Impact of working memory load on FMRI resting state pattern in subsequent resting phases

Abstract

Background: The default-mode network (DMN) is a functional network with increasing relevance for psychiatric research, characterized by increased activation at rest and decreased activation during task performance. The degree of DMN deactivation during a cognitively demanding task depends on its difficulty. However, the relation of hemodynamic responses in the resting phase after a preceding cognitive challenge remains relatively unexplored. We test the hypothesis that the degree of activation of the DMN following cognitive challenge is influenced by the cognitive load of a preceding working-memory task.

Methodology/principal findings: Twenty-five healthy subjects were investigated with functional MRI at 3 Tesla while performing a working-memory task with embedded short resting phases. Data were decomposed into statistically independent spatio-temporal components using Tensor Independent Component Analysis (TICA). The DMN was selected using a template-matching procedure. The spatial map contained rest-related activations in the medial frontal cortex, ventral anterior and posterior cingulate cortex. The time course of the DMN revealed increased activation at rest after 1-back and 2-back blocks compared to the activation after a 0-back block.

Conclusion/significance: We present evidence that a cognitively challenging working-memory task is followed by greater activation of the DMN than a simple letter-matching task. This might be interpreted as a functional correlate of self-evaluation and reflection of the preceding task or as relocation of cerebral resources representing recovery from high cognitive demands. This finding is highly relevant for neuroimaging studies which include resting phases in cognitive tasks as stable baseline conditions. Further studies investigating the DMN should take possible interactions of tasks and subsequent resting phases into account.

Figure 1. Reaction times and accuracy rate.

Reaction times (mean±standard error) and accuracy rate (percentage of correct answers, mean±standard error) of 25 subjects. Asterisks denote where pair-wise comparison is significant.

Figure 2. Default-mode network extracted by TICA.

Top: sagittal, coronal and transversal view (Image displayed in radiological convention). Bottom: rendered 3d-view with transparent voxeldata from frontal-left- (left) and behind-left-view (right). Major clusters with coordinates, Z-score and Brodman area (BA) are: PCC (−4, −32, 28), Z = 5.86, BA 31 and (−6, −52, 12), Z = 3.87, BA 30; Precuneus (−8, −72, 28), Z = 6.44, BA 31; Cuneus (10, −74, 30), Z = 7.50, BA 18; Culmen (−8, −48, −4), Z = 3.09, BA 19; vACC (4, 16, −12), Z = 2.86, BA 25; Middle Frontal Gyrus (4, 60, −4), Z = 3.33, BA 10. Clusters exist left and right in similar size.

Figure 3. Mean of activation in the DMN.

Mean of activation in the DMN were calculated for each n-back (lower values: 0-back, 1-back, 2-back) and subsequent resting block (upper values: R0-back, R1-back, R2-back). x-Axis: working memory load, y-Axis: activation as extracted by TICA for the selected component (which can differ in scaling and mean).