Entoptic phenomena (or just entoptics) are visual sensations that occur in the human visual system itself without external stimuli. That means the sensations can be observed even with closed eyes. The figure below gives some examples of the entoptic phenomena. These are usually simple geometric patterns, like lines, grids or nets, dots, zigzags and spirals. Further, it is typical that there is a replication of the patterns. Neurologist as early as the 1920s or even before have studied Entoptic phenomena. There are several ways to induce or cause entoptic phenomena: They can be caused by the use of drugs, extreme fasting, near dead experiences, extensive dancing and other means.
In 1988 Lewis-Williams and Dowson [Lew88] published a paper with the visionary title ‘The signs of all times’ that put entoptic phenomena into an archeological context. The authors classified the different patterns into a number of classes and could show that these patterns occur as motifs in Paleolithic rock art.
Other authors, like [ Bradley89], [Dron95] took this theory forward and showed that Neolithic art shows similar links and can be explained by means of entoptic phenomena. Typical examples of motifs that can be ascribed to entoptics are the carvings in the tombs of the Irish Boyne valley, ie. Knowth, Newgrange and Dowth or the French Gavrinis.