Although it is possible to have an hallucination involving almost any kind of imagery, it has long been known that there are remarkable similarities between the hallucinations of different people, under different circumstances. Hallucinations were first classified during the last century during a period when many artists and writers experimented with hashish and opium as an aid to experiencing them. In 1926 Kluver began a series of investigations into the effects of mescaline and described four constant types. These were first the grating, lattice or chessboard, second the cobweb type, third the tunnel, cone or vessel, and fourth the spiral. As well as being constant features of mescaline intoxication in different people, Kluver found that these forms appeared in hallucinations induced by a wide variety of conditions.
In the 1960s, when many psychedelic drugs began to be extensively used for recreational purposes, research into their effects proliferated. Leary and others tried to develop methods by which intoxicated subjects could describe what was happening to them. Eventually Leary and Lindsley developed the 'experiental typewriter' with twenty keys representing different subjective states. Subjects were trained to use it but the relatively high doses of drugs used interfered with their ability to press the keys and so a better method was needed.
A decade later Siegel gave subjects marijuana, or THC, and asked them simply to report on what they saw. Even with untrained subjects he found remarkable consistencies in the hallucinations. In the early stages simple geometric forms predominated. There was often a bright light in the center of the field of vision which obscured central details but allowed images at the edges to be seen more clearly, and the location of this light created a tunnel-like perspective. Often the images seemed to pulsate and moved towards or away from the light in the center of the tunnel. At a later stage, the geometric forms were replaced by complex imagery including recognizable scenes with people and objects, sometimes with small animals or caricatures of people. Even in this stage there was much consistency, with images from memory playing a large part.
On the basis of this work Siegel constructed a list of eight forms, eight colors, and eight patterns of movement, and trained subjects to use them when given a variety of drugs (or a placebo) in controlled environment. With amphetamines and barbiturates the forms reported were mostly black and white forms moving aimlessly about, but with THC, psilocybin, LSD and mescaline the forms became more organized as the experience progressed. After 30 minutes there were more lattice and tunnel forms, and the colors shifted from blue to red, orange to yellow. Movement became more organized with explosive and rotational patterns. After 90 - 120 minutes most forms were lattice-tunnels; after that complex imagery began to appear with childhood memories and scenes, emotional memories and some fantastic scenes. But even these scenes often appeared in a lattice-tunnel framework. At the peak of the hallucinatory experience, subjects sometimes said that they had become part of the imagery. They stopped using similes and spoke of the images as real. Highly creative images were reported and the changes were very rapid. According to Siegel [Sie77] at this stage 'The subjects reported feeling dissociated from their bodies.'
The parallels between the drug-induced hallucinations and the typical spontaneous OBE should be obvious. Not only did some of the subjects in Siegel's experiments actually report OBEs, but there were the familiar tunnels and the bright lights so often associated with near-death experiences. There was also the 'realness' of everything seen; and the same drugs which elicited the hallucinations are those which are supposed to be conducive to OBEs.
There have been many suggestions as to why the tunnel form should be so common. It has sometimes been compared to the phenomenon of 'tunnel vision' in which the visual field is greatly narrowed, but usually in OBEs and hallucinations the apparent visual field is very wide; it is just formed like a tunnel. A more plausible alternative depends on the way in which retinal space is mapped on cortical space. If a straight line in the visual cortex of the brain represents a circular pattern on the retina then stimulation in a straight line occurring in states of cortical excitation could produce a sensation of concentric rings, or a tunnel form. This type of argument is important in understanding the visual illusions of migraine, in which excitations spread across parts of the cortex.
Another reasonable speculation is that the tunnel has something to do with constancy mechanisms. As objects move about, or we move relative to them, their projection on the retina changes shape and size. We have constancy mechanisms which compensate for this effect. For very large objects, distortions are necessarily a result of perspective, and yet we see buildings as having straight wall and roofs. If this mechanism acted inappropriately on internally generated spontaneous signals, it might produce a tunnel-like perspective, and any hallucinatory forms would also be seen against this distorted background.
In drug-induced hallucinations there may come a point at which the subject becomes part of the imagery and it seems quite real to him, even though it comes from his memory. The comparison with OBEs is interesting because one of the most consistent features of spontaneous OBEs is that the experiencers claim 'it all seemed so real.' If it were a kind of hallucination similar to these drug-induced ones then it would seem real. Put together the information from the subject's cognitive map in memory, and an hallucinatory state in which information from memory is experienced as though it were perceived, and you have a good many of the ingredients for a classical OBE.
But what of the differences between hallucinations and OBEs? You may point to the state of consciousness associated with the two and argue that OBEs often occur when the person claims to be wide awake, and thinking perfectly normally. But so can hallucinations. With certain drugs consciousness and thinking seem to be clearer than ever before, just as they often do in an OBE. An important difference is that in the OBE, the objects of perception are organized consistently as though they do constitute a stable, physical world. But such is not always the case; there are many cases which involve experiences beyond anything to be seen in the physical world.
Consideration of imagery and hallucinations might provide some sort of framework for understanding the OBE. It would be seen as just one form of a range of hallucinatory experiences. But (and this is a big but) if the OBE is basically an hallucination and nothing actually leaves the body, then paranormal events ought not necessarily to be associated with it. People ought not to be able to see distant unknown places or influence objects while 'out of the body'; yet there are many claims to such an effect.
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