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Geometric Shapes
Geometric designs are the most common petroglyphs representing entoptic motiffs -- the light images created within a human optical system when in a trance or altered state of conciousness.
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Editor's Note:
Entoptic is probably the most interesting word I've run into. It means, within the eye. Some petroglyphs do seem to have been made while the maker was in an altered mindset.
Entoptic phenomena are visual effects whose source is within the human eye itself. (Occasionally, these are called entopic phenomena, which is probably a typographical mistake.)
In Helmholtz's words: "Under suitable conditions light falling on the eye may render visible certain objects within the eye itself. These perceptions are called entoptical."
Overview
Entoptic images have a physical basis in the image cast upon the retina. Hence, they are different from optical illusions, which are caused by the visual system and characterized by a visual percept that (loosely said) appears to differ from reality. Because entoptic images are caused by phenomena within the observer's own eye, they share one feature with optical illusions and hallucinations: the observer cannot share a direct and specific view of the phenomenon with others.
Helmholtz[1] commented on entoptic phenomena which could be seen easily by some observers, but could not be seen at all by others. This variance is not surprising because the specific aspects of the eye that produce these images are unique to each individual. Because of the variation between individuals, and the inability for two observers to share a nearly identical stimulus, these phenomena are unlike most visual sensations. They are also unlike most optical illusions which are produced by viewing a common stimulus. Yet, there is enough commonality between the main entoptic phenomena that their physical origin is now well understood.
Entoptic phenomenon. (2023, September 20). In Wikipedia. https://en.wikipedia.org/wiki/Entoptic_phenomenon
