This study was an fMRI pilot study conducted in 4-6th grade elementary school children using an event-related design. The authors believe this study to be the first fMRI study conducted in Korean children with ADHD using such a design. From the study results, the suppression of the interfering response was associated with an increased activation of the right frontal lobe in medicated ADHD children. This finding suggested that MPH affected the activation of the right frontal cortex, which correlates with improvement of IS during cognitive challenge.
Vaidya et al.23
reported that ADHD patients lacked right frontal lobe activation during RI and exhibited decreased activity of the left inferior frontal lobe during IS. Further, Bush et al.24
reported decreased cingulate cortex activity and atypical fronto-striatal activity during IS in adults with ADHD. Several studies25,26
have also reported that ADHD children exhibit relatively decreased fronto-striatal activity compared to other normal children. However, some studies have also reported increased activity or no difference.4
These controversial findings may stem from differences in the event-related paradigm. In a recent study, Bush et al.27
reported increases in the activity of the dorsal anterior midcingulate cortex, right dorsolateral prefrontal cortex, and both parietal lobes in adult ADHD patients on OROS MPH for 6 weeks. Also, during IS, ADHD patients are known to show atypical activation of the frontal lobe and caudate nucleus when compared to normal control group.23
In this study, altered activation of the caudate nucleus was not found; however, decreased activity of the frontal lobe was observed when MPH was discontinued as compared to when patients were on MPH. In particular, when subjects were on MPH, greater activation of the frontal lobe was observed during IS than when neutral stimulus was given (). The authors could conclude that MPH has an effect on the right frontal lobe during IS test which corresponds with the results of Bush et al.27
; however, changes in the right frontal lobe activity during an RI task with MPH administration were not seen in this study.
Scheres et al.5
reported that MPH had an affect on response time of RI rather than on IS and that this affect was unrelated to dose. On the other hand, in this study, only response time significantly different between on and off MPH during IS, a finding which is in contrast to previous studies. Further, although medication was discontinued for a week, considering inconsistent cognitive data with previous studies, the residual effect of long term medication treatment cannot be excluded. Finally, a dose-response relationship was not verified in this study.
In the current study, subjects exhibited symptomatic improvements with MPH; however, improved executive function of the frontal lobe was not observed except for the response time of IS. Further, MPH-induced increases in frontal lobe activity were identical to those in previous studies.23,28
have reported that the lateral frontal lobe may be the center for interference control in adults. This study replicated previous study results and suggested the possibility of concluding that MPH can normalize ADHD's cognitive control center. But unlike Bush et al.'s results,24,27
decreased activity in the anterior cingulate in adult with ADHD during IS was not observed.
The dopamine system affect on the fronto-sriatal network,12
as related to the RI model, can be activated by MPH treatment; however, this activated fronto-striatal network alone cannot fully explain improved IS. The role of the parieto-temporal network in IS improvement is also important.30,31
Unlike previous studies,5,32
the effect of MPH related to IS as opposed to RI was observed in this study.
Comprehensive results from the previous fMRI studies using event-related designs on cognitive controls25,30,33
have revealed the activation of widespread regions of the frontal cortex during RI. Particularly striking is the predominance of right-hemisphere activation, especially within the inferior frontal region. Recently, Bush et al.27
focused on changes in the right frontal lobe activity during IS in adults with ADHD on medication. This study also revealed increased right frontal lobe activity similar to that of healthy children during incongruent stimulus while being treated with MPH.
This study was limited in that it utilized a sample size that was too small to generalize the relationship between MPH and increased activity of the right frontal lobe. Further studies utilizing a larger sample size are needed. Moreover, there was no normal control group for head-to-head comparisons. A second limitation was that subjects were recruited from a population that had been undergoing treatment in clinical practice. That was why the second MRI scan was done one week after discontinuation of medication. However, it was impossible to completely exclude the possibility of a residual effect of longterm medication. Thus in further studies, drug-naïve patients should be selected for such studies and the first MRI should be performed prior to medication initiation and a subsequent MRI can proceed after the initiation of medication.