The brain’s swan song: hyperactivity near death
We often think of death as flipping a switch: one minute you’re there, next all lights go out. But this is a simple caricature of the dying process: sparks of activity still linger in the brains of those undergoing cardiac arrest, in whom both breath and heartbeat flutter and abruptly halt. Researchers have long thought that these sad, sparse bouts of activity characterize the brain’s descent into permanent unconsciousness. However, a new study suggests that the complete opposite – a surge of heightened connectivity – paradoxically marks the final step towards death. Although a long (and I mean LOOOONG!) stretch, the authors propose that the observation may partially underlie the enigmatic near-death experience (NDE).
Reports of NDE are nothing new. The luckily revived few often re-emerge from “the other side” with realer-than-real stories of long tunnels, intensely vivid visions and meetings with those bygone. NDEs are treated by some as proof of an afterlife, or by others, the existence of a “mind” beyond the brain and body. Spiritual connotations aside, the biological underpinnings remain mysterious, although abnormal dopamine and glutamate transmission may be involved (and probably everything else - the brain IS dying!). Here, the authors turned the focus away from individual neurotransmitters, and instead asked: after the heart stops, what happens to the oscillating waves of neural activity in the brain?
Jimo Borjigin et al. 2013. Surge of neurophysiological coherence and connectivity in the dying brain. PNAS. doi: 10.1073/pnas.1308285110
Researchers fitted 9 rats with electrodes to measure their brain waves – rhythmic brain activity generated by feedback connections between large numbers of neurons that differ in frequency. Alpha activity, for example, is often detected during relaxed wakefulness, while the faster theta activity is linked to cognitive processing. Gamma waves - the most recently discovered component - are particularly interesting to cognitive neuroscientists (and pseudo-science marketers) studying consciousness.
Why? The low gamma band, oscillating at 25-55Hz, has long been linked to visual consciousness, or the perception and awareness of visual stimulation. It seems to promote associative learning, and is also present during REM sleep (and slow wave sleep/deep sleep as well), which involves dreaming and complex visuals. Gamma bands also appear during transcendental mental states, as measured in Tibetan monks told to generate feelings of compassion as they meditated. Some even propose that gamma bands are behind the heightened sense of consciousness and bliss following a meditative bout. Sounds pretty magical, eh? As things goes, it's also a tough band to measure with EEG - in fact, there are even skeptics who doubt its existence.
Back to the study. After fitting rats with electrodes, researchers monitored changes in each brain wave component as the rats passed through three states: awake, under ketamine-induced anesthesia and after cardiac arrest. Unsurprisingly, after the loss of heartbeat and oxygen flow, the strength (“power”) of all brain wave frequencies measured tanked – except for low gamma bands, which spiked in power and became the dominant frequency in the spectrum as you can see above.
After cardiac arrest, gamma waves also showed higher levels of synchrony – that is, the neural activity in various brain regions became more “in tune”, even compared to an awake state. This high level of coherence between different brain regions is often associated with a highly “aroused” brain – that is, a state in which high levels of information processing may occur. Thus the authors concluded that the brain might exist in a hyper-conscious state for tens of seconds after the heart stops.
Sounds a bit too philosophical? I feel you. Where to start? First, the data really doesn’t tell us much. We already know that for a brief time following clinical death (which will most likely be redefined in the future), the brain remains active – so that’s nothing new. The increase in gamma wave power and synchrony is intriguing, especially since it appeared in all 9 rats (but really, just 9?), and the magnitude of the changes were large. But to link those changes to hyper-consciousness (what does that even mean?) and near-death experiences (NDEs) is going a step too far.
For one, there is absolutely no direct proof that gamma waves reflect NDEs. It has never been recorded in people there-and-back-again. While it’s true that high power gamma activity is often measured during conscious brain activity (and dreaming), its presence does not "lead to" conscious perception. Hence we can’t conclude, for example, that the rats were experiencing heightened awareness like NDEs – if they even have the ability to - because they show increased gamma oscillation. Along the same lines, higher gamma activity in the visual cortex does not necessarily mean there is more visual awareness and sensation. It may let you watch your life flash before your eyes, or it might just be a random quirk in the brain before all lights go out.
I'm not bashing research on consciousness. I just dislike interpretations that take data completely out of the realm of scientific discussion. I'd perk up if the authors repeated this experiment on people who have undergone cardiac arrest and experienced NDEs, and found the same pattern of changes in gamma waves. But even then it wouldn’t really tell us much. Now if only we had the ability to experimentally manipulate gamma (or any other) bands and "implant" an NDE in those still alive...
Note: I'd love for EEG experts out there pitch in. How hard is it to measure and isolate gamma band from noise? What conclusions (if any) would you make out of this study?
Borjigin J, Lee U, Liu T, Pal D, Huff S, Klarr D, Sloboda J, Hernandez J, Wang MM, & Mashour GA (2013). Surge of neurophysiological coherence and connectivity in the dying brain. Proceedings of the National Academy of Sciences of the United States of America PMID: 23940340