So today I found a neuroscience review dealing with a far-out topic: the neuroscience underlying near death experiences (NDE) and other “paranormal” phenomena such as experiences seeing bright lights, meeting the dead or being convinced that you are dead.

According to Mobbs and Watt (2011), approximately 3% of Americans declare that they have had a near-death experience, which typically involve the feeling that one’s soul has left the body, is approaching a bright light and/or goes to another blissful and peaceful state. The authors go on to suggest that NDE experiences are “the manifestations of normal brain function gone awry, during a traumatic, and sometimes harmless event.”

Although it is hard to study NDE experiences objectively, few scientific studies of NDE do exist. For example, patients of diabetes sometimes report NDE during episodes of hipoglycaemia that occur during the rapid-eye movement (REM) sleep cycle (Note: REM is also a common marker of dreaming.) Obviously, the patient is not in danger, yet they recount many of the classic features of NDE. In addition, Owen and colleagues reported that roughly half (51.7%) of the patients who recounted NDE in their study were not in medical danger or near death. So how is it that the mind can create the illusion of death or being near it? Below I will list the basic features of NDEs, as described by Mobs and Watt (2011).

Basic features of near death experiences (NDEs):

• An awareness of being dead: One of the most common symptoms reported in NDEs, but not limited to NDEs. An awareness of being dead also occurs in the Cotard or “walking corpse” syndrome and has been associated with parietal and prefrontal cortex and has been described following trauma as well as during advanced stages of multiple sclerosis and typhoid disease.
• Out of body experiences: These experiences are described as feelings that one is floating outside of the body. In some cases these involve “autoscopy” or seeing one’s body from above. These experiences are also common during sleep paralysis, a part of the REM sleep cycle that resulst in paralysis while the person is still aware of the external world. Wilder Penfield, a notorious neurosurgeon, argued that the true perception during out of body experiences has a neural basis. In support of this notion, scientists have demostrated that out of body experiences can be induced by stimulating the right temporoparietal junction (see work by Olaf Blanke and colleages). Moreover, the authors suggest that out of body experiences result from a failure of multisensory information integration from one’s body, resulting in the disruption of elements of self-representation.
• A tunnel of light: The perception that one is moving down a dark tunnel and surfacing into a world of light is also associated with NDEs. However, these experiences can be artificially induced as well. For example, pilots flying at G-force experience a similar phenomena called hypotensive syncope that causes tunnel-like peripehral to central visual loss to develop over 5-8 seconds. Importantly, others have suggested that visual activity during retinal ischemia, which happens when the blood and oxygen supply to the eye is depleted, may underlie the light of the end of the tunnel effect. The tunnel of light effect is also found in other visual disorders like glaucoma, which also results in loss of peripheral vision leading to tunnel vision that is associated with feelings of fear and oxygen loss (i.e. hypoxia). Finally, it is important to remember an important neuroscience concept: the organization of the visual cortex, which is divided into cells that process peripheral and central vision. Excitation of these cells will result in a central bright light and a dark periphery (tunnel effect).
• Meeting deceased people: Hallucinations and visions are also present in patients suffering from neurological disorders like Alzheimer’s and progressive Parkinson’s. Patients that have seen headless corpses or dead relatives have been linked to pallidotomy lesions. A role for abnormal dopamine functioning, a neurotransmitter implicated in dream and hallucinations, has been postulated. Intrestingly, some of the components of this feeling have been found to be artificially induced. For example, electrical stimulation of the area adjacent to the angular gyrus can result in feeling a sense of presence (of someone else). As mentioned before, vision plays an important role, as macular degeneration (damage to the center of the visual field) can also result in vivid visual hallucinations. A theory that has emerged from these findings is that these hallucinations may occur because of the compensatory over-activation of brain areas nearby the damaged area (which has been reported in other neurological illnesses) or by the action of these areas trying to make sense of the noise coming from the damaged area.
• Positive emotions: Feelings of pure bliss, euphoria and acceptance of death are also common in reports of NDEs. However, many medicinal and recreational drugs like ketamine, for example, can mimic these positive emotions and visions. At varying doses, ketamine (an NMDA antagonist that also binds to mu opioid receptors) can produce hallucinations, out of body experiences, euphoria and dissasociation. Finally, let’s not forget that reward and opioid systems may be modulated by environmental factors like the perception of threat or danger. Throughout evolution, these systems have come into play in order to aid the survival of the species. Presumably, these systems are also evoked and work differently when one is under the illusion of danger, such as in NDEs.

As summarized above, the study of the complex phenomena implicated in NDEs is barely understood. However, new theories arise that help account for some of these. Recent theories have implicated the basic arousal systems in the midbrain, especially the locus coeruleus (a key source of noradrenaline/norepinephrine) as a substrate for some of these phenomena. Moreover, noadrenaline has been linked to arousal related to fear, stress and is highly connected to limbic regions such as the amygdala an hippocampus. Other basic midbrain systems such as the periaqueductal gray and the ventral area have also been singled out as likely candidates (directly or indirectly) mediating other features of NDEs such as positive emotions and hallucinations.

But still, NDEs are just creepy.

Sources:

Mobbs and Watt. 2011. There is nothing paranormal about near-death experiences: how neuroscience can explain bright lights, meeting the dead, or being convinced you are one of them. Trends in Cognitive Sciences. 15 (10): 447-9.