“In “The Signs of All Times,” [1988, David] Lewis-Williams and [T.A.] Dowson declare that . . . they have discovered . . . a “neurological bridge” that could take us back to the Paleolithic Age. That bridge is the human nervous system, which they claim was the same then as now. They say that when drugs, fatigue, pain, insistent rhythms, or other stimuli induce a trance, the nervous system produces a pattern of hallucinations derived from it and not from cultural clues. The pattern is the same for all people in all cultures at all times. Therefore, Paleolithic hunters had the same pattern of hallucinations during trances that we do.

In particular the authors mean visions derived from the structure of the optic system. They call such visions “entoptic phenomena.” One example of entoptic phenomena is the jagged lines or herringbone patterns that some people see on the edge of their vision as a prelude to a migraine. Citing a considerable amount of modern research on the effects of mescaline and LSD, Lewis-Williams and Dowson identify six principal entoptic forms: a grid, parallel lines, dots, zigzags, nested curves, and filigrees. They also say that there are three stages of a hallucinatory trance, although they are not necessarily sequential or completely distinct. A subject experiences the entoptic forms and only those forms in the first stage. During the second stage, the subject tries to make sense of the entoptic forms by, for example, seeing a grid as a chessboard. And in the third and final stage, which is usually accompanied by the feeling of flowing through a swirling vortex, the subject experiences hallucinations that are so powerful they seem real. . . . Lewis-Williams and Dowson exhibit several charts showing that the entoptic forms appear in both San and Paleolithic art. More than that, various images from the art of both cultures appear to apply to each of the three stages of a trance.”

—Gregory Curtis, The Cave Painters: Probing the Mysteries of the World’s First Artists, 2006.