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Keywords:

  • ADD/ADHD;
  • fMRI;
  • expectancy;
  • fronto-cerebellar

Background:  Attention deficit hyperactivity disorder (ADHD) is a prevalent neuropsychiatric disorder in childhood with established problems in cognitive control and associated fronto-striatal circuitry. More recently, fronto-cerebellar circuits have been implicated in this disorder. Both of these circuits are important in predicting the occurrence and timing of behaviorally relevant events and in detecting violations of these predictions. Therefore, we hypothesized that the ability to predict the occurrence of frequent events would be compromised in ADHD, as well as the ability to adapt behavior when expectancy was violated.

Methods:  We used rapid, mixed-trial, event-related functional magnetic resonance imaging (fMRI) to examine cognitive and neural processes in two independent samples of children and adolescents with ADHD and matched controls. Subjects performed a variation of a go-no/go task where the predictability of stimulus identity (what) and timing (when) was manipulated.

Results:  Behaviorally, children and adolescents with ADHD had increased variability in reaction times, and decreased benefit in reaction time when events were predictable. Differences in accuracy between groups were most reliable for temporally unpredictable trials. Functional imaging results from both samples showed that relative to the control children and adolescents, individuals with ADHD had diminished cerebellar activity to violations of stimulus timing and diminished ventral prefrontal and anterior cingulate activity to violations in stimulus timing and identity.

Conclusions:  These findings are consistent with the view that disruptive behaviors in inappropriate contexts, a major criterion in diagnosing ADHD, may be related to an impaired ability to predict temporal and contextual cues in the environment, thus hindering the ability to alter behavior when they change. This ability requires intact fronto-cerebellar, as well as fronto-striatal circuitry.