Errors of interpretation and modeling: a reply to Grinband et al

Abstract

Grinband et al., 2011 compare evidence that they have collected from a neuroimaging study of the Stroop task with a simulation model of performance and conflict in that task, and interpret the results as providing evidence against the theory that activity in dorsal medial frontal cortex (dMFC) reflects monitoring for conflict. Here, we discuss several errors in their methods and conclusions and show, contrary to their claims, that their findings are entirely consistent with previously published predictions of the conflict monitoring theory. Specifically, we point out that their argument rests on the assumption that conflict must be greater on all incongruent trials than on all congruent trials-an assumption that is theoretically and demonstrably incorrect. We also point out that their simulations are flawed and diverge substantially from previously published implementations of the conflict monitoring theory. When simulated appropriately, the conflict monitoring theory predicts precisely the patterns of results that Grinband et al. take to present serious challenges to the theory. Finally, we note that their proposal that dMFC activity reflects time on task is theoretically weak, pointing to a direct relationship between behavior (RT) and neural activity but failing to identify any intervening psychological construct to relate the two. The conflict monitoring theory provides such a construct, and a mechanistic implementation that continues to receive strong support from the neuroimaging literature, including the results reported by Grinband et al.

Fig. 1

Conflict model simulation results. Simulated conflict and error likelihood are plotted separately for corresponding models of the flanker task (A,C) and Stroop task (B,D). Data for the flanker task model are reanalyzed from previously published simulations (Yeung et al., 2004), in which trials were divided into bins corresponding to stepwise increases in simulated RT (processing cycles). The Stroop model was based on an earlier implementation of the verbal response version of the task in which errors are very rare (Botvinick et al., 2001), with parameters modified to simulate the increased error rates observed empirically in Grinband et al.’s manual response task. For this simulation, trials were divided into decile bins according to RT. For both models, simulated conflict shows a monotonic increase with RT for both incongruent and congruent trials (A,B), whereas error likelihood shows a monotonic decrease for incongruent trials in which errors were relatively frequent, and no change for congruent trials in which errors were rare or absent (C,D). Replicating Grinband et al. and our previous analyses, simulated conflict is plotted for correct response trials only.