Pressure Hallucinations and Patterns in the Brain1

Whirling domains of geometric patterns seen after pressure on the eyeball, known as phosphenes, have been observed for over 2000 years, but their source of origin remains a mystery. The study tested the feasibility of phosphene generation in the cortex through the action of decreased retinal input. This decrease is elicited by the retinal ischemia that accompanies prolonged pressure. A simple Wilson-Cowan network model of interactions between excitatory and inhibitory neurons receiving tonic thalamic inputs was used. Assuming that inhibitory neurons normally receive disproportionately more retinal input, it was predicted and demonstrated by phase plane analysis of a reduced twoneuron-pair model that decreased retinal input would cause the uniform steady state of the cortex to become unstable via a pitchfork bifurcation, resulting in elementary cortical pattern formation. In a circular, one-dimensional network of 102 neurons, simple stripe patterns were produced by the proposed mechanism. Under the complex logarithmic visuo-cortical mapping, similar two-dimensional patterns could account for commonly observed binocular pressure phosphenes.