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The question of consciousness after death is a common theme in society and culture in the context of life after death. Scientific research has established that the mind and normal waking consciousness are closely connected with the physiological functioning of the brain, the cessation of which defines brain death. However, many people believe in some form of life after death, which is a feature of many religions.
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There are multiple lines of evidence that support the hypothesis that the functioning of the brain causes the phenomenon of mind; besides the general correlations between brain activity and mental activity that can be observed and documented through functional imaging, the emprical evidence also comes from the affects that manipulations and damages to the brain have on the mind, which strongly suggest that the brain processes in question are not just mere correlations, but the physical basis or causes of mental phenomena. The reasoning behind this is based on manipulablity or interventionist theories of causation.
What follows are some examples of the evidence for the scientific perspective above. 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. Visual perception is handled by the occipital lobe, which once damaged often leads to blindness (see cortical visual impairment). Various different mental defects have been documented.
Psychopharmaceuticals can be used to temporarily alter perception, mood, consciousness, cognition, and behavior through manipulation of neurotransmission. The evidence for a causal relationship between brain activity and mental activity is further supported by data from the effects of electrical stimulations on the brain, for example scientist Steven Pinker writes that electrical stimulation during surgery can cause a person to have hallucinations that are indistinguishable from reality, such as a song playing in the room or a childhood birthday party. (In addition, see transcranial magnetic stimulation, optogenetics, lesion, and cortical stimulation mapping).
In addition to all of the above, 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, and is lost and recovered sporadically in the course of their lifespan—during deep sleep, syncope and sometimes coma, because of a lack of communication between cerebral neurons. Consciousness depends on the integration and functioning of several interconnected networks in the brain, two that are most important are the cerebral cortex—the gray matter that covers the outer layer of the brain, and the other is a structure located in the brainstem, called the reticular activating system (RAS) which keeps the cortex "activated" and aroused by encouraging increased brain activity which is needed in order for the brain to process information in such a way that can give rise to awareness. (for more information see neural correlates of consciousness and disorders of consciousness).
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. And during brain death, all brain function halts permanently. As characteristic of all biological cells, brain cells die once deprived of oxygenated blood, destroying the brain. According to the current mainstream neuroscientific view, the mind fails to survive brain death and ceases to exist.
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.
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, Steven Novella 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.