"5.8 The Fourth Plateau
Information pertaining to the fourth plateau (roughly, above 15mg/kg) is limited, and what I have gathered will be presented as a general overview. Fourth plateau doses are similar to fully dissociative (but pre-anaesthetic) doses of ketamine.
Please note that dosages in these ranges are approaching the danger zone, and under no circumstances should anyone take this much DXM without a sober assistant who can take you to the hospital if the need arises! Many people neglect "trip sitters" with psychedelics. While you can probably get away with this with LSD (provided you remain in control enough not to do something stupid), with DXM there may be moments of such total confusion that you can wander into trouble without knowing what's going on. Additionally, the danger of adverse physiological effects, although not great, is worth paying attention to. Finally, psychotic breaks are most frequent at fourth plateau doses (and of course increase as the dosage increases).
Generally, people entering the fourth plateau report that they lose all contact with their bodies, often suddenly. This can be somewhat frightening. In particular, the sense of breathing is one of those missing, and people have occasionally interpreted this as evidence that they were dead. The surrounding environment may be evenly colored (usually grey or white), or it may appear vividly realistic, or cartoon-like, or anywhere in between these.
Many users have reported experiences very similar to "out of body" and "near death" experiences. In such cases, many report that they have contacted other beings, whose reaction to the user is usually somewhere between curiosity and amusement. Contact with "superior being(s)" has also been reported, sometimes as a raw force, sometimes personified in some way. In the reports given to me, the "superior being" image is more often female than male.
Delusions can become fairly involved at this plateau; the crucial factor seems to be whether or not the individual realizes that the belief or thought is drug-induced. Some people, especially those more experienced at this level, have reported that although they were aware that their thoughts were delusional, they didn't really care at the time. In general these delusions are fairly harmless (e.g., "I am a flower in the middle of a field").
Typically an individual in this plateau won't be moving at all, which can be frightening to observers. In many ways this state resembles dreaming. If someone in this plateau does attempt to move, his or her attendants should be very sure that he or she is conscious of these actions, and not responding to a delusional environment.
Somewhat surprisingly, many cognitive abilities are still intact. Basic computational skills and long-term memory recall do not seem to be particularly affected. It is also possible for the "body" (actually body and some parts of the mind) to undergo fairly complex tasks while the conscious mind is dissociated.
One individual wrote the following of the fourth plateau trip, and I think it is a good explanation both of the trip and of its possible origins:
I've come to the conclusion that DXM is almost unique in it's ability to create a truly "alien" experience - one in which major aspects of one's humanity can become entirely irrelevant. Most obviously, one's body can be left behind; even forgotten. The experience of becoming or encountering bizarre life-forms seems at least somewhat common, as are weird, horizonless landscapes or space-scapes. I think alot of this "alieness" comes from having so many of one's ties to the familiar severed. When your body is gone, your mind loses its sense of how "big" or how "small" you are in relation to your surroundings. Hence hallucinations of huge things like galaxies, or of being as large as a mountain, as small as an atom, etc. I think the brain also misses subtle clues like the sensation of breathing, blood flowing through the veins, etc. - things which help remind you that you're human. And at some point, even your memories of the familiar may be suppressed. "