Two complementary analyses were performed on the start-cue and sustained signals (see Experimental Procedures). A conjunction analysis was used to map the number of task conditions during which any given voxel was significantly activated. We chose to supplement our conjunction analysis with a fixed-effects analysis in which the statistical reliability of signals in each condition contributed to the overall statistical strength of the final map to show that the same areas that were activated strongly, on average over tasks, were also activated consistently. The conjunction analysis required that all the task-related effects survive a statistical criterion. The fixed-effects analysis provides a measure of statistical strength of effect across studies, making it amenable to standard region of interest (ROI) definition procedures. The fixed-effects map could potentially be dominated by only a few of the task conditions; therefore, the fixed-effects statistical map needed to show strong similarity to the conjunction map. Strong overlap between the conjunction and fixed-effects analyses would also indicate that the obtained results are robust to differences in the analysis stream.
maps the number of task conditions for which start cue-related activity passed the statistical threshold at each voxel. Several regions showed start-cue activity across many tasks. is a fixed-effects map of the main effect of time for start-cue activity at a voxel. This statistical map highlights very similar regions as the conjunction map (compare to ), making it highly unlikely that the observed effects were driven by only a few of the conditions. A region on the border of the dorsal anterior cingulate/medial superior frontal cortex (dACC/msFC) showed start-cue activity for all task conditions (). The left and right anterior insula/frontal operculum (aI/fO) contained voxels that showed start cue-related activity for nine of ten conditions. The right temporoparietal junction (TPJ) contained a few voxels with significant start-cue activity in all ten tasks. Voxels in the intraparietal sulcus (IPS) bilaterally, right lateral frontal cortex, left and right extrastriate visual cortex, left and right fusiform cortex, and the precuneus all showed start-cue activity in nine of ten tasks. Anterior prefrontal cortex (aPFC) showed start-cue activity in six of ten tasks on the left and seven of ten tasks on the right.
Cross-Study Analyses of Start Cue-Related Activity
Start-Cue Activity Regions of Interest
Separate analysis of the statistical maps for each of the individual studies demonstrated the cross-study activations not to be artifacts of averaging across groups of subjects, because each of the subject groups showed very similar patterns of joint activity (Duncan and Owen, 2000
). To describe regional start-cue signals, we identified the top 15 most reliably activated ROI from the fixed-effects statistical map (; ; see Experimental Procedures) and plotted the responses averaged over voxels within these regions.
shows time courses of start cue-related activity for ten tasks in representative regions of interest. The dACC/msFC and aI/fO are explored separately in a later section. The right TPJ (Talairach coordinates—53, −47, 16), right frontal cortex (41, 3, 36) and left IPS (−31, −59, 42) showed significant time courses of start-cue activity for all ten tasks (Talairach and Tournoux, 1988
). For several of the task conditions in each of these regions, blood oxygenation level-dependent (BOLD) activity does not return to baseline by the seventh time point, because transient start-cue activity “transitions onto” positive sustained activity (also see ).
Regionwise Start-Cue Time Courses
Brain regions with significant sustained activity across many different tasks may be generally important for task-set maintenance. shows the number of conditions for which sustained activity was significant at any given voxel (see Experimental Procedures). Both the dACC/msFC and bilateral aI/fO contained some voxels with significant positive sustained activity in eight of ten task conditions, as did the left inferior parietal lobule (IPL) and right middle temporal cortex. The fixed-effects statistical map for sustained activity () again showed great similarity to the conjunction map. Several regions showed positive sustained activity in seven tasks, among them right IPL and left middle temporal cortex. Right aPFC showed positive sustained activity in up to six tasks, while left a PFC carried sustained signals in four tasks. Several regions consistently showed negative sustained activity, decreased below baseline, across tasks. The vmPFC showed the most reliable negative sustained activity. Extrastriate visual cortex and the precuneus also carried negative sustained activity, but less consistently so. From the fixed-effects statistical map, we derived the top 15 ROI for sustained activity (; ) and plotted the responses averaged over voxels within these regions.
Cross-Study Analyses of Sustained Activity
Sustained Activity Regions of Interest
shows the profiles of sustained activity across ten tasks in three representative ROI. A region in left parietal cortex (−51, −51, 36) showed positive sustained activity for all ten tasks, but interestingly the activity was only individually statistically significant (p < 0.05) for the six task conditions in which subjects responded by pressing a button instead of speaking. Regionwise, negative sustained activity in vmPFC (1, 31, −2) reached statistical significance in nine of ten tasks.
Brain regions forming part of a core task-set system likely also receive trial-by-trial performance feedback, such as error feedback. Data for two of the ten task conditions (#8, #10; same subjects) contained sufficient errors to perform an error analysis (see Experimental Procedures). shows brain regions with significant differences between correct and error trials. The most reliable error-related activity was found in the dACC/msFC and bilateral aI/fO, followed by bilateral cerebellar regions and left and right aPFC, right dlPFC, and right IPL. The top 13 ROI were derived from the ANOVA map for error-related activity (; ), and the responses were averaged over the voxels within these regions.
Error-Related Regions of Interest
In , correct and error time courses from three regions, other than the dACC/msFC and aI/fO, are displayed. The regions in right dlPFC (43, 22, 34), right aPFC (27, 50, 23), and the left cerebellum (−32, −65, −29) all showed greater trial-related activity for the error trials than for the correct trials, as did all other error-related regions of interest.
Conjunction of Start-Cue, Sustained, and Error-Related Activity
One reasonable configuration of a task-set system () would have regions carrying out core executive functions for the configuration of task sets active during both the instantiation and maintenance of task sets, while also receiving performance feedback. Hence, to identify core regions of such a putative human task-set system, we conjoined the regions of interest for start-cue, sustained, and error-related activity.
shows the overlap of thresholded statistical maps for start-cue, sustained, and error-related activations. Only three regions of the brain, dACC/msFC (−1, 10, 46), left aI/fO (−35, 14, 5), and right aI/fO (36, 16, 4), met the statistical thresholds set for all three types of potentially task set-related activity (; ). Bilateral aPFC (R—27, 50, 23; L—−28, 51, 15) also showed three task-set signal types, but for a smaller subset of tasks, and hence did not meet criterion for core task-set regions.
Conjunction of the Fixed-Effects Maps for Start-Cue, Sustained, and Error-Related Activity
Overlap Regions: Three Task-Set Signal Types
The start-cue regions in right TPJ, left and right IPS, right frontal cortex, and the precuneus did not overlap with sustained or error-related regions. Regions of positive sustained activity in bilateral middle temporal cortex and left IPL also showed no overlap with other types of task-set signals. shows the start-cue, sustained, and error-related activity profiles for the regions of interest in the dACC/msFC and bilateral aI/fO defined by the conjunction of different types of task-set signals. While some may think that other effects such as “arousal” could also account for sustained signals, the convergence across three different task-set signals makes such alternative explanations unlikely.