A novel approach to evaluating the neural processes of children with attention-deficit/hyperactivity disorder (ADHD) reveals that, contrary to convention, the condition does not necessarily represent a learning impairment as much as it does a decision-making impairment.
"Individuals with ADHD cannot be characterized by an impaired learning rate per se, in contrast to what has been suggested by theoretical models," the authors, led by Tobias U. Hauser, PhD, University College London's Wellcome Trust Centre for Neuroimaging, write.
"Rather, they show a less fine-grained decision process and explore more frequently."
The study was published online August 20 in JAMA Psychiatry.
Novel Imaging Approach
ADHD has been linked to poor decision-making and impaired learning, the authors note. Models of ADHD have suggested that these deficits "may be caused by impaired reward prediction errors (RPEs) [which] are signals that indicate violations of expectations and are known to be encoded by the dopaminergic system."
However, the investigators note that the "precise learning and decision-making deficits and their neurobiological correlates in ADHD are not well known."
For the study, 20 adolescents, aged 12 to 16 years, with ADHD and 20 healthy control individuals underwent psychiatric tests while being screened with functional magnetic resonance imaging (fMRI) and electroencephalography (EEG) simultaneously.
The combination of the 2 modalities allows for an evaluation that overcomes the weaknesses of either method, Dr. Hauser told Medscape Medical News.
"fMRI, for instance, has a very poor temporal resolution, whereas EEG lacks in spatial resolution," he said. "With the combination of both, we were able to not only localize the impairments but also tell at what time this impairment occurs," he said.
"This also gives us more knowledge of whether this is an early or late part of a 'thought process.' "
In the testing, the investigators used the novel comparison of 2 computational psychiatric models ― an advanced Rescorla-Wagner model, shown to have success in demonstrating a participant's learning in probabilistic reversal learning tasks, and a more flexible Bayesian learning model, which assumes a more precise, fine-grained learning process.
"To our knowledge, our study is the first which uses these computational learning models to understand the decision-making impairments in combination with multimodal imaging," Dr. Hauser said.
"By using such an approach, we were able to better understand the mechanisms behind the decision-making difficulties and their neural correlates in ADHD."
The challenges, performed by participants while undergoing the simultaneous neuroimaging techniques, generally involved learning through trial and error which of 2 images resulted in a better reward of money, with the goal of gaining as much money as possible. The reward probabilities were occasionally changed, requiring participants to adjust accordingly.
The results showed no significant differences between the groups in mean reaction times, reaction time variability, and number of misses. However, the ADHD participants earned marginally less money (P = .08).
Adolescents with ADHD showed more simplistic learning, and they also had increased exploratory behavior compared with the healthy participants (P = .02).
The fMRI meanwhile showed impairments in RPE processing, or signals that suggest violations of expectations, which have been linked to ADHD in previous models of the condition.
RPE impairments were observed in the medial prefrontal cortex ― an area largely associated with decision-making ― during the cue as well as outcome presentation.
Although previous research has also implicated the prefrontal cortex in ADHD, the imaging showed the precise moment of the impairment, which occurred at an early stage, approximately half a second after the feedback.
Not only is deficient RPE medial prefrontal cortex a possible cause of suboptimal choice selections, reflected in more exploratory behavior, but the imaging also helped pinpoint which areas of the medial prefrontal cortex are affected.
"The regions in the medial prefrontal cortex that we found to be impaired in ADHD are adjacent to the core regions known to process RPEs," the authors write.
"This suggests that individuals with ADHD may not process RPEs differently in the RPE core regions. Rather, it seems as if RPEs are processed in a less-extended area."
On a behavioral level, the findings offer a possible explanation for decision-making challenges and potential strategies for overcoming them, Dr. Hauser said.
"Adolescents with ADHD possibly make poorer and more impulsive decisions when they have only limited time to decide or if they are under other kinds of pressure," he explained.
"These weaknesses could be overcome if people with ADHD could learn to thoughtfully reflect potential gains and costs of their decisions."
The study is important in adding to the understanding of the underlying mechanisms of ADHD, which is so essential in developing interventions to help with coping and compensation of deficits, said Cathryn A. Galanter, MD, director of the child and adolescent psychiatry training program at SUNY Downstate/Kings County Hospital Center, in Brooklyn, New York.
"These findings are intriguing and need replication," she told Medscape Medical News.
"While it is too early to draw clinical implications from these results, they do further contribute to neuroscience evidence of what we see clinically, that children with ADHD may have different learning styles and thus may need benefit to different approaches to help them learn."
"Further studies are needed to help us understand the underlying processes of learning and decision-making of children with ADHD," said Dr. Galanter.
The study received support from the Swiss National Science Foundation (grant 320030_130237) and the Hartmann Muller Foundation (grant 1460). Dr Hauser is supported by the Swiss National Science Foundation, and coauthor Suzanne Walitza has received speakers' honoraria from Eli Lilly, Janssen-Cilag, and AstraZeneca in the past 5 years. Dr. Galanter reports no relevant financial relationships.
JAMA Psychiatry. Published online August 20, 2014. Abstract
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Cite this: ADHD Not a Learning Disability? - Medscape - Aug 27, 2014.