Author: Flora Liu, MD
The Daily Dose
Anesthesiologists routinely guide patients through transitions between consciousness and unconsciousness, yet the brain mechanisms responsible for these changes remain incompletely understood.
A session presented at the 2026 IARS and SOCCA Annual Meeting examined how anesthetic drugs alter brain activity, why consciousness may fluctuate even when anesthetic concentrations remain stable, and how emerging EEG measurements could improve the assessment of awakening and neurologic recovery.
Separating Consciousness From Drug Effects
Andrew McKinstry-Wu, MD, PhD, discussed the difficulty of distinguishing the brain activity associated with unconsciousness from the direct pharmacologic effects of anesthetic drugs.
When anesthetic concentration is changing, alterations in the EEG may reflect either the drug level or the patient’s state of consciousness. This makes it challenging to identify a reliable neural marker that specifically indicates whether a patient is conscious.
Researchers attempted to address this problem by maintaining a steady concentration of xenon anesthesia. Xenon is radiopaque, allowing its concentration in the brain to be measured noninvasively through serial CT imaging.
While xenon concentrations remained stable, participants completed a Go/No-Go task and experienced spontaneous transitions between responsiveness and unresponsiveness. Researchers recorded EEG activity during these transitions.
Several commonly evaluated EEG measurements did not reliably distinguish the two states. Frontal peak alpha activity and dynamic criticality were not significantly different when participants were responsive compared with when they were unresponsive.
Statistical criticality, however, decreased during unresponsiveness and increased when consciousness returned. This measurement describes how closely brain activity operates near a transition point between highly ordered and disordered activity.
Because statistical criticality changed even when anesthetic concentration remained stable, it may become a useful EEG marker of consciousness that is less affected by drug concentration.
The Criticality Hypothesis
The criticality hypothesis proposes that the healthy conscious brain operates near a critical point between excessive order and excessive disorder.
At this point, the brain may be best able to process information, communicate across networks, and respond flexibly to changing conditions. General anesthesia commonly moves brain activity away from this critical state and into a more suppressed or subcritical condition.
The return of consciousness may therefore involve restoration of the brain’s ability to operate near this critical point rather than simply removal of the anesthetic drug.
Propofol as a Prognostic Tool
Catherine Duclos, PhD, discussed how responses to propofol may help predict recovery in patients with coma or disorders of consciousness.
Traditional assessments often depend on whether patients follow commands or demonstrate purposeful behavior. These evaluations may be unreliable in patients who have severe motor impairment, fluctuating arousal, or an inability to communicate.
Dr. Duclos presented the Adaptive Reconfiguration Index, an EEG-based measurement that evaluates how brain networks reorganize when exposed to propofol.
The index examines changes in network hubs and directed functional connectivity. Patients with brain injuries who retain the ability to reorganize their neural networks coherently during propofol administration may have a greater potential to recover consciousness.
Paradoxical Responses to Sedation
Propofol does not affect every brain-injured patient in the same way. Although it normally suppresses neural activity, some patients with disorders of consciousness may demonstrate paradoxical signs of improved network organization.
One possible explanation involves disruption of the brain’s mesocircuit. Loss of normal striatal inhibition may cause excessive activity within the globus pallidus interna, which then produces excessive inhibition of the thalamus.
Under certain circumstances, GABAergic medications may alter this abnormal circuitry and temporarily improve network organization or responsiveness.
These findings reinforce that anesthetic effects are not uniform. A patient’s response may depend on:
• The baseline organization of brain networks
• The location and severity of the injury
• The integrity of thalamocortical pathways
• Individual genetic characteristics
• The patient’s underlying state of consciousness
This variability may eventually allow anesthetic challenges to be used as diagnostic or prognostic tests in selected neurologically injured patients.
Motor Behavior and the Return of Consciousness
Paola Calderon, MD, PhD, examined how motor behavior may reveal important differences between reflexive movement and voluntary action during emergence from anesthesia.
Anesthesiologists often assess recovery by asking a patient to follow a verbal command. However, movement alone does not necessarily prove that the patient has regained meaningful consciousness.
Reflexive movements may occur with little activation of brain regions involved in planning or intentional behavior. In contrast, voluntary and goal-directed actions require coordinated activity across cortical and subcortical networks.
Research demonstrated that reflexive movements produced minimal activation in the anterior lateral motor cortex. Voluntary behaviors produced much greater activity, particularly in the superficial cortical layers involved in communication, planning, and coordinated motor action.
Arousal Units
Dr. Calderon introduced the concept of “arousal units,” which are integrated patterns of brain and body activity associated with recovery from anesthesia.
These patterns may combine:
• Motor behavior
• EEG activity
• Autonomic changes
• Cortical activation
• Goal-directed responses
Rather than relying on a single movement or response, clinicians may eventually assess consciousness by evaluating several coordinated signals.
This approach could help distinguish a simple reflex from a meaningful sign of awareness or intentional behavior.
Potential Clinical Applications
Improved markers of consciousness could have important applications in several settings.
In the operating room, they could help anesthesiologists determine whether patients are adequately unconscious without relying entirely on anesthetic concentration or traditional processed EEG numbers.
In the post-anesthesia care unit, they could assist in evaluating delayed awakening and distinguishing residual anesthetic effects from neurologic injury.
In the intensive care unit, they could help identify covert consciousness in patients who appear behaviorally unresponsive but retain organized brain-network activity.
They may also improve prognosis after traumatic brain injury, cardiac arrest, stroke, or prolonged coma.
The Future of Consciousness Monitoring
No single EEG measurement or behavioral test currently provides a complete assessment of consciousness. Consciousness is not simply an on-or-off condition, and responsiveness may not always accurately represent awareness.
Future monitoring will likely combine:
• EEG spectral activity
• Measures of brain criticality
• Functional connectivity
• Network reconfiguration
• Motor-cortex activity
• Autonomic responses
• Purposeful behavioral testing
Anesthetic drugs may also become valuable experimental and diagnostic tools. By observing how the injured brain responds to controlled sedation or stimulation, clinicians may gain insight into preserved neural pathways and the potential for recovery.
Understanding the return of consciousness requires more than measuring how much anesthetic remains in the body. It requires examining whether brain networks have regained the ability to communicate, reorganize, respond to stimulation, and generate purposeful behavior.
These discoveries may eventually lead to more accurate anesthesia monitoring, earlier recognition of covert consciousness, and better prognostication for patients with severe neurologic injuries.
Thank you to The Daily Dose and the International Anesthesia Research Society for allowing us to summarize this important session.