In my last post I linked to a study of electrical activity in the brains of lab rats undergoing cardiac arrest. The study's authors suggest that this end-of-life electrical surge may help to explain near-death experiences.
I've since had a chance to read the study and some critical arguments and counterarguments more carefully. I find this whole area of research very interesting, and it does seem to me that this study offers the first possibility of a naturalistic explanation for some aspects (though not all aspects) of NDEs.
It's worth noting that related work has been carried out by a a different group of researchers, who have recorded electrical activity in human patients at the point of death. Although the methodology is different, making direct comparisons problematic, this research reportedly has found that "about half of patients who succumb in the intensive care unit display an ELES [end-of-life electrical surge], and the waveform is at a higher frequency than that observed in animals." So there appears to be at least preliminary support for the idea that this phenomenon is not limited to rodents, but extends to human beings.
In a published letter questioning the relevance of the study to NDEs, Bruce Greyson, Edward F. Kelly, and W.J. Ross Dunseath make several points. First, they say that no one knows the rat's state of subjective experience at the point of death. This is true, but in my opinion, irrelevant. The argument is not that rats are having NDEs, but that the electrical surge in the mammalian brain, when combined with the subjective, self-aware consciousness apparently unique to humans, can result in an NDE.
Next, Greyson and his colleagues argue* that
the activity observed following cardiac arrest represents a tiny fraction of the total neuroelectric power present just before arrest ... and thus it is misleading to describe these rat brains as being “hyperaroused.” All that can be concluded is that activity of unknown functional significance occurred at a few places in the EEG frequency spectrum in the context of near-total obliteration of activity accompanying the waking state.
This point, however, is effectively countered by the study's authors in a published reply. The authors - Jimo Borjigin, Michael M. Wang, and George A. Mashour - write:
Greyson et al. state that it is misleading to describe the rat brain as hyperaroused because the EEG power at near death was only a small fraction of that of the waking state in our study. However, they neglect to note that we showed that the power of EEG signals associated with consciousness increased in every rat we tested ... Our entire study is devoted to demonstrating the presence of electrical fingerprints of consciousness in the near-death brain. We report increased power and global synchrony in the gamma bandwidth, two neurophysiologic features associated with conscious processing. Moreover, this gamma band exhibits an eightfold increase in top-down information processing (thought to be a key element of consciousness) and fivefold increase in bottom-up information flow (thought to represent sensory information processing) at near death. In addition, we found tight coupling of gamma bands with both theta and alpha bands, yet another indicator of conscious information processing in the postarrest brain.
This is my own reading of the study, as well - though admittedly I'm far from being a neuroscientist! Still, it seems clear from their research paper that Borjigin et al. were specifically investigating the presumed mechanisms of conscious awareness:
Neuronal oscillations in the gamma range (>25 Hz) have been associated with waking consciousness, altered states of consciousness during meditation, and rapid eye movement sleep. Conscious perception is associated not only with an increase in gamma power, but also with long-range synchronization of gamma (30–80 Hz) oscillations. Furthermore, current perspectives on the neural correlates of consciousness suggest that cortical and thalamocortical effective connectivity is critical for conscious processing ... In addition to gamma waves, theta rhythms also play an important role in information processing in the brain. Theta oscillations are important for synaptic plasticity, information coding, and working memory. Studies have shown that the theta band increases in power during both verbal and spatial memory tasks in the mammalian cortex and is a feature of cognitive control across species. Increased theta power has also been detected in human subjects during meditation. Recent studies suggest that cross-frequency coupling between theta and gamma rhythms may play a functional role in long-range neuronal communication, perception, and memory tasks. These findings prompted us to examine electrical oscillations of the brain by focusing on EEG power, global EEG coherence, directional brain connectivity, and cross-frequency coupling of gamma and theta waves during waking, anesthesia, and following cardiac arrest.
So they were indeed focusing on aspects of neurological activity that are currently believed to play key roles in consciousness. What did they find?
In all nine of the test animals, the initial phase of cardiac arrest "exhibited elevated gamma oscillations near 130 Hz in all six EEG channels." This lasted for about 4 seconds on average. The next pahse "exhibited theta oscillations mixed with high-frequency gamma oscillations across all channels." This lasted for about 6 seconds on average. After a 1.7 second "delta blip," the third major stage commenced, lasting about 20 seconds and "dominated by EEG signals in the low-gamma frequency range (30-50 Hz) that were highly synchronous and appeared to be coupled to theta oscillations across all EEG channels." After that, brain activity faded out.
