Prehistoric art from all over the world is often characterised by similar kinds of abstract geometric patterns. There is an abundance of spirals, zigzag patterns, crosses, grids, and other visual forms. Many of these motifs are repeated across independent cultural contexts, yet they do not appear to be images of natural phenomena. One possibility is that they symbolise some of the essential visual features of altered states of consciousness, which can be induced by shamanic trance. 1 Indeed, there is abundant evidence that the ritualised experience of altered states played a significant role in many prehistoric societies around the world. The similarities of the prehistoric patterns could thus be explained in terms of a common mechanism of altered states, for instance the workings of our species-specific brain. This proposal has gained support in the archaeological community with the extensive work of Lewis-Williams and his colleagues, who even extrapolate this idea to explain the art of the Upper Palaeolithic.