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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. The raw EEG (i.e., ‘brain waves') for each area is shown in the Figure below. On top (Panel A), the recording ranges from about 1hr before death to 30mins afterwards. At this coarse time scale you can basically see a sudden decrease in brain activity after cardiac arrest - everything seems to flatline at the moment of death. However, if we now zoom in on the moment just after death (Panels B and C below), 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 and conscious awareness.
In this study, the neuroscientists distinguish four distinct stages of brain death. Cardiac arrest stage 1 (CAS1) reflects the time (~4 seconds) between the last regular heartbeat and the loss of a oxygenated blood pulse (i.e. clinical death). The next stage (CAS2) lasts about 6 seconds, and ends with a burst in low-frequency brain waves (so-called 'delta blip'). The third death stage, CAS3, lasts approximately 20 seconds at which point there is no more evidence of meaningful brain activity at the final stage, CAS4.
These stages seem to reflect an organized series of distinct brain states, rather than a gradual fade out of brain activity. First, we see a sudden transition from the anaesthetised state with an increase in fast brain waves. It is as if the brain is suddenly shaken from the effects of anaesthesia at the moment of death. Next, brain activity settles into a period of slower brain waves during CAS2. Perhaps most surprisingly, recordings are then dominated in CAS3 by brain waves more commonly associated with normal wakefulness during life (so-called gamma activity). In further analyses, the researchers also show that this ‘afterlife' brain activity is also highly coordinated across brain areas and different wavelengths. These are the neural hallmarks of high-level cognitive activity. In sum, these data suggests that long after clinical death, the brain enters a brief state of heightened activity that is normally associated with wakeful consciousness.
Heightened awareness just after death
Interestingly, the authors even suggest that the level of activity observed during the final active death stage (CAS3) not only resembles the waking state, but might even reflect a heightened state of conscious awareness similar to the "highly lucid and realer-than-real mental experiences reported by near-death survivors". This is a pretty bold claim that critically depends on their quantification of 'consciousness'. They argue that in the final stage of brain death there is actually more evidence for consciousness-related activity than during normal wakeful consciousness. But how can we quantity ‘consciousness-related activity'?
To date, there is no handy index of consciousness that we can use to infer the true state of awareness. And even if we could derive such a consciousness metric in humans (see here), how could we relate it to the rodent experience? Research in animals can only ever hint at human experience, including near-death experiences.
Nevertheless, as the authors note, this research demonstrates a surge in brain activity after death that is consistent with active cognitive processing. This suggests 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. Then 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.
[this post is adapted from Death Wave at The Brain Box]
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
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National Geographic
The Independent
