Convergent Effects of Different Anesthetics on Cortical Oscillations: A Step Closer to Understanding Consciousness
- Yiğit Kurtuluş
- Sep 11
- 3 min read
This report, titled "Convergent effects of different anesthetics on changes in phase alignment of cortical oscillations" by Bardon, Ballesteros, Brincat, et al., investigates how various anesthetic drugs—despite their diverse molecular mechanisms—lead to a common outcome: loss of consciousness (LOC). The researchers propose that alterations in the phase alignment of brain waves (oscillations) might be a shared neural mechanism behind this effect.
What Did the Researchers Aim to Find?
The study explored how two very different anesthetics—ketamine (an NMDA receptor blocker) and dexmedetomidine (an alpha-2 adrenergic agonist)—affect brain activity in rhesus macaques. These two drugs act on different parts of the brain but both reliably induce unconsciousness at high doses.
To uncover potential common effects on brain function, the team focused on the prefrontal cortex (PFC)—a brain region central to cognition and consciousness. They implanted electrode arrays in left and right dorsolateral (L-Dor, R-Dor) and ventrolateral (L-Ven, R-Ven) areas of the PFC and recorded local field potentials (LFPs). The monkeys’ responsiveness was tracked using a simple lever-press task, and loss of response was used as a sign of LOC.
The researchers looked specifically at:
Phase Locking Value (PLV) and coherence: Measures of how synchronized brain signals are.
Phase offset: The angular difference between two brain waves’ peaks. They compared these during awake states, under anesthetic doses, and under sub-anesthetic doses to see how changes in brainwave alignment correlate with loss of consciousness.
What Did They Discover?
1. Increased Low-Frequency Synchronization
Both ketamine and dexmedetomidine increased brain activity in low-frequency bands—particularly delta (1–4 Hz) and theta (4–8 Hz) waves. There was a significant increase in phase locking across all PFC regions, suggesting more synchronized brain activity at slow frequencies under anesthesia.
2. Disrupted Within-Hemisphere Communication
Although overall low-frequency synchrony increased, there was reduced alignment between neighboring brain areas within the same hemisphere. For instance, the L-Dor and L-Ven regions became more out-of-sync, showing larger phase offsets. This misalignment might fragment communication and block the integration of information within a hemisphere.
3. Strengthened Cross-Hemisphere Synchronization
In contrast, homologous areas across hemispheres (e.g., L-Dor vs. R-Dor) became more phase-aligned, with phase offsets shifting closer to perfect synchrony. This pattern is opposite of what’s usually seen in awake and alert brains, where cross-hemisphere coordination is more variable.
4. Distance-Based Effects
Anesthesia enhanced the natural tendency for longer distances between electrodes to be associated with lower phase locking and greater phase offset. This supports the idea that traveling waves—slow waves moving across the cortex—may be amplified under anesthesia and contribute to these effects.
5. Dose-Dependent Impact
At sub-anesthetic doses, both drugs produced weaker and more variable changes, though the overall trends remained the same. Phase locking and offset shifts were smaller, indicating that these brainwave changes scale with dose and level of consciousness.
Why Does This Matter?
The study suggests that a dynamic re-alignment of cortical communication—with fragmentation within hemispheres and synchronization across them—may be a shared neural mechanism for how anesthetics cause unconsciousness.
Rather than focusing solely on each drug’s molecular target, this research highlights the network-level effects of anesthesia, such as how brain circuits organize themselves during unconsciousness.
These brainwave shifts might:
Be used as biomarkers for unconsciousness.
Help guide real-time monitoring of anesthesia depth in medical settings.
Reduce risks of overmedication, especially in vulnerable populations (like those with dementia or anxiety disorders).
What Might Be Behind These Effects?
The researchers also point toward possible biological explanations, such as:
Cortical layer 5 pyramidal neurons, known to influence long-range cortical communication.
Thalamocortical loops, which link deep brain structures to the cortex and are crucial for consciousness.
These networks may be particularly sensitive to changes in low-frequency synchrony and could be disrupted in a similar way by multiple drugs.
What Are the Limitations?
While compelling, this study does not establish causality—that is, it doesn't prove that the observed changes cause unconsciousness. Further studies are needed using a wider variety of anesthetic agents and deeper interventions to test this mechanism directly.
Additional reading: https://bigthink.com/neuropsych/unconscious-brains-conscious-minds/
