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The Ketamine Model of the Near Death Experience

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A Central Role for the NMDA Receptor


The Ketamine Model of the Near Death Experience:
A Central Role for the NMDA Receptor

Dr. Karl L. R. Jansen, MD, PhD, MRCPsych.
Psychiatrist
The Maudsley Hospital
Denmark Hill
London SE5 8AZ
United Kingdom.

K@BTInternet.com

Near-death experiences (NDE's) can be reproduced by ketamine via blockade of receptors in the brain (the N-methyl-D-aspartate, NMDA receptors) for the neurotransmitter glutamate. Conditions which precipitate NDE's (hypoxia, ischaemia, hypoglycaemia, temporal lobe epilepsy etc.) have been shown to release a flood of glutamate, overactivating NMDA receptors resulting in neuro ('excito') toxicity. Ketamine prevents this neurotoxicity. There are substances in the brain which bind to the same receptor site as ketamine. Conditions which trigger a glutamate flood may also trigger a flood of neuroprotective agents which bind to NMDA receptors to protect cells, leading to an altered state of consciousness like that produced by ketamine. This article extends and updates the theory proposed in 1990 (Jansen, 1990b).

The near-death experience (NDE) is a phenomenon of considerable importance to medicine, neuroscience, neurology, psychiatry, philosophy and religon (Stevenson and Greyson, 1979; Greyson and Stevenson, 1980; Ring, 1980; Sabom, 1982; Jansen, 1989a,b, 1990b). Unfortunately, some scientists have been deterred from conducting research upon the NDE by claims that NDE's are evidence for life after death, and sensationalist media reports which impart the air of a pseudoscience to NDE studies. Irrespective of religous beliefs, NDE's are not evidence for life after death on simple logical grounds: death is defined as the final, irreversible end. Anyone who 'returned' did not, by definition, die - although their mind, brain and body may have been in a very unusual state.

There is overwhelming evidence that 'mind' results from neuronal activity. The dramatic effects on the mind of adding hallucinogenic drugs to the brain, and the religous experiences which sometimes result, provide further evidence for this (Grinspoon and Bakalar, 1981). One of the many contradictions which 'after-lifers' can not resolve is that "the spirit rises out of the body leaving the brain behind, but somehow still incorporating neuronal functions such as sight, hearing, and proprioception" (Morse, 1989, original italics).

All features of a classic NDE can be reproduced by the intravenous administration of 50 - 100 mg of ketamine (Domino et al., 1965; Rumpf ,1969; Collier, 1972; Siegel,1978, 1980,1981; Stafford, 1977; Lilly, 1978; Grinspoon and Bakalar, 1981; White, 1982; Ghoniem et al., 1985; Sputz, 1989; Jansen, 1989a,b, 1990b, 1993). There is increasing evidence which suggests that the reproduction of NDE's by ketamine is unlikely to be a coincidence. This evidence includes the discovery of the major neuronal binding site for ketamine, known as the phencyclidine (PCP) binding site of the NMDA receptor (Thomson et al., 1985), the importance of NMDA receptors in the cerebral cortex, particularly in the temporal and frontal lobes, the key role of these sites in cognitive processing, memory, and perception, their role in epilepsy, psychoses, hypoxic/ischaemic and epileptic cell damage (excitotoxicity), the prevention of this damage by ketamine, the discovery of substances in the brain called 'endopsychosins' which bind to the same site as ketamine, and the role of ions such as magnesium and zinc in regulating the site (Anis et al., 1983; Quirion et al., 1984; Simon et al., 1984; Benveniste et al., 1984; Ben-Ari,1985; Thomson, 1986; Coan and Collingridge, 1987; Collingridge, 1987; Contreras et al., 1987; Rothman et al., 1987; Mody et al., 1987; Quirion et al., 1987; Westbrook and Mayer, 1987; Sonders et al., 1988; Barnes,1988; Choi,1988; Monaghan et al., 1989; Jansen et al., 1989a,b,c, 1990a,b,c, 1991a,b,c, 1993).

