A Model for the Origin and Properties of Flicker-Induced Geometric Phosphenes

We present a model for flicker phosphenes, the spontaneous appearance of geometric patterns in the visual field when a subject is exposed to diffuse flickering light. We suggest that the phenomenon results from interaction of cortical lateral inhibition with resonant periodic stimuli. We find that the best temporal frequency for eliciting phosphenes is a multiple of intrinsic (damped) oscillatory rhythms in the cortex. We show how both the quantitative and qualitative aspects of the patterns change with frequency of stimulation and provide an explanation for these differences. We use Floquet theory combined with the theory of pattern formation to derive the parameter regimes where the phosphenes occur. We use symmetric bifurcation theory to show why low frequency flicker should produce hexagonal patterns while high frequency produces pinwheels, targets, and spirals

From Stroboscope to Dream Machine: A History of Flicker-Induced Hallucinations

When early neurophysiologists, like William Grey Walter (1910-1977), started using intermittent photic driving in electroencephalography, they were struck by a wide range of visual hallucinations that were reported. In current neuroscience, the phenomenon is used mainly to model hallucinations that are related to altered neuronal activity between the thalamus and the visual cortex, such as the Charles Bonnet syndrome. However, during the psychedelic 1960s, Brion Gysin (1916-1986), a painter and a poet, became interested in the hallucinations and designed his own stroboscope or dream machine, as a means for spiritual enlightenment. This article traces back the history of flicker-induced hallucinations from the early use of stroboscopes in neurophysiology to the dream machine. Copyright (C) 2009 S. Karger AG, Base