As San shamans dance, their supernatural power, or ‘potency’, builds up until it reaches a breaking point and flies out of the body. At this point, they ‘die’, a metaphor for travelling to another realm where spirits dwell in the same way as the soul travels after leaving after physical death. However, there are methodological issues concerning the use of 20th century ethnographies, from peoples who did not make rock art, as analogous to interpret art elsewhere that is hundreds or even thousands of years older. Lewis-Williams argues that the trance dance correlates with symbolism in the rock art. Features depicted in images that, it is argued, relate to altered states of consciousness, include nasal bleeding and the ‘arms back’ posture. Lewis-Williams and Megan Biesele (known for her work with the Ju/'hoan people) showed that the gap between different groups of San and different traditions of rock art could be bridged because of similar terms and concepts centred around the dance used by both the /Xam San in the south and the Ju/'hoansi San people in the north. Building on the work of previous scholars such as Lorna Marshall and Daniel McCall regarding a 'pan-San' belief system, Biesele and Lewis-Williams together suggested that the conceptual linguistic terms and ritual observances similar to the Ju/'hoansi and |Xam could be used to understand the complexity of the images. Indeed, Lewis-Williams writes that   The idea of a conceptual belief system was expanded upon using neuropsychology. Together with Thomas Dowson, Lewis-Williams explored the relationship between universal neuropsychological patterns in the wiring of the human brain and practices in shamanistic societies. Using data produced from laboratory experiments with hallucinogens, they proposed a neuropsychological model with multiple stages of hallucinations experienced during altered states of consciousness. Simply put, the model demonstrates the relationship between altered states of consciousness and the subjective interpretation of hallucinations. The premise of the neuropsychological model is that there is a difference between cultural imagery and neurologically produced visual patterns known as entoptic phenomena. During ASCs, which can be induced in a number of ways, the first stage of hallucination experienced by a subject contains only entoptic phenomena, such as the scintillating scotoma experienced by migraine sufferers. The second stage begins when the hallucinations are construed by the subject into culturally familiar content. The implication of this is that entoptic phenomena will be understood differently in different cultures.