You are talking about 3 different phenomena here (emotional numbness, fading consciousness through medical anaesthesia, and silence within anechoic chambers). As your title seems to indicate you are actually asking about emotional numbess, I will base my answer on that.
Emotional numbess has been talked about, although not in the same context, in the answers to Can calmness happen during the fight-flight response? and How do certain individuals, like Quang Duc, develop the ability to remain calm when enduring significant nociceptive pain?
Dissociation is the phenomenon involved with emotional numbess as well as physical numbness.
Whilst some define dissociation as a combination of 2 distinct psychological mechanisms (Brown, 2006), the psychobiological mechanism of dissociation is little understood; but, when someone is dissociated from the event, the detachment of conscious awareness from the event can make the person appear very calm.
A person can start to dissociate when a topic of discussion starts which that person is not comfortable with.
Dissociation is a psychological defence mechanism (Cardeña, 1994) which helps the person to get through the situation with as little harm as possible. Think about it like throwing a switch on conscious awareness.
The neuroscience within dissociation is discussed extensively within Krause-Utz, et al. (2017)
Dissociation is a complex heterogeneous phenomenon. It has been defined as a “disruption of and/or discontinuity in the normal, subjective integration of one or more aspects of psychological functioning, including – but not limited to – memory, identity, consciousness, perception, and motor control”. This definition implicates a wide range of psychological symptoms (e.g., depersonalization, derealization, emotional numbing, and memory fragmentations) and somatoform symptoms (e.g., analgesia) [...]
Up to now, the precise neural/neurobiological underpinnings of dissociation remain elusive. Yet, a growing number of neuroimaging studies in DDD, DID, and D-PTSD have implicated dissociative symptoms in altered brain structure and function.
There is a lot of information in the article regarding altered brain structure and function; and reading it will give the full details, but as a snippet:
Aside from functional alterations, several studies reported structural abnormalities in clinical samples with high trait dissociation, although these structural findings are still quite heterogeneous.
In depersonalization disorder, reduced gray matter volumes (GMV) in right thalamus, caudate, and cuneus, and increased GMV in the left dorsomedial PFC and the right somato-sensoric regions were observed . As abovementioned, these areas have been implicated in dissociation.
In DID, reduced volumes in the amygdala and hippocampus and parahippocampus were found, although discrepant findings of normal amygdala and hippocampal volumes compared to healthy controls were also reported. Smaller hippocampal volumes may be related to early life trauma: the hippocampus has a high density of glucocorticoid receptors and is highly sensitive to a heightened release of the stress hormone cortisol—therefore, chronic traumatic stress may lead to cell damage in this area. Smaller hippocampal volumes were also found in healthy individuals with childhood trauma, who did not develop a disorder. Reduced hippocampal volumes in PTSD may therefore stem from a history of trauma rather than specific to the diagnosis. In a recent study, comparing PTSD patients with versus without dissociative subtype, no significant group differences in amygdala, hippocampus, and parahippocampus volumes were observed. Yet, patients with D-PTSD showed increased GMV in right precentral and fusiform gyri and reduced GMV in right inferior temporal gyrus. Severity of depersonalization and derealization was positively correlated with GMV in the right middle frontal gyrus. Another study in PTSD found positive associations between trait dissociation and GMVs in medial/lateral PFC, orbitofrontal, temporal polar, parahippocampal, and inferior parietal cortices—brain regions associated with emotion regulation.
Brown, R. J. (2006). Different types of “dissociation” have different psychological mechanisms. Journal of Trauma & Dissociation, 7(4), 7-28. doi: 10.1300/J229v07n04_02
Cardeña, E. (1994) The domain of dissociation. In: Dissociation: Clinical and theoretical aspects, Edited by: Lynn, S. J. and Rhue, J. W. 5–31. New York, NY: Guildford Press.
Krause-Utz, A., Frost, R., Winter, D., & Elzinga, B. M. (2017). Dissociation and alterations in brain function and structure: implications for borderline personality disorder. Current psychiatry reports, 19(1), 6. doi: 10.1007/s11920-017-0757-y pcmcid: PMC5283511