Removing the effect of response time on brain activity reveals developmental differences in conflict processing in the posterior medial prefrontal cortex

Abstract

In functional magnetic resonance imaging (fMRI) studies, researchers often attempt to ensure that group differences in brain activity are not confounded with group differences in mean reaction time (RT). However, even when groups are matched for performance, they may differ in terms of the RT-BOLD relationship: the degree to which brain activity varies with RT on a trial-by-trial basis. Group activation differences might therefore be influenced by group differences in the relationship between brain activity and time on task. Here, we investigated whether correcting for this potential confound alters group differences in brain activity. Specifically, we reanalyzed data from a functional MRI study of response conflict in children and adults, in which conventional analyses indicated that conflict-related activity did not differ between groups. We found that the RT-BOLD relationship was weaker in children than in adults. Consequently, after removing the effect of RT on brain activity, children exhibited greater conflict-related activity than adults in both the posterior medial prefrontal cortex and the right dorsolateral prefrontal cortex. These results identify the RT-BOLD relationship as an important potential confound in fMRI studies of group differences. They also suggest that the magnitude of the RT-BOLD relationship may be a useful biomarker of brain maturity.

Figure 1

Age range of the sample.

Figure 2

Effects of response conflict on brain activity. In both adults (left panels) and children (right panels), incongruent trials evoked greater activity than congruent trials in a widespread network of frontal and parietal regions. All maps are thresholded at p < 0.005 and 30 contiguous voxels.

Figure 3

Effects of trial-by-trial variations in reaction time (RT) on brain activity. In adults (left panels), relatively slow RTs were associated with increased activity throughout the fronto-parietal network. In contrast, the RT-BOLD relationship was reduced in children (right panels) who exhibited only weak effects of RT on brain activity, which were confined to lateral parietal cortex. All maps are thresholded at p < 0.005 and 30 contiguous voxels.

Figure 4

Region of interest analyses in the posterior medial frontal cortex (pMFC). Panel A: The pMFC ROI, overlaid on the ch2bet template in MNI space. Panel B: Conflict-related activity in pMFC did not differ for children and adults before removing the effect of RT on brain activity (t(37) = 1.65, n.s.). However, after removing for this effect, children showed significantly greater conflict-related activity than adults (t(37) = 2.59, p = 0.014). Error bars show the standard error of the mean. Panel C: Developmental trajectory of the conflict effect in the pMFC using the original data. Panel D: Developmental trajectory of the conflict effect in the pMFC using the RT-equated data.

Figure 5

Region of interest analyses in the right dorsolateral prefrontal cortex (DLPFC). Panel A: The right DLPFC ROI, overlaid on the ch2bet template in MNI space. Panel B: Conflict-related activity in the right DLPFC did not differ for children and adults before removing the effect of RT on brain activity (t(37) = 1.23, n.s). However, after removing this effect, children showed significantly greater conflict-related activity than adults (t(37) = 2.84, p = 0.0072). Error bars show the standard error of the mean. Panel C: Developmental trajectory of the conflict effect in the right DLPFC using the original data. Panel D: Developmental trajectory of the conflict effect in the right DLPFC using the RT-equated data.

Figure 6

Region of interest analyses in the left dorsolateral prefrontal cortex (DLPFC). Panel A: The left DLPFC ROI, overlaid on the ch2bet template in MNI space. Panel B: Conflict-related activity in the left DLPFC did not differ for children and adults before removing the effect of RT on brain activity (t(37) = −0.17, n.s.). After removing this effect, children showed marginally significantly greater conflict-related activity than adults (t(37) = 1.86, p = 0.071). Error bars show the standard error of the mean. Panel C: Developmental trajectory of the conflict effect in the left DLPFC using the original data. Panel D: Developmental trajectory of the conflict effect in the left DLPFC using the RT-equated data.