for other prevalent learning disabilities; this framework focuses on different learning skills in the understanding of learning disabilities and emphasizes the diagnostic significance of "secondary" symptoms. This approach highlights the need for development of "neurocognitive" tests to probe the function of components of each neural system and improve strategies for explanation, diagnosis and support of developmental disorders.

Contribution of implicit sequence learning to spoken language processing: some preliminary findings with hearing adults

Spoken language consists of a complex, sequentially arrayed signal that contains patterns that can be described in terms of statistical relations among language units. Previous research has suggested that a domain-general ability to learn structured sequential patterns may underlie language acquisition. To test this prediction, we examined the extent to which implicit sequence learning of probabilistically structured patterns in hearing adults is correlated with a spoken sentence perception task under degraded listening conditions. Performance on the sentence perception task was found to be correlated with implicit sequence learning, but only when the sequences were composed of stimuli that were easy to encode verbally. Implicit learning of phonological sequences thus appears to underlie spoken language processing and may indicate a hitherto unexplored cognitive factor that may account for the enormous variability in language outcomes in deaf children with cochlear implants. The present findings highlight the importance of investigating individual differences in specific cognitive abilities as a way to understand and explain language in deaf learners and, in particular, variability in language outcomes following cochlear implantation.