If certain types of religious experience may be constants, what about the imagery associated with altered states of consciousness? The case is solid and the effects have been studied for more than 2000 years with the first known recorded descriptions coming from ancient Greece. Normally known as "Entoptic Phenomena", the images derive from effects within the optic system – the eye and those elements of the nervous system and brain connected with it.
They're the whorls, catenaries, circles, spider-webs and spirals you see when vision is affected by fatigue, illness, injury or drugs. The term "entoptic" is used to embrace two more specialised groups, "phosphenes" and "form constants", "Pressure phosphenes", for example, can be generated mechanically, for example by pressing on the eyeball.
Studies of the imagery perceived during hallucinations were conducted during the late nineteenth century and eventually resulted in a work by Heinrich Klüver, 'Mescal: The "Divine" Plant and Its Psychological Effects' (1928), drawing on several medical; studies in Europe and the USA. Noting that the imagery described was fairly uniform, Kluver went on to classify four basic groups of "form constants" gratings and filigrees; cobwebs; tunnels and cones; and spirals. Later work with electrical stimulation, sensory deprivation, cocaine and LSD has extended the classification of images but these core classes have stayed constant.
The word "Entoptic" was coined for these phenomena by C W Tyler, a researcher into vision, and appears in the title of his paper 'Some New Entoptic Phenomena' (1978); It was adopted by Lewis-Williams and Dowson in 'The Signs of All Times' (1988), which contains a fairly typical description of the development of entoptic perception:
Thus far, the clinical evidence is solid, and many researchers have extended it, seeing entoptic imagery in, for example, some of the illuminations for the work of Hildegard of Bingen, particularly the bright and dark stars and corona-catenary effects in her visions of "Anima". (It's generally agreed that Hildegard was a migraine sufferer).
More controversially, various arch&ealig;ologists and anthropologists have postulated that these images form the basis for images in Palćolithic art. 'The Signs of All Times', compares shamanistic rock art of the Coso Indians of California and the Southern San of South Africa. Some commentators felt that the work stretched evidence beyond its limits in order to fit an "all-encompassing theory" that's probably invalid, but the idea is supported by Richard Bradley. 'Deaths and Entrances' (1989), maybe drawing its title from a Dylan Thomas poem, draws its evidence from "Megalithic" (Neolithic) tombs in southern Brittany and makes a very good case for the stones in the temple-tomb at Gavr'inis, Brittany. Some were re-used, new decoration being added that avoided obliterating the older images, indicating that the imagery was important.
The Gavr'inis patterns fit well with the "entoptic phenomena" theory and are reminiscent of rock engravings from Australia, Southern Africa and South-West America, which might indicate some "universal" experience on the part of the makers. The evidence for some universal "Mother Goddess" cult is relatively weak and it's more probable that small local cults were involved. However, an entoptic origin for the images is not inconsistent with their being part of Goddess worship; construal of the images is what would give them meaning to their makers.
There's reasonable evidence that some of the images do indeed indicate a female force and these are very similar to "classic" entoptic imagery. One such is the type of zig-zag line used to indicate snakes at Gavr'inis, which appear as far away as Jomon period Japan. Snakes can be seen to "emerge" from the earth, and the snake image suggests (female) fertility. The case is far from proven, but it's persuasive if you've seen the stones and pottery.
Further evidence for the universality of experience, if not meaning, comes from studies of trance and hallucinations. Caveat: one commentator with a healthy distaste for "new age" writing on the topic would call what follows "mystical material". That may in fact be a fair cop, but in my defence it is not " presented as scientific work", which is her main criticism of the genre.
Erika Bourguignon (with colleagues) has studied many societies widely distributed across the globe. She found a " form of ritualisation of dissociational states" in the vast majority of them. In her summary she suggests that trance is "apparently a universal human capacity, which many societies … have utilized" Although a few of the societies studied were large (upwards of a million people) most of the sample were small, so it's not really valid to extrapolate to, say, Europe, but the findings are certainly impressive.
Other findings are also significant. The methods used to enter trance (sensory deprivation, rhythmic stimulation, drugs and so on) have pretty much a set form in each society, and the types of imagery are also fairly pre-determined: practitioners build up an expectation set that allows the evocation (or perhaps construal) of certain types of vision.
Gerardo Reichel-Dolmatoff has studied the Tukano Indians of Columbia. Their drug-induced imagery is conditioned to see jaguars and other creatures from their belief system and drawings of visions seen in trance are filled with patterns like phosphenes. Laboratory investigations of yajé, their main hallucinogen, however, find that non-natives report different experiences: flying, body transformations, visual images of scenes and animals and geometric designs of bright colours.
Cultural determination of image construal is easy to explain: an example unconnected with entoptics will suffice. Looking out of a window I can see an oddly shaped branch in long grass (I experience it simply as an image). It looks like a squirrel, tail raised, about to leap (I elaborate it into an iconic form). A friend comes into the room and I ask "have you noticed my squirrel?" (the image now appears to derive from memory). All well and good, but my friend might have seen a peacock preparing to fan its tail, had I not conditioned her construal of the image - it's now, however, difficult for her to see anything other than a squirrel. A trivial example but entirely natural and in my own experience pretty much universal.
Unlike the societies studied by archæologists, the Tukano can tell anthropologists what the designs mean. Theories that certain symbols in palæolithic art are "male" pr "female" symbols have been widely challenged, but with the Tukano this is exactly the case – very many of their symbols have sexual meanings. Of course it's dangerous to assert that the imagery at Gavr'inis has the same meaning. A zigzag that might to me say "snake" and therefore perhaps be a mother-earth figure, is sort of related - it's the anaconda canoe from the Tukano creation story - but has a different underlying meaning. I think it fair, however, to suggest on this evidence that the imagery a Gavr'inis was entoptically derived and that it had some specific meaning to its creators.