Conditional Differences in Mean Reaction Time Explain Effects of Response Congruency, but not Accuracy, on Posterior Medial Frontal Cortex Activity

Abstract

According to the conflict-monitoring model of cognitive control, the posterior medial frontal cortex (pMFC) plays an important role in detecting conflict between competing motor responses. Consistent with this view, pMFC activity is greater in high-conflict trials (e.g., incongruent trials and errors) than in low-conflict trials (e.g., congruent trials and correct responses) of distractor interference tasks. However, in both low- and high-conflict trials, pMFC activity increases linearly with reaction time (RT). Thus, heightened pMFC activity in high-conflict trials may simply reflect the fact that mean RT is longer in high-conflict than in low-conflict trials. To investigate this hypothesis, we reanalyzed data from a previously published fMRI study in which participants performed an event-related version of the multi-source interference task. Critically, after controlling for conditional differences in mean RT, effects of response congruency on pMFC activity were eliminated; in contrast, effects of response accuracy on pMFC activity remained robust. These findings indicate that effects of response congruency on pMFC activity may index any of several processes whose recruitment increases with time on task (e.g., sustained attention). However, effects of response accuracy reflect processes unique to error trials. We conclude that effects of response accuracy on pMFC activity provide stronger support for the conflict-monitoring model than effects of response congruency.

Keywords: brain–behavior correlation; conflict monitoring; error monitoring; fMRI; reaction time.

Figure 1

Overlap between effects of congruency and effects of reaction time (RT) on BOLD amplitude as revealed by whole-brain analyses. Significant congruency effects (incongruent > congruent) are highlighted in red; significant RT effects (slow RT > fast RT) are indicated in green. The conjunction of these contrasts is highlighted in yellow. All activations are overlaid on the Ch2bet template in MNI space. Activations are displayed using a height threshold of p ≤ 0.005 and an extent threshold of k ≥ 30 voxels.

Figure 2

Region of interest analyses in the posterior medial frontal cortex. (A) Posterior medial frontal cortex region of interest (pMFC; x = 2, y = 16, z = 46), overlaid on the Ch2bet template in MNI space. (B) RT-regression analysis. Activity in the pMFC ROI was significantly greater in incongruent than in congruent trials [t(20) = 5.06, p < 0.001] but did not differ between incongruent and RT-equated congruent trials [t(20) = −0.39, p = 0.7]. (C) RT-matching analysis. Activity in the pMFC ROI was significantly greater in incongruent than in congruent trials in the RT-subsampled data [t(20) = 3.17, p = 0.005] but not in the RT-matched data [t(20) = −0.44, p = 0.67]. Furthermore, the congruency effect in the pMFC ROI was significantly greater in the RT-subsampled data than in the RT-matched data [F(1, 20) = 9.33, p = 0.006]. Error bars denote ± 1 SE of the mean.

Figure 3

Mean reaction time (RT) for congruent and incongruent trials in the full data set (open circles), the RT-subsampled data set (filled squares), and the RT-matched data set (filled circles). Errors bars denote ± 1 SE of the mean.

Figure 4

Effects of response accuracy on brain activity before (left panels) and after (right panels) statistically correcting for conditional differences in mean RT. For both contrasts, error-related pMFC activity remained robust after controlling for RT differences between conditions. All activations are overlaid on the Ch2bet template in MNI space. Activations are displayed using a height threshold of p ≤ 0.005 and an extent threshold of k ≥ 30 voxels.