Summing up, the authors write:
High-gamma waves near 130 Hz increased in power, whereas other frequency bands did not change during [the first phase of cardiac arrest] ... During [the second phase,] lower-frequency bands showed marked reduction in power, except for a narrow band of theta that persisted for the entire [second phase] ... The [third phase] was distinguished by a clear increase in absolute power of low gamma and associated with persistent theta bands ... Compared with waking and anesthetized states, the [third phase] was associated with dramatically reduced EEG power at all frequencies except the low-gamma ... bands, which exhibited a significant increase.
Moreover, "in addition to the increase of gamma power, a large increase of mean coherence for gamma oscillations was detected at near-death ... The global coherence of low-gamma waves at near-death exhibited a more than twofold increase during [the third phase] relative to waking and anesthetized states ... Theta oscillations [during the third phase] also exhibited increased mean coherence compared with anesthesia and waking states ... Our results clearly indicate that mammalian brain activities become transiently and highly synchronized at near-death."
And then there's the issue of "feedback connectivity, reflecting causal information transfer." Remarkably, "the levels of connectivity for all rats at near-death were nearly as high as waking for all frequency bands (except the delta bands) and significantly higher than anesthesia ... Low-gamma waves exhibited the highest directed connectivity levels at near-death: fivefold higher in the feedforward and eightfold higher in the feedback direction compared with the waking state ... Thus, cardiac arrest induces a level of cortical directed connectivity in the near-death brain that far exceeds that observed during the waking state."
Again, I'm no neuroscientist, but it certainly sounds to me as if the study demonstrates that the near-death rats were indeed "hyperaroused" in precisely those regions and activities of the brain that are believed to be critical to consciousness.
As their third point, Greyson and his colleagues observe that monitoring of human patients during cardiac arrest has shown "a slowing and attenuation of EEG activity," not a surge. In rebuttal, Borjigin and his colleagues point out that human monitoring is done with scalp electrodes , while the rat study was done with intracranial electrodes. And as mentioned above, roughly half the human patients in a different clinical study reportedly have demonstrated an electrical surge at the point of death.
After these three objections, which strike me as rather weak, Greyson and his colleagues move to stronger ground, observing that many NDEs involve veridical observations of events that occurred well after the first 30 seconds of cardiac arrest (the only time period when an electrical surge could be detected); and that "about a quarter" of NDEs occur under general anesthesia, while the rat study indicated that brain activity under anesthesia bears little resemblance to the cardiac arrest activity.
The study's authors don't specifically respond to these points, other than to say that "other conditions that produce NDEs should be explored in future studies."
Although Borjigin et al. clearly feel they've made a breakthrough in explaining NDEs, I think the phenomenon will remain challenging as long as the veridical observations cannot be satisfactorily addressed, and as long as the brain surge that apparently occurs at death cannot be detected in patients under anesthesia (who also have NDEs). Still, the study does greatly complicate the now-standard argument that NDEs must be of extracerebral origin because the near-death brain is too medically compromised to generate structured thought. It now has to be admitted that the dying brain can briefly function at a surprisingly high level - a higher level, in some respects, than that associated with waking life.
It's also worth mentioning the phenomenon of terminal lucidity, in which dying patients have a last moment of surprising mental clarity after a long period of dementia or delirium. Could terminal lucidity be the result of an end-of-life electrical surge?
More important, could many aspects of the NDE (though not, presumably, the veridical observations) likewise be part of a narrative constructed by the brain in a last surge of energy? Borjigin and his colleagues write:
NDE represents a biological paradox that challenges our understanding of the brain and has been advocated as evidence for life after death and for a noncorporeal basis of human consciousness, based on the unsupported belief that the brain cannot possibly be the source of highly vivid and lucid conscious experiences during clinical death. By presenting evidence of highly organized brain activity and neurophysiologic features consistent with conscious processing at near-death, we now provide a scientific framework to begin to explain the highly lucid and realer-than-real mental experience reported by near-death survivors.
*In all quotations, footnotes have been omitted.