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The question of consciousness after death is a common theme in society and culture. Across the ancient world, most ethnic groups held afterlife beliefs about preservation of consciousness after the death of the physical body. Following the advent of scientific method, consciousness has been associated with physiological function of the brain, whose cessation of function defines death. However, many people continue to believe in some form of consciousness after death, and this is a feature of many religious belief systems.
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According to neuropsychology, the mind or psyche, as well as consciousness and personality, is a product of the functioning brain. During brain death, all brain function halts permanently. The implication is that the mind fails to survive brain death and ceases to exist.
Individual cases in which brain functioning has been disrupted have been used to support this point of view. In the case of Phineas Gage, a 25-year-old man survived destruction of one or both frontal lobes by a projectile iron rod and went on to manifest pronounced changes in personality, suggesting a correlation between brain states and mental states. Similar examples abound; neuroscientist David Eagleman describes the case of another individual who exhibited escalating pedophilic tendencies at two different times, and in each case was found to have tumors growing in a particular part of his brain. The amygdala processes reactions to violations concerning personal space, and these reactions are absent in persons in whom the amygdala is damaged bilaterally. Monkey mothers who have amygdala damage show a reduction in maternal behaviors towards their infants, often physically abusing or neglecting them. The acquisition of memory and knowledge has a bio-chemical basis. Psychopharmaceuticals can be used to temporarily alter the mind through manipulation of neurotransmission. Visual perception is handled by the occipital lobe, which once damaged often leads to blindness (see cortical visual impairment). The insular and the cingulate cortices, especially, are involved in consciousness and self-awareness. Also, see lobotomy.
In addition, neuropsychology links mental development in individual organisms with brain development. In intelligent animals, human and non-human alike, consciousness as we know it begins to appear at neonatal stages, is lost and recovered sporadically in the course of their lifespan—during deep sleep, syncope and sometimes coma, as lack of communication between cerebral neurons—and is lost permanently once their brains are dead. Brain death is a chronic disorder of consciousness characterized by lack of response to external stimuli, lack of observed activity and lack of observed behavior.
In the process of clinical death, the heart stops working and pumping blood to the brain, thereby cutting the brain's essential supply of oxygen and of other less urgent nutrients. In dogs, measurable brain activity ends within 20 to 40 seconds. As characteristic of all biological cells, brain cells die once deprived of oxygenated blood, destroying the brain.
Some people who have undergone cardiopulmonary resuscitation report experiencing such sensations as detachment from the body, feelings of levitation, and the presence of a light, happening after cardiac arrest. These are commonly referred to as near-death experiences or NDEs by those who study such phenomenon. It has been suggested (e.g., by David Fontana) that NDEs indicate postmortem consciousness. Sam Parnia comments that consciousness isn't disrupted immediately after clinical death and describes the process of death as "essentially a global stroke of the brain. Therefore, like any stroke process one would not expect the entity of mind/consciousness to be lost immediately."
Stuart Hameroff and Deepak Chopra suggest that at death or during NDE, "it is conceivable that the quantum information which constitutes consciousness could shift to deeper planes and continue to exist purely in space-time geometry, outside the brain, distributed nonlocally", as a "quantum soul" apart from the body.
In 2008, academic neurosurgeon Eben Alexander had an NDE while attached to an electroencephalogram which demonstrated a total lack of neural activity. After resuscitation, he found he was able to identify the face of a deceased biological sister whom he had previously not known existed (he had been raised in an adoptive family). Prior to his NDE, Alexander had been non-religious. Afterwards, he gained a definite belief in the existence of an afterlife, and went on to write a book about his life-altering experience. Alexander states that the elaborate, highly detailed, and even prescient experiences he had while his brain activity was clinically non-existent provide definitive proof that consciousness can exist without the need for a functional brain.
Sam Harris, Oliver Sacks, and Michael Shermer have pointed out that it is not entirely clear that Dr. Alexander had the experience during coma. They argue that the experience could have occurred when he was returning from the coma, while his neocortex was coming back "online" and returning to full function. Further criticisms include contesting the claim that the coma led to complete neural inactivity.
Such near-death experiences have been described in medical journals as hallucinatory, and such prescient information supposedly gained from NDEs as merely coincidental and dubious. Ketamine, a dissociative hallucinogen, has been shown to replicate compounds of near-death experiences. Lucid dreaming too induces experiences quite similar to those of NDEs. The imagery in NDEs varies within cultures. Rick Strassman advanced the hypothesis that a massive release of the psychedelic dimethyltryptamine (DMT) from the pineal gland prior to death or near-death was the cause of the near-death experience phenomenon.