Characteristic Features of the Near-Death Experience

There is no internationally determined and agreed set of criteria which define the NDE, no list of 'research diagnostic criteria' similar to those provided by the American Psychiatric Association (APA) for psychiatric disorders. This lack has allowed some critics of neurobiological models to dismiss these models because some particular criterion which they believe to be important may not have been fully accounted for by the model being proposed, although it may well be that a consensus, statistical definition of the key features of the NDE would not include those features - just as, for example, the APA definition of schizophrenia (1980) represents an international consensus and avoids the sectarian views of a few, or inclusion of obscure cases which do not meet the general rule. For example, Gabbard and Twemlow (1989) argued that Saavedra -Aguilar and Gomez-Jeria's neurobiological hypothesis (1989), which was based on temporal lobe electrical abnormalities, did not have general validity because Gabbard and Twemlow had identified 5 cases in which hypoxia and stress did not appear to be a triggering factor (temporal lobe epilepsy, and many acute psychoses, can occur spontaneously without any apparent triggering factors). These cases are certainly not adequate grounds for the dismissal of neurobiological models.

Ketamine administered by intravenous injection, in appropriate dosage, is capable of reproducing all of the features of the NDE which have been commonly described in the most cited works in this field, and the following account is based upon these (Domino et al., 1965; Rumpf, 1969; Collier, 1972; Siegel,1978, 1980, 1981; Stafford, 1977; Lilly, 1978; Grinspoon and Bakalar, 1981; White, 1982; Ghoniem et al., 1985; Sputz, 1989; Jansen, 1989a, b,1990b, 1991c, 1993).

Important features of NDE's include a sense that what is experienced is 'real' and that one is actually dead, a sense of ineffability, timelessness, and feelings of calm and peace, although some cases have been frightening. There may be analgesia, apparent clarity of thought, a perception of separation from the body, and hallucinations of landscapes, beings such as 'angels', people including partners, parents, teachers and friends (who may be alive at the time), and religous and mythical figures. Transcendant mystical states are commonly described. Memories may emerge into consciousness, and are rarely organised into a 'life review' (Greyson, 1983).

Hearing noises during the initial part of the NDE has also been described (Noyes and Kletti, 1976a; Morse et al., 1985; Osis and Haraldsson, 1977; Greyson and Stevenson, 1980; Ring, 1980; Sabom, 1982). Ring (1980) classified NDE's on a 5 stage continuum: 1.feelings of peace and contentment; 2.a sense of detachment from the body; 3. entering a transitional world of darkness (rapid movements through tunnels: 'the tunnel experience'); 4. emerging into bright light; and 5. 'entering the light'. 60% experienced stage 1, but only 10% attained stage 5 (Ring, 1980). As might be expected in a mental state with a neurobiological origin, more mundane accounts also occur, e.g. children who may 'see' their schoolfellows rather than God and angels (Morse, 1985). It is clear that NDE's are not as homogeneous as some have claimed.

Ketamine and Phencyclidine

Ketamine is a short-acting, hallucinogenic, dissociative anaesthetic related to phencyclidine (PCP). Both drugs are arylcyclohexylamines - they are not opioids and are not related to LSD. In contrast to PCP, ketamine is relatively safe, an uncontrolled drug in most countries, and remains in use as an anaesthetic for children (White et al., 1982). Anaesthetists attempt to prevent patients from having NDE's (emergence phenomena) by the co-administration of benzodiazepines and other sedative substances which produce 'true' unconsciousness rather than dissociation (Reich and Silvay, 1989).

Ketamine produces an altered state of consciousness which is very different from that of the 'psychedelic' drugs such as LSD (Grinspoon and Bakalar, 1981). It can reproduce all features of the NDE, including travel through a dark tunnel into light, the conviction that one is dead, 'telepathic communion with God', hallucinations, out-of-body experiences and mystical states (see ketamine references above). If given intravenously, it has a short action with an abrupt end. Grinspoon and Bakalar (1981, p34) wrote of: '...becoming a disembodied mind or soul, dying and going to another world. Childhood events may also be re-lived. The loss of contact with ordinary reality and the sense of participation in another reality are more pronounced and less easily resisted than is usually the case with LSD. The dissociative experiences often seem so genuine that users are not sure that they have not actually left their bodies.'

