Navicular entoptic phenomena

In a section of The Mind in the Cave titled Construing universals Lewis-Williams introduces a neurological concept called the navicular entoptic phenomenon aka the fortification illusion. Its form has, he says, been established by laboratory research. Further: This is a scotoma frequently experienced by migraine sufferers. There are some wonderful colour illustrations of the phenomenon in Oliver Sacks' book, Migraine. This is how Lewis-Williams describes it: In its more elaborate form, this percept comprises two elements: an outer arc characterized by iridescent flickering bars of light or zigzags, and, within the arc, a lunate area of invisibility - a 'black hole' that obliterates veridical imagery. Beyond the area of invisibility is the centre of vision ... The navicular entoptic phenomenon appears again and again in the rock art of the San in southern Africa, particularly in rock engravings; it is less common in the paintings. For a long time it was not recognised for what it is: people thought the crescent-shaped figures were boats and wondered why a desert people should draw them over and over. Sometimes the phenomenon is seen as a bee hive, perhaps because those entering trance states frequently experience aural hallucinations reminiscent of the buzzing of bees: it seems that some San link this aural experience to their simultaneous, shimmering visual hallucinations ... and believed they were both seeing and hearing bees swarming over honeycombs. Sometimes the flickering outer curve is construed as the flashing of antelope legs; sometimes, too, these animals, or others, are showing emerging from 'behind' the navicule in the same way that so many of the animals painted in the caves of France and Spain are painted coming out of the rock upon which they seem to float. Another class of these images as painted by the San show therianthropic beings emerging from the inner curve of the navicule, out of the area of invisibility in the centre of vision. Lewis-Williams writes: It seems that some painters took the area of invisibility within the arc ... to be an entrance into the spirit world and a gateway to transformation; in this way the area of invisibility paralleled the vortex. San shamans, who could be either men or women, do not use psychotropic drugs to have their visions, rather, they enter trance states by intense concentration, audio-driving, prolonged rhythmic movement and hyperventilation; i.e. through music, dance and chanting. Sometimes a shaman will fall down trembling violently in a cataleptic fit; sometimes they suffer nose bleeds, and then, since they are healers, the blood is smeared on those they wish to cure. In a deep trance their spirits leave their bodies through the tops of their heads. It is unclear, however, if, in their culture, there is a relationship between migraine and the navicular entoptic phenomena.