Endogenous human brain dynamics recover slowly following cognitive effort

Abstract

Background: In functional magnetic resonance imaging, the brain's response to experimental manipulation is almost always assumed to be independent of endogenous oscillations. To test this, we addressed the possible interaction between cognitive task performance and endogenous fMRI oscillations in an experiment designed to answer two questions: 1) Does performance of a cognitively effortful task significantly change fractal scaling properties of fMRI time series compared to their values before task performance? 2) If so, can we relate the extent of task-related perturbation to the difficulty of the task?

Methodology/principal findings: Using a novel continuous acquisition "rest-task-rest" design, we found that endogenous dynamics tended to recover their pre-task parameter values relatively slowly, over the course of several minutes, following completion of one of two versions of the n-back working memory task and that the rate of recovery was slower following completion of the more demanding (n = 2) version of the task.

Conclusion/significance: This result supports the model that endogenous low frequency oscillatory dynamics are relevant to the brain's response to exogenous stimulation. Moreover, it suggests that large-scale neurocognitive systems measured using fMRI, like the heart and other physiological systems subjected to external demands for enhanced performance, can take a considerable period of time to return to a stable baseline state.

Figure 1. Summary of experimental paradigm.

Figure 2. Task-activated brain regions and the recovery of fractal scaling of endogenous oscillations after task performance.

(a) Within-group map of activated (red) and deactivated (blue) regions from a contrast of n-back versus zero-back (control) trials of the working memory task. Axial slice locations are in mm coordinates of the MNI stereotaxic template. The left of the image is the right of the brain. Threshold for significance was at the cluster level and set such that one false-positive cluster was expected under the null hypothesis (equivalent p = 3.6×10⁻³). (b–e) Post-task recovery of fractal scaling (ΔH) for low and high working memory loads, extracted from activated regions clusters 1–3 and deactivated regions cluster 1 (see table 1 for anatomical description). Error bars are between-subject standard deviations. Note that in the immediate post-task period, values of H were lower than before task performance, indicating a relative loss of long-range autocorrelations or long memory properties in the endogenous dynamics. Endogenous dynamics tended to recover their pre-task parameter values quite slowly over the course of several minutes following completion of the task and the rate of recovery was faster following completion of the less demanding version of the working memory task.