Why can't I feel sound under anesthesia?

The purpose of anesthesia is to put the brain into an unconscious state in which stimuli such as sounds are not perceived
.
In this state, neurons in the auditory cortex are still stimulated by sound, but the latter is not perceived
by the brain.
Scientists from the Institut Pasteur, CNRS and Université Paris-Saclay have revealed a new neural mechanism that accompanies the transition
from a conscious perceptual state to an unconscious state under anesthesia.
A cutting-edge optical imaging technique, multiphoton microscopy, was used to observe the activity
of nearly 1,000 neurons in the auditory cortex during the transition from wakefulness to anesthesia in a mouse model.
The results showed that in the waking state, some combinations of neurons responded to sounds, while others were spontaneously active (indicating that brain activity was ongoing).

But under anesthesia, the collection of neurons that respond to sound is indistinguishable
from neurons that are spontaneously active.
In the unconscious state produced by anesthesia, the cerebral cortex masks sensory input
with its own "spontaneous" activity.

The findings, published in the September 28, 2022 issue of the journal Nature Neuroscience, open up new possibilities
for modeling alert states.

Although the mechanisms by which the ear receives sound are beginning to be clearly understood, we know very little
about those mechanisms related to the perception and interpretation of sound and auditory perception.
Auditory neuroscience, and sensory perception in general, is grappling with several problems
in this regard.
One is about the mechanism that distinguishes between conscious perception in waking states and sound processing in the brain in unconscious states such as sleep or anesthesia
.

Why is the auditory cortex activated when sound reaches the ears of a person or animal being anesthetized, whereas under anesthesia, humans have no conscious perception of sound?

This problem has not been solved before, because existing neural activity measurements can only provide information
about the activity of individual neurons or small groups of neurons recorded individually in the vast neural networks that make up the cortex.
These data fail to provide an overview of
neural network coordination activities.
Other datasets collected at the brain level were used to infer the average activity of these large networks, but no details
were provided about them.
Therefore, it is impossible to extract enough information about the activity of the cerebral cortex to measure fundamental differences
in sensory responses in both awake and under anesthesia.

In this study, a team
led by Brice Bathellier (Dynamics of the Auditory System and Multisensory Processing/Inserm) and Alain Destexhe (Paris-Saclay Institute for Neuroscience/CNRS/University) of the Institut Pasteur Institute of Hearing.
Paris-Saclay used an optical recording technique — calcium imaging through multiphoton microscopy — to monitor the activity of nearly 1,000 neuronal collections in the auditory cortex of
mice while awake and anesthetized.
The scientists then mathematically studied the contours
of neurons being activated when they hear a series of different sounds.
These novel observations show that although neurons are activated by sound both awake and anesthetized, the combination of neurons in both states is completely different
.

They also showed that the cerebral cortex is a continuously active structure
even in the absence of stimulation.
By comparing this "spontaneous" activity with that caused by sound, the scientists made further observations
.
In the awake state, the combination of neurons activated by sound is different from the combination of neurons that are activated spontaneously, while in the state of anesthesia, the combination of neurons excited by sound is also systematically and spontaneously activated
.
Thus, although the auditory cortex does respond to sound stimuli under anesthesia, its response is indistinguishable from its own internal activity
.

These observations propose a mechanism that can explain the paradox of auditory perception under anesthesia and, by extension, the paradox of sensory perception
.
"The activation of neurons in the anesthetized subcortex is not only structurally very different from what is observed while awake; What's more, it is almost indistinguishable
from the spontaneous activity of the cerebral cortex.
Thus, for the rest of the brain, the neural responses of the auditory cortex under anesthesia are actually "silent" because they are drowned out by their own "noise
.
" These results also show that one of the conditions for conscious perception is that the cortex is able to activate combinations of neurons that differ from spontaneous activation," explains
Brice Bathellier, the study's final co-author.

In a sense, the cortex when awake is more "creative" because it produces new sounds directed at sounds, a specificity that seems to be absent
at anesthesia.
Whether this conclusion also applies to other states such as sleep remains to be seen," notes
Alain Destexhe, the study's final co-author.

These results also provide new insights into the mechanisms of conscious perception of sound or other sensory signals and pave the way
for new models of conscious and unconscious information processing in other regions of the cerebral cortex.