A lack of default network suppression is linked to increased distractibility in ADHD - PubMed (original) (raw)
A lack of default network suppression is linked to increased distractibility in ADHD
Catherine Fassbender et al. Brain Res. 2009.
Abstract
Heightened distractibility in participants with ADHD as indexed by increased reaction time (RT) variability has been hypothesized to be due to a failure to sufficiently suppress activation in the default attention network during cognitively demanding situations. The present study utilized fMRI to examine the relationship between intra-individual variability (IIV) in task RT and suppression of BOLD response in regions of the default network, using a working memory paradigm and two levels of control tasks. IIV was calculated separately for thirteen healthy control and twelve children with ADHD, Combined Type. Children with ADHD displayed significantly more RT variability than controls. Neural measures showed that although both groups displayed a pattern of increasing deactivation of the medial prefrontal cortex (PFC) with increasing task difficulty, the ADHD group was significantly less deactive than controls. Correlations between IIV and brain activation suggested that greater variability was associated with a failure to deactivate ventromedial PFC with increasing task difficulty. T-tests on brain activation between participants with ADHD with low versus high IIV implicated a similar region so that high variability was associated with greater activity in this region. These data provide support for the theory that increased distractibility in at least some participants with ADHD may be due to an inability to sufficiently suppress activity in the default attention network in response to increasing task difficulty.
Figures
Figure 1
Graphs of IIV (represented by sigma in panel A, tau in panel B, and the sum of sigma and tau in panel C) for the ADHD and HC groups across MST, AT and VSAT tasks. Individual results for RT variability are displayed by the dot plot and mean and standard deviation are represented by the vertical line.
Figure 2
Regions in default mode attention network resulting from the HC and ADHD “OR” within-group activation map. Bar plots illustrate the signal change associated with each group (HC and ADHD) during AT and VSAT tasks in each functionally defined region from the OR within-group contrast map. Medial PFC [bar plots: 1) superior medial PFC, 2) ACC and 3) ventromedial PFC]. HC children displayed significant deactivation in medial PFC during AT and VSAT tasks, with a tendency to increase deactivation for the VSAT working memory task (1, 2 and 3). Activation in medial PFC in ADHD children did not significantly differ from MST baseline in any region except in a superior region of medial PFC (1), which they significantly deactivated during the VSAT, although not to HC levels. In fact, examination of the bar plot reveals that children with ADHD deactivated the ACC during the VSAT to a similar degree as HC children during the simpler AT condition. Repeated measures ANOVA revealed a main effect of group for all medial PFC regions (1, 2 and 3) and a main effect of condition for a superior region of medial PFC (1) and ACC (2). Thus the HC group tended to deactivate medial PFC more than the ADHD group in general and deactivation was significantly boosted with increasing task difficulty. Posterior cingulate/ precuneus (bar plot 4): The bar plot reveals that both HC and ADHD participants significantly deactivated posterior cingulate/ precuneus during AT and VSAT tasks (4). Therefore both groups deactivated posterior cingulate extending into the precuneus equally for the AT and VSAT conditions.
Figure 3
Scatter plots illustrating the relationship between IIV and brain activation in all participants combined (on the left) and ADHD participants alone (on the right).
Figure 4
Scatter plots illustrating the relationship between change in IIV from AT to VSAT tasks and change in activation between AT and VSAT tasks in all participants combined (on the left) and ADHD participants alone (on the right).
Figure 5
HC versus ADHD between-group VSAT activation map combined with the HC within-group activation map. Regions in orange are those regions from the HC within-group map. Regions in green were derived from the between-groups map and the yellow regions represent areas that these two maps overlap. Significant between-group differences were seen in medial PFC only, with overlap between maps in rostral ACC.
Figure 6
Low versus high IIV between-group VSAT activation map. Between-group activation was examined in ROIs based on the centers of mass of within-group regions from the HC and ADHD maps. One small cluster of activation was noted in the ventromedial PFC ROI only. ADHD participants with high levels of IIV showed greater levels of activity (that is, less deactivation) in this region. The ROI is represented in range and the cluster in yellow in this figure.
Figure 7
Experimental Paradigm The paradigm used included three levels of task difficulty: A) the MST (Match-to-Sample Task) required participants to respond “yes” by a right-hand button press if the number on the top of a screen matched one presented on the bottom or “no” by a left-hand button press if the numbers displayed were different; B) the AT (Addition Task) required participants to respond “yes” by a right-hand button press if the sum of the top two numbers equaled the number in parentheses and “no” by a left-hand button press if the sum differed from the number in parentheses; C) the VSAT (Visual Serial Addition Task) was the most difficult level of task. Each number on the top of the screen is added to the top number from the previous trial, and the sum was compared to the number on the bottom in parentheses. Again participants responded “yes” if the sum was correct by right hand button press and “no” the sum was incorrect by left hand response.
References
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