A psychologist with experience of LSD described ketamine as 'experiments in voluntary death' (Leary, 1983, p375). Sputz (1989, p65) noted:'one infrequent ketamine user reported a classic near-death experience..."I was convinced I was dead. I was floating above my body. I reviewed all of the events of my life and saw a lot of areas where I could have done better". The psychiatrist Stanislav Grof stated: "If you have a full-blown experience of ketamine, you can never believe there is death or that death can possibly influence who you are" (Stevens, 1989, p481-482). 'Ketamine allows some patients to reason that ...the strange, unexpected intensity and unfamiliar dimension of their experience means they must have died..' (Collier, 1981, p552).

Attempts to explain NDE's as hallucinations are sometimes rejected by spiritualists because many persons insist upon the reality of their experiences (Osis and Haraldsson, 1977; Ring, 1980). However, 30% of normal subjects given ketamine were certain that they had not been dreaming or hallucinating, but that the events had really happened (Rumpf et al., 1969; see also Siegel, 1978). What is a hallucination ? " a hallucination has the immediate sense of reality of a true perception .....transient hallucinatory experiences are common in individuals without mental disorder" (APA, 1980). The apparently clear sensorium of some persons who have had NDE's has also been used to argue that the NDE is 'real' and not a hallucination (Osis and Haraldsson, 1977; Ring, 1980). It is thus important to note that hallucinations in schizophrenia typically occur in clear consciousness and are believed to be real (APA, 1980). A personal conviction of the 'reality' of an NDE does not invalidate scientific explanations. Some users of LSD have claimed that their minds are clearer than usual, and that the LSD world is real while the 'normal' world is a veil of illusion (Grinspoon and Bakalar, 1981). Cardiac arrest survivors have been reported as describing their resuscitation in detail (Sabom, 1982). Ketamine can permit sufficient sensory input to allow accounts of procedures during which the patient appeared wholly unconscious (Siegel, 1981).

Glutamate, NMDA and Sigma Receptors, and the Hippocampus

Most large neurones in the cerebral cortex use glutamate as their neurotransmitter. Glutamate, an excitatory amino acid, is central to the function of the hippocampus, temporal and frontal lobes (Cotman et al., 1987; Fagg and Foster, 1983; Greenamyre et al., 1984; Monaghan, Bridges and Cotman, 1989; Jansen et al., 1989c, 1990a) and plays a vital role in all cognitive processes involving the cerebral cortex, including thinking, memory and perception (Monaghan, Bridges and Cotman, 1989; Oye et al., 1992).

The major neuronal binding site for ketamine is called the PCP receptor, which is itself attached to the NMDA receptor (Monaghan, Bridges and Cotman, 1989). As they are part of the same macromolecular complex, the two terms are sometimes used interchangeably. It was formerly believed that the sigma and PCP sites were the same entity, but it is now clear that sigma receptors are very different, have a unique distribution in the CNS, and are not a form of opioid receptor (Walker et al., 1990; Jansen et al., 1991b) .

There was initially some debate as to whether the hallucinogenic properties of ketamine were due to NMDA or sigma receptors (Jansen, 1990b). These effects are now largely attributed to NMDA receptor blockade (Krystal et al., 1994). Sigma ligands with a high degree of specificity (e.g. (+)pentazocine) do not produce NDE's at doses where most of the binding is to sigma rather than NMDA and/or kappa opioid receptors (sigma receptor ligands frequently have affinity for NMDA and/or kappa opioid receptors at higher doses) (Musacchio et al., 1990; Walker et al., 1990).

When glutamate is present in excess, neurones die via a process called excitotoxicity. Conditions which have been proven to lead to excessive release of glutamate include hypoxia/ischaemia, epilepsy and hypoglycaemia (e.g. Rothman, 1984; Rothman and Olney, 1986, 1987). Blockade of PCP receptors prevents cell death from excitotoxicity (e.g. Rothman et al., 1987). The brain may thus have a protective mechanism against a glutamate flood: release of a counter-flood of substances which block PCP receptors, preventing neuronal death. Considering the sophistication of the brain's many known defences, and the vulnerability of neurones to hypoxia, a protective mechanism against excitotoxicity seems very likely. This is the only speculation in the process outlined above: the other statements are strongly supported by experimental evidence (Benveniste et al.,1984; Simon et al., 1984; Ben-Ari, 1985; King and Dingledine, 1986; Rothman et al., 1987; Westerberg et al., 1987; Hoyer and Nitsch, 1989). A peptide called a-endopsychosin, which binds to the PCP receptor, has been found in the brain (Quirion et al., 1984). Certain ions such as magnesium and zinc also act as endogenous PCP channel blockers (Thomson, 1986; Westbrook and Mayer, 1987; Cotman, Monaghan and Ganong, 1988), and it is possible that these ions are centrally involved in producing NDE's.

