The conflict-monitoring model of cognitive control posits that the pMFC detects the presence of conflict between incompatible motor responses (Botvinick et al., 2001
; Yeung et al., 2004
). Consistent with this view, pMFC activity is greater in high-conflict trials (e.g., incongruent trials and incorrect responses) than in low-conflict trials (e.g., congruent trials and correct responses) of distractor interference tasks (Botvinick et al., 1999
; MacDonald et al., 2000
; Orr and Weissman, 2009
). However, pMFC activity increases linearly with RT in both low- and high-conflict trials (Weissman et al., 2006
; Yarkoni et al., 2009
). Thus, effects of response conflict on pMFC activity may simply reflect the fact that mean RT is greater in high-conflict than low-conflict trials. The present results provided partial support for this hypothesis: congruency effects in the pMFC were wholly eliminated after controlling for differences in mean RT between incongruent and congruent trials. However, not all effects of response conflict on pMFC activity could be explained by conditional differences in mean RT. Specifically, we observed greater pMFC activity for errors than for correct responses, even after controlling for conditional differences in mean RT.
We used two independent strategies to control for conditional differences in mean RT. First, we used an RT-regression method to estimate activity that would have been observed in congruent trials whose RT equaled the mean RT in incongruent trials. This strategy considers all of the available data, thereby maximizing statistical power. However, this analysis assumes that the RT-BOLD relationship is predominantly linear. Our data supported this assumption. Nevertheless, we sought to confirm the results of this analysis using a method that makes no assumptions about the form of the RT-BOLD relationship. Specifically, we selected RT-matched pairs of incongruent and congruent trials, while excluding trials that could not be matched across conditions. Importantly, the results of the RT-regression and RT-matching analyses showed remarkable agreement with regard to pMFC activity: in both cases, controlling for conditional differences in mean RT wholly eliminated congruency effects in the pMFC (Figure ). Thus, both approaches supported the conclusion that congruency effects in the pMFC could be explained by conditional differences in mean RT.
In contrast, results from the two methods diverged with regard to parietal activity. Correcting for RT differences using RT-regression eliminated congruency effects in the posterior parietal cortex; however, correcting for these differences using RT-matching did not. This difference may reflect the fact that the RT-regression analysis assumes a linear RT-BOLD relationship, while the RT-matching method does not. The two methods also may have differed because the RT-regression method considered all correct trials, while the RT-matching method discarded trials that could not be matched. In sum, our results do not rule out a role for the posterior parietal cortex in detecting response conflict independent of time on task.
Our finding that congruency effects in the pMFC could be explained by conditional differences in mean RT presents an interpretive difficulty for the conflict-monitoring model. On the one hand, it could be consistent with the model if heightened demands on conflict detection in incongruent (relative to congruent) trials lead to both greater RT and greater pMFC activity. On the other hand, it could be inconsistent with the model if heightened demands on some other cognitive process whose recruitment increases with RT lead to greater pMFC activity in incongruent trials. Along these lines, Yarkoni et al. (2009
) argued that greater pMFC activity in trials with slow RTs reflects heightened demands on processes that sustain attention until a response is made. Consistent with this view, the pMFC has been implicated in sustained attention (Murtha et al., 1996
; Dosenbach et al., 2006
) and various other processes whose recruitment likely also increases with RT, such as autonomic arousal (Critchley et al., 2003
) and cognitive effort (Mulert et al., 2005
). For these reasons, the present findings indicate that congruency effects in the pMFC may be explained equally well by heightened demands on conflict detection or a variety of other processes whose recruitment increases with RT. These findings do not rule out the conflict-monitoring model. However, they indicate that effects of response congruency on pMFC activity do not provide direct support for the model.
In contrast, effects of response accuracy on pMFC activity were more consistent with processes that detect response conflict than with other processes whose recruitment varies with RT. Specifically, such effects persisted in the pMFC after controlling for RT differences between errors and correct responses (Figure ). Thus, in the present study, effects of response accuracy on pMFC activity provided stronger support for the view that the pMFC plays a role in detecting response conflict than effects of response congruency. Nevertheless, our data do not rule out the possibility that error-related pMFC activity reflects other processes that are uniquely recruited in error trials, such as emotional reactions (Kiehl et al., 2000
) or heightened attention (Posner and Petersen, 1990
; Orr and Weissman, 2009
) following suboptimal performance. Future studies could explore this issue with tasks that include a larger number of errors. Such tasks would not only maximize statistical power for comparing errors to correct responses, but also allow researchers to test focused hypotheses regarding error-related activity.
Although the present research focuses on the role of the pMFC in detecting response conflict, the issue raised here is germane to a wide range of studies in cognitive neuroscience. Indeed, conditional differences in mean RT are a ubiquitous confound in brain imaging studies of human cognition. For example, this confound is present in work investigating differential brain activity for high versus low memory load (Cohen et al., 1997
), task-switch versus task-repeat trials (Dove et al., 2000
), and difficult versus easy moral judgments (Greene et al., 2004
). In such studies, conditional differences in brain activity are often attributed to a specific cognitive process but might, in fact, reflect any of several processes whose recruitment varies with RT. Nonetheless, a recent survey of neuroimaging studies revealed that only 9% modeled trial-by-trial variations in RT (Grinband et al., 2008
). Some of these studies reported that conditional differences in brain activity could be explained by conditional differences in mean RT (Christoff et al., 2001
; Epstein et al., 2007
) while others reported that this was not the case (Dobbins and Han, 2006
; Simons et al., 2006
). The fact that so few studies have explicitly tested this important hypothesis underscores the need for future research in this area.
In summary, we found that conditional differences in mean RT explained effects of response congruency, but not response accuracy, on pMFC activity in an event-related version of the MSIT. These findings indicate that effects of response accuracy on pMFC activity provide stronger support for the conflict-monitoring model than effects of response congruency. More broadly, they emphasize the importance of controlling for conditional differences in mean RT in all functional neuroimaging studies of cognition.