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Contextual abnormalities of saccadic inhibition in children with attention deficit hyperactivity disorder
1The Neurophysiology nd Neurovisual Research Unit, Mental Health Research Institute of Victoria, Parkville, Victoria, Australia 3052. sheree@neuro.mhri.edu.au
Insights
Children with attention-deficit hyperactivity disorder (ADHD) show deficits in both executive function and inhibitory control. These two functions appear to be independent in ADHD, with medication improving executive function but not inhibitory control.
Area of Science:
- Neuroscience
- Developmental Psychology
- Child Psychiatry
Background:
- Executive function deficits are common in children with attention-deficit hyperactivity disorder (ADHD).
- These deficits are hypothesized to stem from disruptions in behavioral inhibition systems.
- Understanding the interplay between executive and inhibitory functions in ADHD is crucial.
Purpose of the Study:
- To compare executive and inhibitory functions in unmedicated and medicated children with ADHD, healthy children, and healthy adults.
- To investigate the relationship between executive function, behavioral inhibition, and ADHD.
- To determine if medication impacts executive and inhibitory functions differently in ADHD.
Main Methods:
- Executive functions assessed using a spatial working memory task.
- Inhibitory functions measured via an ocular motor paradigm assessing fixation control.
- Fixation offset effect (FOE) used to infer inhibitory control, modulated by target probability (80% vs. 20%).
Main Results:
- Healthy children performed worse on spatial working memory than adults but showed similar fixation control.
- Unmedicated ADHD group had more spatial working memory errors; medicated group showed normal performance.
- Both unmedicated and medicated ADHD groups failed to modulate FOE based on context, indicating impaired inhibitory control.
Conclusions:
- Deficits in executive function and inhibitory control appear independent in children with ADHD.
- Context-based modulation of fixation release is not governed by the same systems as executive function.
- Stimulant medication improved executive function in ADHD but did not restore inhibitory control.
Abstract:
Abnormalities of executive function are observed consistently in children with attention-deficit hyperactivity disorder (ADHD), and it is hypothesised that these arise because of disruption to a behavioural inhibition system. Executive and inhibitory functions were compared between unmedicated and medicated children with ADHD (combined type), age-matched healthy children and healthy adults. Executive functions were measured using a test of spatial working memory shown previously to be sensitive to ADHD and to stimulant medication. Inhibitory functions were measured using an ocular motor paradigm that required individuals to use task context to control the release of fixation. Context was set according to the probability that a target would appear at either of the two locations. In one block, targets appeared on 80% of trials. In the other block, targets appeared on 20% of trials. The ability to control the release of fixation was inferred from the fixation offset effect (FOE), or the difference in saccade latency when the current fixation is offset 200 ms prior to the onset of the saccade target (gap condition), compared with when there is no offset (overlap condition). Although the healthy children made more errors on the spatial working memory task than the healthy adults, there was no difference between the two groups in their ability to control fixation using context. Both showed a larger FOE when target probability was low. As expected, the unmedicated ADHD group made more errors on the spatial working memory test than the healthy children, although spatial working memory performance was normal in the medicated ADHD group. However, both the unmedicated and medicated ADHD groups were unable to modulate the FOE according to context, and this was due to their inability to voluntarily inhibit saccades when there was a low target probability. These data suggest that the context-based modulation of fixation release is not controlled by the same systems that control executive function. Furthermore, deficits in executive function and inhibitory control appear independent in children with ADHD.