Abstract
Near-death experiences (NDE) are episodes of enhanced perception with impending death, which have been associated with increased high-frequency (13–100 Hz) synchronization of neuronal activity, which is implicated in cognitive processes like perception, attention and memory. To test whether the NDE-associated high-frequency oscillations surge is related to cardiac arrest, recordings were made from the hippocampus of anesthetized rats dying from an overdose of the sedative chloral hydrate (CH). At a lethal dose, CH caused a surge in beta band power in CA3 and CA1 and a surge in gamma band power in CA1. CH increased the inter-regional coherence of high-frequency oscillations within and between hippocampi. Whereas the surge in beta power developed at non-lethal chloral hydrate doses, the surge in gamma power was specific for impending death. In contrast, CH strongly suppressed theta band power in both CA1 and CA3 and reduced inter-regional coherence in the theta band. The simultaneously recorded electrocardiogram showed a small decrease in heart rate but no change in waveform during the high-frequency oscillation surge, with cardiac arrest only developing after the cessation of breathing and collapse of all oscillatory activity. These results demonstrate that the high-frequency oscillation surge just before death is not limited to cardiac arrest and that especially the increase in gamma synchronization in CA1 may contribute to NDE observed both with and without cardiac arrest.
Highlights
Near-death experiences (NDE) are “intense psychological experience of debated nature, characterized by an atypical state of consciousness occurring during an episode of apparent unconsciousness and usually in life-threatening conditions” (Palmieri et al, 2014)
To test whether the increase in high-frequency oscillations induced by cardiac arrest (Borjigin et al, 2013) is dependent on the method of death, we examined whether similar EEG changes would happen during death by an overdose of chloral hydrate (CH)
In this study we found that death induced by an overdose of CH was associated with hyper-synchronization in beta/gamma bands of hippocampal CA1 and CA3 areas, just before cardiovascular death, confirming that the high-frequency oscillation surge is not dependent on method of death and may well underlie NDEs
Summary
Near-death experiences (NDE) are “intense psychological experience of debated nature, characterized by an atypical state of consciousness occurring during an episode of apparent unconsciousness and usually in life-threatening conditions” (Palmieri et al, 2014). In anesthetized rats killed by high potassiuminduced cardiac arrest a similar increase in gamma band (γ, 30–100 Hz) frequency oscillations was reported, which was suggested to reflect a heightened state of conscious awareness (Borjigin et al, 2013). Synchronization of neuronal activity takes place in various brain areas in different frequency bands: theta (θ: 3–8 Hz), alpha (α: 8–13 Hz, most prominent in the visual cortex), beta (β: 13–30 Hz) and γ. These neural oscillations provide temporal frame for the information processing related to the perception and memory (Mizuseki et al, 2009; Amemiya and Redish, 2018). Loss of consciousness, e.g., under isoflurane anesthesia is associated with disruption of high-frequency (>13 Hz) oscillations and inter-regional coherence (Imas et al, 2005, 2006) and sedation by barbiturates is linked to increase in β oscillation power (Akeju and Brown, 2017)
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