What can coma patients report about their experiences - if anything? Coma is correlated to a significant inactivity of the cerebral cortex and the reticular activating system, while other parts of the brain - e.g. the limbic system - might still show considerable activity.

My question is:


  Are there cases where people recovered from a coma and could later plausibly report experiences they made during the coma?


What kind of experiences would they typically report - if any? And how fine-grained can descriptions of such experiences be?

Examples:


"I felt that I were alive." 
"I felt horrible/relaxed." 
"I sometimes felt hungry." 
"I felt that my mother was there."
"I somehow felt she touched me." 
"I somehow heard her say my name."


Most surely, elaborate thoughts and complex emotions can not be recalled and plausibly reported (because they could not take place).


  What would such reports tell us about the neural basis of
  consciousness?


Are there forms of recoverable coma where the cerebral cortex and/or the reticular activating system is completely mute, i.e. without any measurable coordinated neural activity?

Do other parts of the brain necessarily show no measurable coordinated neural activity when the cerebral cortex and/or the reticular activating system doesn't?
The majority of people who recovered from coma did not report any experiences that occurred while they were in coma (clinically unresponsive, with absent or minimal brainstem reflexes, and with severely abnormal EEG). However, some people who had cardiac arrest or came very close to clinical death but were successfully revived reported later that they had conscious perceptual experiences during these critical periods, when they were in coma. This kind of experiences is called “near-death experience (NDE)” by Moody [1]. About 10% to 20% of such critical individuals reported near-death experiences (NDEs) [2]. NDEs include many experiences, such as seeing tunnel, light, and stars; out-of-body experiences; pleasant feelings; entering a door/gate, heaven, or hell; seeing deceased relatives, religious figures, or god; a life review; a conscious return into the body; etc. [2-5]

However, NDE is still an unsettled subject. Some believe that cumulated evidence so far shows that NDEs are real, that the mind and consciousness can exist without and outside the brain, and that the current understanding that the brain is the basis of consciousness and the mind is not correct [6-10].

Others believe that NDEs are neurophysiologic phenomena that occur while the brain is functioning in an abnormal state [5,11]. Evidence is that many mental phenomena that occur in NDEs can occur or be induced to occur in other non-coma, neurophysiologic states, such as sleep [4], meditation [12], simple partial seizures [13], certain drug-induced altered states of consciousness [11,14], and electrical stimulation and transcranial magnetic stimulation of the brain[15]. Interestingly, some investigators found that a surge of neurophysiological coherence and connectivity occurred within the first 30 sec after cardiac arrest and preceded isoelectric electroencephalogram in experimental rats [16]. Whether this also occurs in humans and is the basis of NDE occurrences or not needs further investigations.

The most crucial aspect of NDEs is whether the reported experiences occurred during coma/cardiac arrest or not. It is possible that those experiences occurred when the patients have recovered but were still in a confused or dream-like state and that the patients wrongly attributed the time of those experiences to when they were still in coma/had cardiac arrest; their reports were just sincere fallacy. However, this can be investigated in NDE experiencers who have out-of-body experiences and report seeing the scene and event when they are in coma/have cardiac arrest. If these NDE experiencers report (without the help from other people) the scene and events where and when they are in coma/have cardiac arrest correctly, it means their NDEs indeed occur during coma/cardiac arrest and, indeed, normal consciousness can exist without and outside a normally functioning brain – our understanding of the neural basis of consciousness has to be greatly revised from the root. AWARE [17] was the study that was designed to investigate this. Its results, however, were inconclusive. AWAREII [18] is a similar study that is expected to finish this year (2020). It is interesting to see how its results will come out.

So far, however, our present understanding of the neural basis of consciousness has been vindicated in clinical and experimental settings all around the world every day for a long time. It predicts that no conscious experiences, complex thoughts, or complex emotions can occur when one is in coma (clinically unresponsive, with absent or minimal brainstem reflexes, and with severely abnormal EEG), and no definite incidents of conscious experiences during coma have ever been reported. 

References.


Moody, R. A. (1975). Life After Life. New York, NY: Bantam Press.
Kopel J. Near-death experiences in medicine. Proc (Bayl Univ Med Cent). 2019 Jan;32(1):163-164. doi: 10.1080/08998280.2018.1542478.
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Charland-Verville V, Jourdan JP, Thonnard M, Ledoux D, Donneau AF, Quertemont E, et al. Near-death experiences in non-life-threatening events and coma of different etiologies. Front Hum Neurosci. 2014;8:203. doi: 10.3389/fnhum.2014.00203. 
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van Lommel P. Near-death experiences: The experience of the self as real and not as an illusion. Ann N Y Acad Sci. 2011 Oct;1234:19-28. doi: 10.1111/j.1749-6632.2011.06080.x. 
Martial C, Cassol H, Charland-Verville V, Pallavicini C, Sanz C, Zamberlan F, et al. Neurochemical models of near-death experiences: A large-scale study based on the semantic similarity of written reports. Conscious Cogn. 2019 Mar;69:52-69. doi: 10.1016/j.concog.2019.01.011. 
Gordon WV, Shonin E, Dunn TJ, Sheffield D, Garcia-Campayo J, Griffiths MD. Meditation-induced near-death experiences: A 3-year longitudinal study. Mindfulness (N Y). 2018;9(6): 1794-1806. doi: 10.1007/s12671-018-0922-3. 
Hoepner R, Labudda K, May TW, Schoendienst M, Woermann FG, Bien CG, et al. Ictal autoscopic phenomena and near death experiences: A study of five patients with ictal autoscopies. J Neurol. 2013 Mar;260(3):742-749. doi: 10.1007/s00415-012-6689-x. 
Timmermann C, Roseman L, Williams L, Erritzoe D, Martial C, Cassol H, et al. DMT Models the Near-Death Experience. Front Psychol. 2018 Aug 15;9:1424. doi: 10.3389/fpsyg.2018.01424. 
Blanke O, Thut G. Chapter 26. Inducing Out-of-body Experience. In: oup–Tall Tales about Mind and Brain (Typeset by SPi, Delhi). 2006 Jul 12.
Borjigin J, Lee U, Liu T, Pal D, Huff S, Klarr D, et al. Surge of neurophysiological coherence and connectivity in the dying brain. Proc Natl Acad Sci U S A. 2013 Aug 27;110(35):14432-7. doi: 10.1073/pnas.1308285110. /
Parnia S, Spearpoint K, de Vos G, Fenwick P, Goldberg D, Yang J, et al. AWARE - AWAreness during REsuscitation-a prospective study. Resuscitation. 2014 Dec;85(12):1799-1805. doi: http://dx.doi.org/10.1016/j.resuscitation.2014.09.004. 
AWARE II. NHS Health Research Authority.