A core system for the implementation of task sets

Abstract

When performing tasks, humans are thought to adopt task sets that configure moment-to-moment data processing. Recently developed mixed blocked/event-related designs allow task set-related signals to be extracted in fMRI experiments, including activity related to cues that signal the beginning of a task block, "set-maintenance" activity sustained for the duration of a task block, and event-related signals for different trial types. Data were conjointly analyzed from mixed design experiments using ten different tasks and 183 subjects. Dorsal anterior cingulate cortex/medial superior frontal cortex (dACC/msFC) and bilateral anterior insula/frontal operculum (aI/fO) showed reliable start-cue and sustained activations across all or nearly all tasks. These regions also carried the most reliable error-related signals in a subset of tasks, suggesting that the regions form a "core" task-set system. Prefrontal regions commonly related to task control carried task-set signals in a smaller subset of tasks and lacked convergence across signal types.

Figure 1. Task Set-Related Signals

(A) In response to the task instructions, a task-set system should instantiate and maintain task sets that aid in the configuration of downstream moment-to-moment processes. Such a task-set system should also receive “bottom-up” feedback about ongoing performance. (B) Only the mixed blocked/event-related fMRI design allows the separate extraction of three different types of task set-related signals. Activity time-locked to the start of a task block (yellow) is likely important for the instantiation of task sets. To ensure continued performance success, task sets should be sustained for the length of the task period (red). Error-related signals (blue) can provide performance feedback to the task-set system. (C) The adjusted activity (left) remaining after the extraction of trial-related activity (right) closely resembles our model of cue-related and sustained activity. The start cue occurred on the first MR frame (time to repeat [TR] of EPIBOLD acquisition), the stop cue appeared on MR frame 35. MR frames 36 to 50 were fixation-only baseline.

Figure 2. Cross-Study Analyses of Start Cue-Related Activity

(A) Conjunction image showing the number of studies for which a voxel carried significant start-cue activity. Only voxels with significant start-cue activity in at least four conditions are shown. Activations were displayed on an inflated surface rendering of the human brain, using the CARET program (Van Essen et al., 2001). (B) Fixed-effects analysis map of start-cue activity. (C) ROI derived from the fixed-effects map. All time courses were significant (p < 0.001).

Figure 3. Regionwise Start-Cue Time Courses

Start cue-related activity in representative regions (responses averaged across voxels). Percent BOLD signal change displayed on y axis; MR frames (TR) displayed on×axis. Since the TR differed from 2.5 s for two of the conditions (see Table 6) the hemodynamic responses (finite impulse responses) may not be directly comparable.

Figure 4. Cross-Study Analyses of Sustained Activity

(A) Conjunction image showing voxels with significant sustained activity across conditions. The yellow-to-red scale shows voxels with positive sustained activity in four or more conditions, while the green-to-blue scale shows voxels with negative sustained activity in four or more conditions. (B) Fixed-effects analysis map of sustained signals. (C) ROI derived from the fixed-effects map.

Figure 5. Regionwise Sustained Activity

Percent BOLD signal change of sustained activity for ten task conditions in representative regions (responses averaged across voxels). Asterisk indicates statistically significant sustained activity (p < 0.05) for individual tasks.

Figure 6. Error-Related Activity

(A) Statistical map showing regions with differences in trial-related activity between correct and error trials (ANOVA). (B) Regions of interest for error-related activity. (C) Regionwise error-related data in representative regions of interest (responses averaged across voxels). Percent BOLD signal change displayed on y axis; MR frames (TR) displayed on×axis.

Figure 7. Conjunction of the Fixed-Effects Maps for Start-Cue, Sustained, and Error-Related Activity

(A) Conjunction map of task-set signals. (B) Regionwise data from overlap regions (ROI shown on bottom). For start-cue and error-related time courses, percent BOLD signal change is displayed on the y axis and MR frames (TR) are displayed on the×axis. Asterisk indicates significant (p < 0.05) sustained activity. All start-cue activity was significant (p < 0.001).

Figure 8. Hypothesized Framework of the Human Task-Set System

Hypothetical task-set system consistent with the observed experimental results. Other frameworks may also be consistent. Colors are as in Figure 1B, instantiation signals in yellow, maintenance in red, and feedback in blue.