The underlying neural pathways leading to symptoms of Attention-Deicit/Hyperactivity Disorder (ADHD) remain largely unclear. In order to develop optimal treatments for ADHD, it is crucial to understand the neural mechanisms associated with this disorder. A neural mechanism that may be implicated in ADHD is the """"""""Reward System"""""""", a circuitry that originates in the ventral tegmental area and projects to brain structures such as the striatum, nucleus accumbens, limbic areas, and frontal cortex. This mesocorticolimbic dopamine system has been implicated in the processing of reward information and motivation, and has been hypothesized to play a crucial role in ADHD (in particular the striatum). However, this hypothesis has attracted almost no empirical study. The goal of this proposed study is to test this hypothesis by experimentally investigating whether the Brain Reward System is altered in ADHD, and whether such an alteration is related to impulsivity, one of the main symptoms of ADHD. Impulsivity has been operationalized as the preference for small rewards that is immediately available over larger rewards that will be delivered after a period of delay. By examining activation in limbic areas including ventral striatum and frontal areas during temporal discounting of rewards, the proposal for this pilot fMRI study seeks to elucidate a possible neural mechanism underlying symptoms of impulsivity. A secondary goal of this study is to examine alterations in the Brain Reward System as a function of age, and as a function of age by diagnostic group. A temporal discounting task will be used during which participants will be presented with a series of choices between a smaller sooner reward and a larger later reward. We hypothesize that striatal and frontal activation during temporal discounting is altered in ADHD. Furthermore, we hypothesize that there is a correlation between striatal/frontal activation and preference for the immediate reward. In terms of age, we predict that frontal activation during temporal reward discounting will increase with age, and that this effect may differ across diagnostic groups. For this study, we plan to include 24 medication-na?ve adolescents with ADHD-combined type, 24 adolescents with ADHD-combined type with a medication history, and 24 healthy controls. Participants will be in the age range of 12 - 17, free of learning disabilities, group-matched for sex and age, and will be required to have an IQ above 70. Addressing the pathophysiology of ADHD in youth is necessary in order to develop improved diagnostic criteria and therapeutic strategies for conditions that have major public health implications. Briefly, ADHD is a very common disorder that incurs great cost to the individual (i.e., poor school achievement, poor peer relationships, increased risk of involvement with criminal justice system, poor occupational attainment, as well as increased risk for substance abuse), the family (poor family relationships, financial and emotional costs of judicial involvement), and society (huge costs of involvement in legal system). ? ? ?
| Plichta, Michael M; Scheres, Anouk (2014) Ventral-striatal responsiveness during reward anticipation in ADHD and its relation to trait impulsivity in the healthy population: a meta-analytic review of the fMRI literature. Neurosci Biobehav Rev 38:125-34 |
