impacted their life. Another category, so-called distressing or unpleasant near-death experiences, has been investigated by Greyson and Bush. Explanatory models Explanatory models for the phenomenology and the elements of the NDE can, according to sources, be divided into a few broad categories: psychological, physiological, and transcendental. Christian Agrillo adopts a more parsimonious overview and notes that literature reports two main theoretical frameworks: (1) "biological/psychological" interpretation (in-brain theories), or (2) "survivalist" interpretation (out-of-brain theories). In a study published in 1990, Owens, Cook and Stevenson presented results that lent support to all of these three interpretations. Each model contains a number of variables that are often mentioned, or summarized, by commentators: Psychological theories have suggested that the NDE can be a consequence of mental and emotional reactions to the perceived threat of dying, or a result of expectation. Other psychological variables that are considered by researchers include: imagination; depersonalization; dissociation; proneness to fantasy; and the memory of being born. Physiological theories tend to focus on somatic, biological or pharmacological explanations for the NDE, often with an emphasis on the physiology of the brain. Variables that are considered, and often summarized by researchers, include: anoxia; cerebral hypoxia; hypercarbia; endorphins; serotonin or various neurotransmitters; temporal lobe dysfunction or seizures; the NMDA receptor; activation of the limbic system; drugs; retinal ischemia; and processes linked to rapid eye-movement (REM) sleep or phenomena generated on the border between sleep and wakefullness. A third model, sometimes called the transcendental explanation, considers a number of categories, often summarized by commentators, that usually fall outside the scope of physiological or psychological explanations. This explanatory model considers whether the NDE might be related to the existence of an afterlife; a changing state of consciousness; mystical (peak) experiences; or the concept of a mind-body separation. Several researchers in the field have expressed reservations towards explanations that are purely psychological or physiological. Van Lommel and colleagues have argued for the inclusion of transcendental categories as part of the explanatory framework. Other researchers, such as Parnia, Fenwick, and Greyson, have argued for an expanded discussion about the mind-brain relationship as well as the possibilities of human consciousness. Research – history and background Precursors - parapsychology thanatology and autobiographical accounts Individual cases of NDEs in literature have been identified into ancient times. In more recent times the topic of near-death phenomena was a part of the investigation of paranormal phenomena during the 1880's an 90's.