Scientific Hypotheses and NDE's

Claims that NDE's must have a single explanation (e.g. Ring, 1980), or that a scientific theory must explain all of the experiences ever given the name of NDE (e.g. Gabbard and Twemlow, 1989) are difficult to justify. It is well established that mental phenomena have multiple causes and variable expressions. The NDE is more likely to be the final common expression of several different causes. Even then, the final 'common' expression contains sufficient variability to suggest different types of NDE, for example in Ring's study (1980), only 10% 'enter the light'. A multi-levelled interpretation is thus the most useful. The glutamate hypothesis of the NDE is not intended to apply to every NDE, and is not necessarily incompatible with the theories described below.

Temporal Lobe Epilepsy

It has been claimed that there is some similarity between the phenomena experienced in temporal lobe epilepsy (TLE) and NDE's (Persinger and Makarec ,1987; Saavedra-Aguilar and Gomez-Jeria,1989). Glutamate is the key neurotransmitter in the temporal lobe, particularly in the hippocampus, and is implicated in epilepsy. The neuropathology of epilepsy is believed to result from excito-toxic cell death (Ben-Ari, 1985; King and Dingledine, 1986; Olney, Collins and Sloviter, 1986; Mody and Heinemann, 1987; Cotman, Monaghan and Ganong, 1988).

A neuroprotective system might become active in any excitotoxic situation including epilepsy. The degree of damage, and the mental state, resulting from a glutamate flood may depend on the final balance in each neuronal pathway between excito-toxic forces and neuroprotective mechanisms. Persons who were oxygen deprived for prolonged periods and had a profound NDE, sometimes survived the episode unimpaired (Sabom, 1982). The lack of apparent brain damage may result from a very effective mechanism for glutamatergic blockade in those individuals.

It is also possible that ketamine has its effects by mimicing some of the pathological processes seen in temporal lobe epilepsy. Even though ketamine blocks glutamatergic transmission, and prevents excitotoxic cell death, the effect of ketamine upon the human electroencephalograph (the EEG) suggests that it can be epileptogenic - the final result of ketamine acting in the brain is the result of a complex interplay of forces. There is a reduction in a wave activity, but b, d and q wave activity are increased (Schwartz et al. 1974; Pichlmayr et al., 1984). Ketamine acts both as an anticonvulsant (e.g. McCarthy et al., 1965; Celesia and Chen, 1974; Taberner, 1976; Leccese et al., 1986; Mares et al., 1992) and as a pro-convulsant (Bennet et al., 1973; Gourie et al., 1983; Myslobodsky, 1981). Myslobodsky (1981) reported that ketamine could produce epileptiform EEG patterns in human limbic and thalamic regions, but that there was no evidence that this affected other cortical regions or that fits were likely to occur. This is consistent with the NDE model presented by Saavedra-Aguilar and Gomez-Jeria (1989) involving limited electrical abnormalites in the limbic system. Thus production of NDE's by ketamine is not at odds with proposals that NDE's may result from abnormal electrical activity. Reich and Silvay (1989): " it is hard to draw objective conclusions regarding the anti-convulsant properties of ketamine...animal data are particularly difficult to interpret because of interspecies variations". Ketamine is probably anticonvulsant at NDE producing doses (Myslobodsky, 1981) suggesting that a PCP receptor blocker is released to produce the NDE.

