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| Near-death Experience (Wiki Commons) |
First, to put this research into context, death-related brain activity was examined in rats, not humans. For obvious reasons, it is easier to study the death process in animals rather than humans. In this study, nine rats were implanted with electrodes in various brain regions, anaesthetised then 'euthanized' (i.e., killed). The exact moment of death was identified as the last regular heartbeat (clinical death). Electroencephalogram (EEG) was recorded during normal waking phase, anaesthesia and after cardiac arrest (i.e., after death) from right and left frontal (RF/LF), parietal (RP/LP) and occipital (RO/LO) cortex (see Figure below). Data shown in panel A ranges from about 1hr before death to 30mins afterwards. At this coarse scale you can see some patterns in the waking data that generally reflect high frequency brain activity (gamma band, >40Hz). During anaesthesia, activity becomes synchronised at lower frequency bands (especially delta band: 0.1–5 Hz), but everything seems to flatline after cardiac arrest. However, if we now zoom in on the moment just after death (Panels B and C), we can see that the death process actually involves a sequence of structured stages, including a surge of high-frequency brain activity that is normally associated with wakefulness.
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| Adapted from Fig 1 of Borjogin et al. (2013) |
In the figure above, Panel B shows brain activity zoomed in at 30min after death, and Panel C provides an even closer view, with activity from each brain area overlaid in a different colour. The authors distinguish four distinct cardiac arrest stages (CAS). CAS1 reflects the time between the last regular heartbeat and the loss of oxygenated blood pulse (mean duration ~4 seconds). The next stage, CAS2 (~6 seconds duration) ended with a burst in delta waves (so-called 'delta blip' ~1.7 seconds duration), and CAS3 (~20 seconds duration) continued until there was no more evidence of meaningful brain activity (i.e., CAS4 >30mins duration). These stages reflect an organized series of brain states. First, activity during CAS1 transitions from the anaesthetised state with an increase in high-frequency activity (~130Hz) across all brain areas. Next, activity settles into a period of low-frequency brain waves during CAS2. Perhaps most surprisingly, during CAS3 recordings were dominated by mid-range gamma activity (brain waves ~35-50Hz). In further analyses, they also demonstrate that this post-mortem brain activity is also highly coordinated across brain areas and different frequency bands. These are the hallmarks of high-level cognitive activity. In sum, these data suggests that long after death, the brain enters a brief state of heightened activity that is normally associated with wakeful consciousness.
Heightened awareness just after death
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| Adapted from Fig 2 of Borjogin et al. (2013) |
Nevertheless, as the authors note, this research certainly demonstrates that activity in the brain is consistent with active cognitive processing. The results demonstrate that a neural explanation for these experiences is at least plausible. They have identified the right kind of brain activity for a neural explanation of near-death experiences, yet it remains to be verified whether these signatures do actually relate directly to the subjective experience.
Future directions: The obvious next step is to test weather similar patterns of brain activity are observed in humans after clinical death. Next, it will be important to show that such activity is strongly coupled to near-death experience. For example, does the presence or absence of such activity predict whether or not the person would report a near death experience. This second step is obviously fraught with technical and ethical challenges (think: The Flatliners), but would provide good evidence to link the neural phenomena to the phenomenal experience.
Key Reference:
Borjigin, Lee, Liu, Pal, Huff, Klarr, Sloboda, Hernandez, Wang & Mashour (2013) Surge of neurophysiological coherence and connectivity in the dying brain. PNAS
Related references:
Tononi G (2012) Integrated information theory of consciousness: An updated account. Arch Ital Biol 150(2-3):56–90.
Auyong DB, et al. (2010) Processed electroencephalogram during donation after
cardiac death. Anesth Analg 110(5):1428–1432
Related blogs and news articles:
BBC News
Headquarters Hosted by the Guardian
National Geographic
The Independent
Great summary, Mark. But I'm not sure how this highly synchronized low gamma activity can index "heightened conscious processing" in deeply anesthetized dying rats. Do they emerge from their state of ketamine/xylazine anesthesia ~10 sec after their hearts stop into a conscious state capable of organized thought? Or is it just an artifact of massive calcium influx, as Sam Parnia said in an interview with Ed Yong?
ReplyDeleteIt's fascinating, potentially profound work -- what could be more profound than studying the process of brain death? But I think the authors are too eager to ascribe phenomenological descriptions to brain activity that appears abnormal -- so abnormal that it might be artifactual/epiphenomenal.
A final point: nine out of nine rats followed the same stereotyped sequence of events, which suggests that all humans should experience NDEs when dying of cardiac arrest, which isn't the case in stories told by survivors (as pointed out by Parnia in Ed Yong's piece).
Thanks Neurocritic for your comment. I agree that the neural effects could be related to calcium influx, or other neurophysiological processes associated with cessation of blood supply. But presumably this could also give rise to a phenomenal experience? TMS to visual cortex is pretty much an artefact, but gives rise to a phosphene. At face value, it seems that these results provide evidence that the kind of activity required for conscious experience is present after clinical death. The real test will be relating to it to humans, and to actual NDEs.
ReplyDeleteI also agree that anaesthesia complicates things, but it is interesting that the same effects are evident for different modes of death. Finally, as Parnia notes in Ed’s piece, the highly stereotyped effect in rats does not seem to square with individual differences in NDEs reported by humans, which could potentially indicate an artefact. But it is also possible that this apparent neural death throe (via Ca++ or other process) results in a reflexive burst of activity throughout the system, which could drive some kind of experience that might remain available to consciousness if you were lucky enough to be revived. People might vary in their ability to recall across this transition, just as some people have trouble remembering their dreams. Presumably such experiences are highly vulnerable to major state transitions - sleep to wake should be easier than death to resurrection!
Obviously a lot more work needs to be done, but it certainly seems to be an important step towards characterising how the brain transitions from life to death. As always, beware the hype that surrounds stories like these. The more profound the topic, the more silly things seem to be said…