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Sunday, April 3, 2022

Enhanced Interplay of Neuronal Coherence and Coupling in the Dying Human Brain

R. Vicente, M. Rizzuto, et al. 
Front. Aging Neurosci., 22 February 2022


The neurophysiological footprint of brain activity after cardiac arrest and during near-death experience (NDE) is not well understood. Although a hypoactive state of brain activity has been assumed, experimental animal studies have shown increased activity after cardiac arrest, particularly in the gamma-band, resulting from hypercapnia prior to and cessation of cerebral blood flow after cardiac arrest. No study has yet investigated this matter in humans. Here, we present continuous electroencephalography (EEG) recording from a dying human brain, obtained from an 87-year-old patient undergoing cardiac arrest after traumatic subdural hematoma. An increase of absolute power in gamma activity in the narrow and broad bands and a decrease in theta power is seen after suppression of bilateral hemispheric responses. After cardiac arrest, delta, beta, alpha and gamma power were decreased but a higher percentage of relative gamma power was observed when compared to the interictal interval. Cross-frequency coupling revealed modulation of left-hemispheric gamma activity by alpha and theta rhythms across all windows, even after cessation of cerebral blood flow. The strongest coupling is observed for narrow- and broad-band gamma activity by the alpha waves during left-sided suppression and after cardiac arrest. Albeit the influence of neuronal injury and swelling, our data provide the first evidence from the dying human brain in a non-experimental, real-life acute care clinical setting and advocate that the human brain may possess the capability to generate coordinated activity during the near-death period.

From the Discussion

The findings we report here are similar to the alterations in neuronal activity that have been observed in rodents, where an increase of low gamma band frequencies was observed 10–30 s after cardiac arrest (Borjigin et al., 2013). Our data reveals enhanced relative gamma power compared to other bands along with a decrease in theta. An interesting difference between the two studies can be observed when comparing phase-amplitude coupling (cross-frequency coupling): Post cardiac arrest, delta, theta, and alpha modulate low gamma activity in the rodent (Borjigin et al., 2013), whereas in the human brain, such modulation occurs in all gamma bands and is mostly mediated by alpha waves, to a lesser degree by theta rhythms. The alpha band is thought to critically interfere in cognitive processes by inhibiting networks that are irrelevant or disruptive (Klimesch, 2012). Given that cross-coupling between alpha and gamma activity is involved in cognitive processes and memory recall in healthy subjects, it is intriguing to speculate that such activity could support a last “recall of life” that may take place in the near-death state. Unlike previous reports, our study is the first to use full EEG placement, which allows a more complete neurophysiological analysis in a larger dimension. Further, the data was obtained from an acutely deteriorating patient. Previous human reports were limited to frontal cortex EEG signals that were analyzed by neuromonitoring devices, which may have captured artifacts and the focus was set on critically ill patients in chronic settings (Chawla et al., 2009, 2017). In line with our findings, electrical surges were also reported in these studies after cessation of blood circulation.