A Flood of Endorphins

Carr (1981, 1989) proposed that NDE's resulted from a flood release of endogenous opioids (endorphins). It had been reported that survival time was increased by giving opiate antagonists (e.g. naloxone) in fatal circumstances (Holoday and Faden, 1978). More recently, a sudden increment of b-endorphin has been reported in the brain and body fluids of dogs who are 'conscious' at the moment of death (Sotelo et al., 1995). It is now known that a glutamate flood results in excitotoxic cell death in hypoxia/ischaemia and epilepsy (see above). However, glutamate is an amino acid. Endorphins are unlikely to produce NDE's as they are not potent dissociative hallucinogens (Oyama et al., 1980). Injection of b-endorphin into the CSF has analgesic effects lasting well over 22 hours (Oyama et al.,1980). This does not match the time course of a typical NDE which is relatively brief. Ketamine produces brief, deep analgesia (White et al., 1982) due to NMDA (PCP) receptor blockade ( e.g. Schouenberg and Sjolund, 1986; Parsons et al., 1988). The limited psychotomimetic properties of some opioids (e.g. (-) pentazocine) result from binding to k opioid receptors, and to PCP receptors at higher doses (Pfieffer et al., 1986; Mussachio et al., 1990). However, the effects of (-)pentazocine binding to k receptors, at doses which are relatively selective, are described as 'feelings of cheerfulness and strength' (Belville and Forrest, 1968), a description bearing no resemblance to the dramatic effects of ketamine or NDE's. With higher doses, more marked effects may appear as a result of binding to PCP receptors - but pentazocine is not an endorphin. Claims that sigma- selective (+)isomers of benzomorphan opiates have psychotomimetic effects are not generally supported by human trials, carried out in the 1960's, which demonstrated that it is the (-)isomers which have psychotomimetic properties - and these may prefer PCP receptors rather than sigma sites (review: Mussachio,1990). The naloxone-reversible component is due to k opioid receptor binding, while the naloxone insensitive component is due to PCP (i.e. NMDA) receptor binding, not sigma binding (Walker et al., 1990). The role of opioid receptors in ketamine effects is contoversial (Reich and Silvay, 1989). Naloxone could not reverse the effects of ketamine in humans (Amiot et al., 1985) and dogs (Vaupel, 1983). However, ketamine is supplied as a racemic mixture of (+)and (-) isomers. The controversy may be resolved by studying the separate effects of the isomers, and the doses at which these appear. As doses rise, drugs bind to a wider range of receptors. Ketamine can induce NDE's at doses about four times less than those required for anaesthesia (Stafford, 1977; Lilly, 1979; Grinspoon and Bakalar, 1981; Sputz, 1989). White et al. (1980) reported that it was (+)ketamine which has some opioid binding properties and which produced the most anaesthesia, while (-)ketamine produced more NDE's (described by anaesthetists as 'psychic emergence reactions'). White et al. (1985) went on to show that (+)ketamine is about four times more potent as a hypnotic and analgesic, and has different effects upon the EEG.

Saavedra-Aguilar and Gomez-Jeria (1989) cited animal experiments showing b-endorphin to be epileptogenic to support an argument that b-endorphins produce NDE's (e.g. McGinty et al., 1986; Henriksen et al., 1978). While b-endorphin may have had these effects within the rat paradigms used, opioids usually produce calming, inhibitory effects in humans - not excitation or states resembling epilepsy (Meltzer, 1987). Released peptides probably have protective functions rather than contributing further to excito-toxicity. The finding of Su, London and Jaffe (1988), that some steroids bind to sigma receptors, was cited to suggest that steroids could play a role in NDE's. However, the steroid was progesterone which is not a hallucinogen. Schwartz et al. (1989) reported that the affinity of progesterone for the sigma site is insufficient to result in significant receptor occupancy, except in pregnancy.

Hypoxia and Hypercarbia

1. Hypoxia: Blacher (1980) suggested that hypoxia induced NDE's. This has been criticised by some authors (Sabom, 1982) as studies involving a slow fall in inspired oxygen produced mental clouding rather than NDE's (Henderson et al., 1927). However, these studies are not an accurate model of events in, for example, cardiac arrest. Sudden hypoxia causes an excessive release of glutamate with resulting excitotoxicity, which can be prevented by ketamine (see previous references).

2. Hypercarbia: a CO2-enriched breathing mixture can result in typical NDE phenomena such as bodily detachment and the perception of being drawn towards a bright light. Diverse personality types produced broadly similar reports, suggesting a shared neurological substrate (Meduna, 1950).

