Does anesthesia work through communication breakdown or low local information generation??
Anesthesia Monitor. Image: Kalumet/CC BY-SA-3.0
It is commonly believed that anesthetics interrupt signal transmission between brain areas and thus turn off consciousness, but there could be another reason as well
To receive anesthesia for medical procedures is a blessing. It was only introduced in the 19th century. Developed in the twenty-first century. Whereas in the past patients had to be restrained during surgical operations because the pain could not be eliminated by alcohol or any plant extracts, nitrous oxide, ether or morphine, as precursors of the multitude of modern anasthetics, have massively expanded the possibilities of medical interventions and thus the treatment of patients. Anesthesia switches off the consciousness and the pain sensation, besides, sometimes additionally with agents of the muscle relaxation movement is prevented. Breathing must be ensured.
It is amed that anesthetic substances act on the central nervous system. But what exactly happens that both consciousness and pain sensitivity are switched off, so that the anesthetized person does not wake up and has no memory of it even through interventions that cause severe pain, is not really clarified. Very rarely, despite anesthesia, patients experience waking states in which they feel pain, perceive something and can remember something.
The prevailing theory is that anesthetics inhibit signal transmission between brain areas (transfer entropy), which paralyzes consciousness. But since it is not clear what consciousness is and how it arises, there is only speculation as to why anesthetics produce unconsciousness, pain, memory and motionlessness.
Neuroscientists at the Goethe University and the Max Planck Institute for Dynamics and Self-Organization in Gottingen have found a new explanation, as they report in their article for PLOS Computational Biology. Consciousness is not switched off because communication between areas is weakened or suppressed by censorship, but because fewer signals are produced in certain areas. The lower signal transmission recorded so far was not due to a disturbance of the communication, but to a lack of generated information. If there is not enough information, so the thesis, consciousness can not arise, which also suppresses pain.
Ultimately, the activity of brain areas that are induced to sleep was reduced, so that brain areas that interpret signals as pain no longer had sufficient data for this. But why are the "Source areas" not even woken up by severe pain? And not even if the breathing stops? And why is pain, a primordial biological experience one would think, no longer felt when consciousness fails and communication between areas is restricted??
In two ferrets anesthetized with isoflurane, the scientists examined the local field potentials of neuronal excitations in the "Source areas" of the prefrontal cortex (PVC ) to the primary visual cortex area V1. In contrast to the waking state, the signal exchange between PVC and V1 was less, but from PVC as a source area to V1 as a target area even significantly less, so the top-down information flow decreases. "This indicates that the information available in the source determines the information transfer and not a disturbed signal transmission", say the authors.
Isoflurane depresses certain receptors, but not AMPA and NMDA synapses, which are critical for cross-talk. In the opinion of scientists, it is not censorship or disruption of communication that is responsible for the anesthetic effect of consciousness shutdown, but simply the silencing of the activity of primary areas that hardly produce any signals anymore. This was also matched by the fact that electrophysiological anesthesia leads to more uniform signal behavior with repetitive patterns that are interrupted by sudden activity.
If the disturbance of the top-down information flow leads to the shutdown of consciousness?
Why the activity of the prefrontal cortex and V1 were examined is not clear from the study. If not the communication or the information transfer, but the data production for consciousness or the flow of information, respectively. If pain perception is crucial, then the disruption of the top-down information flow by too little signal production could lead to the shutdown of consciousness.
The authors point to the predictive coding model that consciousness information-theoretically permanently generates and updates hypotheses about the world that predict sensory input at different levels. According to the predictive coding model, a perturbation of top-down information transfer could cause the spread of predictions to hierarchically lower areas to fail. However, the decrease in locally available information and entropy in the prefrontal cortex could also prevent the integration of information in an area that is crucial for generating a coherent model of the world and the associated predictions. If predictions can no longer be made, so the thesis, consciousness expires.
If the approach was transferred to the societal level, it might not be so uninteresting. Social consciousness, which could no longer be so easily suppressed by censorship and other measures, arose precisely through the activity of the members of society, who were exercising themselves. If they withdraw and remain silent in public, the social pain diminishes or narcotization takes place without the need for direct intervention from above.