Brain patterns that support transitions of consciousness at sleep onset also reflect ongoing cognitive task demands, with active task engagement recruiting long-range frontoparietal and auditory cortical synchronization while passive listening drives thalamocortical synchronization during decreasing alertness.
Key Findings
Results
Decreasing alertness during passive listening led to increased synchronization primarily in the parietal lobe, whereas decreasing alertness during active task performance led to increased long-range frontoparietal synchronization.
Participants fluctuated between alert and drowsy states while engaged in either an active or passive auditory task during simultaneous EEG-fMRI recording.
Passive listening during low alertness was associated with increased parietal lobe synchronization.
Active task engagement during low alertness was associated with increased long-range frontoparietal synchronization.
These findings reflect flexible functional reorganization of brain networks when alertness declines but cognitive processes persist.
Results
During decreasing alertness, passive listening (but not active task engagement) was associated with widespread increased synchronization between the thalamus and cortex.
Thalamocortical dynamics that typically change during sleep onset were differentially affected by task type.
Passive listening was associated with widespread thalamocortical synchronization increases as alertness declined.
Active task engagement did not produce the same pattern of thalamocortical synchronization during decreasing alertness.
This dissociation suggests that active cognitive engagement modulates thalamocortical circuits during the wake-sleep transition.
Results
Active task engagement (but not passive listening) led to increased synchronization between the auditory cortex and the rest of the brain during decreasing alertness.
Increased auditory cortex-whole brain synchronization was specific to the active task condition during low alertness periods.
Passive listening did not produce this pattern of auditory cortical synchronization.
This finding suggests that active maintenance of task engagement recruits additional sensory processing networks during drowsiness.
The result supports the hypothesis that active task engagement recruits frontoparietal and sensory processing networks during low alertness.
Results
The study found that brain circuits supporting alertness fluctuations and cognitive processing undergo flexible functional reorganization when alertness declines but cognitive engagement persists.
The study used simultaneous EEG-fMRI to assess individuals fluctuating between alert and drowsy states.
Two conditions were compared: active auditory task performance and passive auditory tone listening.
The overlapping circuits involved include frontoparietal, thalamocortical, and sensory pathways.
The findings suggest the brain has capacity for flexible functional reorganization when alertness levels decline but cognitive processes persist.
Results
The authors hypothesized and found evidence that thalamocortical dynamics that typically change during sleep onset would remain unaffected by active task engagement during low alertness.
The hypothesis stated that during periods of low alertness, active task engagement would recruit additional frontoparietal and sensory processing networks while thalamocortical dynamics would remain unaffected.
Results showed that passive listening (not active engagement) drove thalamocortical synchronization changes.
Active task engagement was instead associated with auditory cortex and frontoparietal synchronization changes.
These patterns suggest that cognitive task demands shape the neural signatures of the wake-sleep transition.
What This Means
This research suggests that the brain does not simply 'wind down' uniformly as a person becomes drowsy — instead, the pattern of brain activity during the transition from wakefulness to sleep depends heavily on whether a person is actively engaged in a task. Using a combination of EEG and fMRI brain imaging, researchers studied people as they naturally fluctuated between alert and drowsy states while either actively performing an auditory task (making decisions about sounds) or passively listening to tones. They found that the brain reorganizes itself differently depending on the cognitive demands placed on it, even as alertness declines.
Specifically, when people were passively listening and became drowsy, the thalamus — a brain region that acts as a relay station and is known to change activity during sleep onset — became more strongly synchronized with the rest of the cortex. This pattern is consistent with classical sleep transition processes. However, when people were actively performing a task and became drowsy, the thalamus did not show this same pattern. Instead, the auditory processing areas and the frontoparietal network (regions associated with attention and executive control) became more strongly connected, suggesting the brain was compensating to maintain task engagement despite declining alertness.
This research matters because it shows that the brain's transition into sleep is not a fixed, uniform process — it is shaped by ongoing cognitive demands. This has implications for understanding how people maintain performance during fatigue, and for studying states of consciousness more broadly. It also suggests that measuring brain states during drowsiness without accounting for what a person is doing could give an incomplete picture of what is happening neurologically.
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Kumar S, Arzi A, Bareham C, Gonzalez-Castillo J, Fernandez I, Tagliazucchi E, et al.. (2026). Brain network dynamics in the wake-sleep transition reorganize according to task engagement.. Imaging neuroscience (Cambridge, Mass.). https://doi.org/10.1162/IMAG.a.1336