A Model for the Origin and Properties of Flicker-Induced Geometric Phosphenes (PDF).
Many people see geometric patterns when looking at flickering lights. The patterns depend on the frequency, color, and intensity of the flickering. Different people report seeing similar shapes, which are called “form constants”.
Flicker hallucinations are best induced using a Ganzfeld (German for “entire field”): an immersive, full-field, uniform visual stimulation. Frequencies ranging from 8 to 30 Hz are most effective.
This effect is used by numerous sound-and-light machines sold for entertainment purposes. Some
of these devices claim to alter the frequency of brain waves. There is
no scientific evidence for this. However, the flickering stimulus may
increase the amplitude of oscillations that are already present in the brain, to the point where geometric visual hallucinations can occur.
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| Figure 1. Illustrations of basic phosphene patterns (form constants) as they appear subjectively (left), and their transformation to planar waves in cortical coordinates (right). |
