Neural correlates of ongoing conscious experience: both task-unrelatedness and stimulus-independence are related to default network activity

Abstract

The default mode network (DMN) is a set of brain regions that consistently shows higher activity at rest compared to tasks requiring sustained focused attention toward externally presented stimuli. The cognitive processes that the DMN possibly underlies remain a matter of debate. It has alternately been proposed that DMN activity reflects unfocused attention toward external stimuli or the occurrence of internally generated thoughts. The present study aimed at clarifying this issue by investigating the neural correlates of the various kinds of conscious experiences that can occur during task performance. Four classes of conscious experiences (i.e., being fully focused on the task, distractions by irrelevant sensations/perceptions, interfering thoughts related to the appraisal of the task, and mind-wandering) that varied along two dimensions ("task-relatedness" and "stimulus-dependency") were sampled using thought-probes while the participants performed a go/no-go task. Analyses performed on the intervals preceding each probe according to the reported subjective experience revealed that both dimensions are relevant to explain activity in several regions of the DMN, namely the medial prefrontal cortex, posterior cingulate cortex/precuneus, and posterior inferior parietal lobe. Notably, an additive effect of the two dimensions was demonstrated for midline DMN regions. On the other hand, lateral temporal regions (also part of the DMN) were specifically related to stimulus-independent reports. These results suggest that midline DMN regions underlie cognitive processes that are active during both internal thoughts and external unfocused attention. They also strengthen the view that the DMN can be fractionated into different subcomponents and reveal the necessity to consider both the stimulus-dependent and the task-related dimensions of conscious experiences when studying the possible functional roles of the DMN.

Figure 1. Dimensions of ongoing conscious experiences.

Conceptual division of ongoing conscious experiences occurring during tasks requiring sustained externally-driven attention according to their “stimulus-dependency” and “task-relatedness” dimensions.

Figure 2. Brain areas associated with the main effect of task-relatedness.

Regions are displayed at p<0.001, uncorrected for multiple comparisons with a minimum cluster size of 15 voxels. Bar graphs illustrate the mean parameter estimates (± standard error of the mean) for each cluster. SD = stimulus-dependent, SI = stimulus-independent, TR = task-related, TU = task-unrelated, L = left, R = right, MPFC = medial prefrontal cortex, PCC = posterior cingulate cortex, aIPL = anterior inferior parietal lobe, pIPL = posterior inferior parietal lobe, Occ. cortex = occipital cortex, Rsp = restrosplenial cortex, PHC = parahippocampal cortex.

Figure 3. Brain areas associated with the main effect of stimulus-dependency.

Regions are displayed at p<0.001, uncorrected for multiple comparisons with a minimum cluster size of 15 voxels. Bar graphs illustrate the mean parameter estimates (± standard error of the mean) for each cluster. SD = stimulus-dependent, SI = stimulus-independent, TR = task-related, TU = task-unrelated, L = left, R = right, MPFC = medial prefrontal cortex, PCC = posterior cingulate cortex, pIPL = posterior inferior parietal lobe, ITG = inferior temporal gyrus, MTG = middle temporal gyrus.

Figure 4. Brain areas associated with mind-wandering.

The brain areas associated with mind-wandering compared to being fully focused on task are shown in the superior panels. The brains areas associated with mind-wandering compared to both task-related interferences and external distractions (conjunction analysis) are shown in the inferior panels. Regions are displayed at p<0.001, uncorrected for multiple comparisons with a minimum cluster size of 15 voxels.

Figure 5. Mean parameters estimates for a rostral portion versus a more caudal portion of the MPFC.

Mean parameter estimates (averaged over all voxels within a 4-mm radius of the peak voxel) for each of the four conditions (relative to the baseline) in two regions of the medial prefrontal cortex. Error bars represent the standard error of the mean. SD = stimulus-dependent, SI = stimulus-independent, TR = task-related, TU = task-unrelated.