BACKGROUND
Previous studies have identified significant variability in attention-deficit / hyperactivity disorder (ADHD) prevalence estimates worldwide, largely explained by methodological procedures. However, increasing rates of ADHD diagnosis and treatment throughout the past few decades have fuelled concerns about whether the true prevalence of the disorder has increased over time.


METHODS
We updated the two most comprehensive systematic reviews on ADHD prevalence available in the literature. Meta-regression analyses were conducted to test the effect of year of study in the context of both methodological variables that determined variability in ADHD prevalence (diagnostic criteria, impairment criterion and source of information), and the geographical location of studies.


RESULTS
We identified 154 original studies and included 135 in the multivariate analysis. Methodological procedures investigated were significantly associated with heterogeneity of studies. Geographical location and year of study were not associated with variability in ADHD prevalence estimates.


CONCLUSIONS
Confirming previous findings, variability in ADHD prevalence estimates is mostly explained by methodological characteristics of the studies. In the past three decades, there has been no evidence to suggest an increase in the number of children in the community who meet criteria for ADHD when standardized diagnostic procedures are followed.

ADHD Prevalence Estimates Across Three Decades: An Updated Systematic Review and Meta-Regression Analysis

The authors conceptualize these memory systems from both behavioural and neurobiological perspectives, guided by three related principles: * First, that our understanding of a wide range of memory phenomena can be advanced by breaking down memory into multiple forms with different operating characteristics. * Second, that different forms of memory representation are supported by distinct brain pathways with circuitry and neural coding properties. * Third, that the contributions of different brain systems can be compared and contrasted by distinguishing between dedicated (or specific) and elaborate (or general) memory systems. A primary goal of this work is to relate the neurobiological properties of dedicated and elaborate systems to their neuropsychological counterparts, and in so doing, account for the phenomenology of memory, from conditioning to conscious recollection.

From Conditioning to Conscious Recollection Memory Systems of the Brain. Oxford Psychology Series, Volume 35.

Most research on the neurobiology of language focuses on language alone. However, new biological functions commonly make use of previously existing mechanisms. Thus, language likely relies on previously existing neurobiological substrates, whether or not these have become further specialized for language. The declarative/procedural model posits that language learning, storage, and use depend heavily on declarative and procedural memory. After all, most, if not all, of language must be learned, and these are the two most important learning and memory systems in the brain. Crucially, both systems are well-studied at many levels in humans and animals, leading to numerous independent predictions about language. Here, I first present background on the neurobiology of the two systems, then lay out ensuing predictions for language, and finally examine evidence testing these predictions. The evidence suggests that language indeed depends on declarative and procedural memory, moreover in interesting and informative ways that advance our understanding of the neurobiology of language.

The Declarative/Procedural Model: A Neurobiological Model of Language Learning, Knowledge, and Use

Neurobiological Basis of Language Learning Difficulties

This study tested the procedural deficit hypothesis of specific language impairment (SLI) by comparing children’s performance in two motor procedural learning tasks and an implicit verbal sequence learning task. Participants were 7- to 11-year-old children with SLI (n = 48), typically developing age-matched children (n = 20) and younger typically developing children matched for receptive grammar (n = 28). In a serial reaction time task, the children with SLI performed at the same level as the grammar-matched children, but poorer than age-matched controls in learning motor sequences. When tested with a motor procedural learning task that did not involve learning sequential relationships between discrete elements (i.e. pursuit rotor), the children with SLI performed comparably with age-matched children and better than younger grammar-matched controls. In addition, poor implicit learning of word sequences in a verbal memory task (the Hebb effect) was found in the children with SLI. Together, these findings suggest that SLI might be characterized by deficits in learning sequence-specific information, rather than generally weak procedural learning.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/desc.12125

Antoine Tremblay

Papers

Grammar predicts procedural learning and consolidation deficits in children with Specific Language Impairment

Procedural learning in Tourette syndrome, ADHD, and comorbid Tourette-ADHD: Evidence from a probabilistic sequence learning task