Serotonin

Like endorphins, serotonergic effects may be contributory but are unlikely to play a central role in the NDE. Psychedelic drugs such as LSD are serotonergic in action and produce a mental state very different from NDE's . There is frequently an overwhelming increase in sensory input from the external environment (Grinspoon and Bakalar, 1981), in contrast to the dissociation produced by ketamine. Psychedelic visual phenomena bear little relationship to the dream-like images of ketamine and the NDE. 'Ego dissolution' experienced on LSD has a different quality from the conviction of having died which may arise with ketamine. Loss of contact with the external environment leading rapidly to the 'tunnel experience' is not a typical psychedelic drug effect, although it may occur.

Psychological

1. Depersonalisation: The NDE may be an adaptive mechanism which alerts one to the threat of death while potentially overwhelming emotion is held at bay. The reality can then be integrated without panic (Greyson, 1983; Noyes and Kletti, 1976a,b). This model is applicable when death is psychologically near as when falling from a cliff. While protecting nerve cells from excitotoxicity is then irrelevant, glutamate and NMDA receptors would be involved in producing the experience as they play a key role in cognition and perception.

2. Regression in the service of the ego: confronting death cuts off the external world resulting in regression to a pre-verbal level. This is experienced as mystical ineffability (Greyson, 1983). Losing contact with the external world is one of the most typical effects of ketamine. This is partially due to blockade of NMDA receptors involved in sensory transmission. NMDA receptors play a central role in the transmission of data from all sensory modalities (Davies and Watkins, 1983; Greenamyre et al., 1984; Headley et al., 1985; Cotman et al., 1987; Cline et al.,1987; Monaghan, Bridges and Cotman, 1988; Kisvardy et al., 1989; Oye et al., 1992).

3. State dependant reactivation of birth memories (Grof and Halifax, 1977). Movement through tunnels towards light may be a memory of being born : a 'near-birth experience'. NMDA receptor blockade could be the mechanism for such a reactivation of primitive memories.

4. Sensory deprivation: memories may normally be suppressed by a 'gate' which admits primarily external signals when we are fully conscious and concentrating upon an external task (Siegel,1980, 1981). If this input is dramatically reduced (e.g. by ketamine or a heart attack) in combination with central stimulation (e.g. by excessive glutamate release during hypoxia, epilepsy, or arising without external provocation), stored perceptions are released and become 'organised' into a meaningful experience by psychodynamic forces in the mind in question (Greyson, 1983). The 'white light' may result from CNS stimulation , and also a possible lowering of the phosphene perceptual threshold (Siegel,1980, 1981). Sensory deprivation can produce profound alterations in consciousness (Lilly, 1961,1978).

The hippocampus is the anatomical location of the 'memory gate' described above. NMDA receptors form the molecular substrate of the gate. NMDA receptors have their highest concentration in the hippocampus, a part of the medial temporal lobe where data from the external world is integrated with internal programs. The NMDA receptor plays an important role in learning, and in the formation and retrieval of memories. The PCP receptor is referred to as a 'gated channel'. Whether the gate is open or closed depends on the degree of excitation - specifically, the position of a magnesium ion in the channel. In simple terms, ketamine blocks this channel and closes the gate to incoming data (Monaghan, Bridges and Cotman, 1989; Morris et al., 1986; Collingridge, 1987; McNaughton and Morris, 1987; Cotman, Monaghan and Ganong, 1988).

Drug-induced hallucinations ?

Administered drugs may explain some cases of NDE's, but in most no drugs were given with effects of this nature (Sabom, 1982).

Conclusions

NDE's can be safely induced by ketamine, and the glutamate theory of the NDE can thus be investigated by experiment. Discoveries in neuroscience suggest a common origin for ketamine experiences and the NDE in events occuring at glutamatergic synapses, mediated by NMDA receptors via their PCP channel component. This hypothesis links most of the neurobiological and psychological theories (hypoxia, a peptide flood, temporal lobe electrical abnormalities, regression in the service of the ego, reactivation of birth memories, sensory deprivation etc.) rather than being an alternative to them. Most of the tenets of the hypothesis are strongly supported by experimental evidence which implicates glutamate and NMDA receptors in the processes which precipitate NDE's. The postulate that anti-excitotoxic agents can flood the brain remains to be clearly established.

Spiritualists have sometimes seen scientific explanations of NDE's as dull and reductionist. However, the exploration of the mind-brain interface is one of the most exciting adventures which humans have ever undertaken. The real reductionism lies in attempts to draw a mystical shroud over the NDE, and to belittle the substantial evidence in favour of an scientific explanation.

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