Which deep brain structures belong to the default mode network?
Resting-state brain scans showed that the default mode network reaches below the cortex into specific thalamic, brainstem and hypothalamic arousal nuclei, while parts of the basal ganglia fluctuate in the opposite direction.
Source
Mapping the subcortical connectivity of the human default mode network
Study at a glance
- Design
- Cross-sectional — Secondary analysis of one 15-minute 7T resting-state fMRI run per person; data were time-aligned across subjects (BrainSync), decomposed into group networks with a tensor method (NASCAR), and the subcortical part of the DMN was quantified in atlas-defined nuclei, with a split-half replication and a seed-based comparison.
- N
- N=168 · 168 healthy Human Connectome Project participants after exclusions, split randomly into two halves of 84 for replication; one post-mortem brainstem specimen was used for tyrosine hydroxylase staining.
- Population
- Healthy young adult volunteers from the Human Connectome Project 7T resting-state release
- Outcome
- Strength and sign of DMN-linked signal in thalamic, brainstem, hypothalamic, basal forebrain and basal ganglia nuclei relative to the cortical DMN
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What they did
The researchers took high-field (7 tesla) resting-state fMRI from 168 healthy Human Connectome Project volunteers, synchronised everyone's time series, and used a tensor decomposition to find group-level networks spanning cortex and subcortex together. They pulled out the subcortical part of the default mode network (DMN), overlaid it on an ultra-high-resolution post-mortem MRI, and measured it in individual nuclei from several atlases. They tested whether the thalamic central lateral nucleus (CL) and the ventral tegmental area (VTA) were more DMN-connected than their surroundings, explored other nuclei, compared against ordinary seed-based correlation, and repeated everything in two random halves of 84 people.
What they found
Subcortical DMN signal was much weaker than cortical signal overall, but the thalamus, caudate and brainstem reached roughly 30% of cortical strength. As predicted, CL and VTA were significantly more DMN-connected than the thalamus and brainstem on average, and exploratory analyses flagged further thalamic, raphe, reticular and hypothalamic nuclei. The putamen and internal globus pallidus were mostly anti-correlated with the DMN, in line with the proposed mesocircuit in which the pallidum inhibits the central thalamus. The two halves produced almost identical maps (spatial correlation of 0.987).
The limits
What it doesn't show
Functional connectivity is correlation, not proof of direct anatomical wiring, and the authors stress that BOLD anti-correlations are not a direct readout of neuronal inhibition, so the mesocircuit is supported but not proven. Subcortical fMRI signal is very noisy and small nuclei are hard to register across people, so the authors warn against voxel-level conclusions; the whole decomposition explained only about 11% of the data's variance. Results come from 7T data in healthy young adults and may not hold on standard 3T clinical scanners or in patients with disorders of consciousness, the group the map is meant to help. Beyond CL and VTA, the extra nuclei are exploratory and hypothesis-generating.
Key terms
- Default mode network (DMN)
- A set of brain regions (posterior cingulate, precuneus, medial prefrontal and inferior parietal cortex) that are most active and strongly coupled at rest and are linked to self-referential thought and awareness.
- Resting-state functional connectivity
- The degree to which spontaneous BOLD fluctuations in different brain regions rise and fall together while a person lies in the scanner doing no task.
- Central lateral nucleus (CL)
- A small nucleus of the central thalamus that receives brainstem arousal input and projects widely to cortex; deep brain stimulation of it has been used to try to restore consciousness.
- Ventral tegmental area (VTA)
- A midbrain region rich in dopamine-producing neurons, implicated in reward and, increasingly, in arousal and consciousness.
- Anti-correlation
- A pattern in which a region's signal tends to go down when the network's signal goes up, and vice versa.
- Mesocircuit hypothesis
- The proposal that consciousness depends on a loop in which the globus pallidus inhibits the central thalamus, which in turn drives the cortex, so reduced striatal and thalamic activity after brain injury suppresses awareness.
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Quiz yourself
What was the main purpose of mapping the subcortical DMN in this study?
Common questions
Why does it matter whether the DMN has subcortical parts?
Subcortical nuclei are small hubs whose stimulation can influence large swaths of cortex, so identifying which ones belong to the DMN points to possible targets for deep brain stimulation or focused ultrasound in patients with disorders of consciousness.
Why not just use a standard seed-based correlation?
Seed maps changed depending on which seed was used and whether the global signal was removed; removing it introduces artificial negative correlations. The tensor method modelled the global physiological signal as its own component, giving cleaner positive and negative patterns that better matched the VTA's known dopamine-cell distribution.
Does this show that CL and VTA cause consciousness?
No. It shows that their resting activity tracks the cortical DMN in healthy people; causal claims need stimulation studies or patient data, which the authors propose as next steps.
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