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David Lewis-Williams – The Mind in the Cave

In the first and ‘lightest’ stage people may experience geometric visual percepts that include dots, grids, zigzags, nested catenary curves and meandering lines………….. They flicker, scintillate, expand, contract, and combine with one another; the types are less rigid than this list suggests. Importantly, they are independent of an exterior light source. They can be experienced with the eyes closed or open; with open eyes, they are projected onto and partly obliterate visual perceptions of the environment. For instance, the so called fortification illusion, a flickering curve with a jagged or castellated perimeter and a black hole of invisibility in the centre, may, with a movement of the head, be positioned over a person standing nearby so that his or her head vanishes in the black hole. This particular percept is associated with migraine attacks and is therefore well-known to sufferers from that condition. Such precepts cannot be consciously controlled; they seem to have a life of their own…………