Neurologists recognize three types of altered states of consciousness: normal sleep, pharmacologically-induced unconscious states, and pathologically altered conscious states (coma or vegetative state). Neurological ASC refers only to the "state" of a person's consciousness, not the content of that state. What anthropologists and archaeologists mean when they refer to ASC is probably something more along the lines of "Altered Patterns of Phenomenal Properties": we are interested in the experiences people have during ASC, particularly heightened and focused imagery. As archaeologists, we are particularly interested in how images from ASC experiences relate to the practices of early religion.
One of the primary tools of the religious specialist called shaman, the visions characterized by (anthropology-defined) Altered States of Consciousness can be achieved through fasting, dehydration, lack of sleep, blood loss, extreme pain, chanting, sweat lodges, and ingesting psychoactive drugs. In this manner, shamans reach trance states in which they may travel to the underworld, gain help from spirits, manipulate the weather, find lost objects or help with pain or fertility problems.
Certain images, shapes and colors are associated with ASC, and representations of these images are known from a wide range of behaviors including paintings on cave walls, decorating ceramic vessels and structure walls, sand paintings, and fetishes associated with shamanic activity. The oldest forms of art, preserved on the walls of caves and in portable art, have abstract forms that some scholars have connected to induced "entoptic" images. Entoptic images are those which are associated with physical activity on the retina and optic nerve, but are not created by visual inputs which can be shared by others.
Experiencing ASC
Three levels of imagery and sensation have been reported by people experiencing different levels of (neurological) ASC. The first includes abstract visual hallucinations: patterns of grids, lattices, honeycombs and webs; sets of parallel lines, dots or stars; spirals, tunnels and vortexes; checkerboards, zigzag patterns, and nested zigzags. Vivid colors are associated with some psychotropic drugs; others strip out the colors leaving only black, white and yellow.
At a second level, the brain transforms these abstract images into known experiences: a wavy line could become a snake or water; zigzags become birds or lightning; parallel lines become humans and change into animals and back again. In the third level, the images become experiential, so that the person experiencing the ASC falls into a tunnel or flies as a bird. These levels are not necessarily accomplished in order: and, it must be pointed out that these images can be produced by normal brain activity during dreaming.
Archaeological Evidence of ASC
Archaeologists identify the use of psychotropic drugs in prehistoric contexts by the discovery of use kits (snuffing kits, tubes, inhaling bowls, effigy pipes). Seeds and plant residues of hallucinogenic plants have been found in ritual spaces; and the chemical analysis of artifacts or human remains can also provide evidence of drug use related to shamanic activity.
Depictions of the kinds of images known to be experienced by people in various forms of (neurological) ASC are also used as supporting evidence for shamanic practices. Representations of hallucinogenic plants are particularly strong evidence, especially when combined with abstract patterns of entoptic figures. Abstract images, including grids, cross-hatching and dot patterns, are the subjects of the earliest forms of art, found in Middle Stone Age sites in Africa and Upper Paleolithic sites in Europe and Australia.
Drugs known to have been used in prehistoric shamanistic societies include a wide range of drugs, including various forms of alcohol all over the world; datura, nicotine-rich tobacco, and peyote in North America; coca leaves and Banisterioposis (ayahuasca) in South America; cannabis in Asia; opium from Europe and the middle east; water lily and mandrake root in ancient Egypt.
Sources
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