Specialization in the default mode: Task-induced brain deactivations dissociate between visual working memory and attention

Abstract

The idea of an organized mode of brain function that is present as default state and suspended during goal-directed behaviors has recently gained much interest in the study of human brain function. The default mode hypothesis is based on the repeated observation that certain brain areas show task-induced deactivations across a wide range of cognitive tasks. In this event-related functional resonance imaging study we tested the default mode hypothesis by comparing common and selective patterns of BOLD deactivation in response to the demands on visual attention and working memory (WM) that were independently modulated within one task. The results revealed task-induced deactivations within regions of the default mode network (DMN) with a segregation of areas that were additively deactivated by an increase in the demands on both attention and WM, and areas that were selectively deactivated by either high attentional demand or WM load. Attention-selective deactivations appeared in the left ventrolateral and medial prefrontal cortex and the left lateral temporal cortex. Conversely, WM-selective deactivations were found predominantly in the right hemisphere including the medial-parietal, the lateral temporo-parietal, and the medial prefrontal cortex. Moreover, during WM encoding deactivated regions showed task-specific functional connectivity. These findings demonstrate that task-induced deactivations within parts of the DMN depend on the specific characteristics of the attention and WM components of the task. The DMN can thus be subdivided into a set of brain regions that deactivate indiscriminately in response to cognitive demand ("the core DMN") and a part whose deactivation depends on the specific task.

Figure 1

Stimuli and trial design. Participants were presented with a search array and asked to memorize only the objects marked with a target item. The targets were either easy to discriminate from the distractors (“easy search”) or not (“difficult search”). WM load was manipulated by changing the number of targets (Load 1, left array; Load 3, right array). The search array was presented for 8 s and the analysis focused on the late encoding predictor (green bar, grey: additional predictors). ITI: intertrial interval.

Figure 2

Activations (red) and deactivations (blue) compared to the baseline revealed by the late encoding predictors (4–6 s after stimulus onset). Group data are projected on the flattened surface reconstruction of the MNI template brain (courtesy of the Montreal Neurological Institute). Activations and deactivations are those exceeding a whole‐brain false discovery rate threshold of q(FDR) < 0.05. (LH: left hemisphere, RH: right hemisphere). CiS: cingulate sulcus, IFS: inferior frontal sulcus, IPS: inferior parietal sulcus, LS: lateral sulcus, MOG: middle occipital gyrus, OTS: occipito‐temporal sulcus, PPC: posterior parietal cortex, RS: rolandic sulcus, SFS: superior frontal sulcus.

Figure 3

Deactivations in the four experimental conditions during late encoding (4–6 s). Statistical maps of the contrasts DS vs. ES (yellow), WM Load 3 vs. 1 (blue, green: overlap), and the interaction of search difficulty × WM load (red) are shown. Averaged time courses of the BOLD response are given for selected ROIs together with the predictors modeling the different task phases. Deactivations are those exceeding a whole‐brain false discovery rate threshold of q(FDR) < 0.05. DS: difficult search, ES: easy search, FG: frontal gyrus, MTG: middle temporal gyrus, SMG: supramarginal gyrus, STG: superior temporal gyrus, green bar: late encoding predictor.

Figure 4

Functional connectivity maps. (A) Instantaneous influence between parts of the brain and right SMG, P < 0.01, corrected. (B) Instantaneous influence between parts of the brain and left SFG, P < 0.01, corrected. (C) Difference maps for instantaneous influence between parts of the brain and left SFG vs. right SMG, P < 0.05, corrected. Yellow indicates regions showing stronger functional connectivity with the left SFG vs. right SMG. Blue indicates regions showing stronger functional connectivity with the right SMG vs. left SFG. The two reference regions are shown in the transversal slices.

Figure 5

Deactivations compared with activations in the four experimental conditions. Averaged time courses from selected ROIs associated with significant decreases (left panel) or increases (right panel) from baseline during late encoding (4–6 s) are shown. FG: frontal gyrus, MFG: middle frontal gyrus, MTG: middle temporal gyrus, PrcS: precentral sulcus, SFG: superior frontal gyrus.