MIGRAINE PHOSPHENES AND THE RETINO-CORTICAL MAGNIFICATION FACTOR

Quantitative observations on the shape and position of migraine phosph enes within the visual field were obtained by controlled ''perimetric' ' drawings of the phosphenes performed every 1-2 min during the aura s tate. The visual field eccentricity of the ''fortification'' or zig-za g patterns scintillating at about 10 Hz was plotted as a function of o bservation time. It is well described by an exponential function of ti me. This exponential function is the product of a first-order linear d ifferential equation determined by the distribution of the retino-cort ical magnification factor across the visual field and a constant diffu sion speed of the cortical pathophysiological process leading to the m igraine phosphene patterns. The observed ''particle'' size of the phos phene pattern and the width of the scotoma trailing the scintillating phosphenes could also be easily predicted from these assumptions. A mo del in which the main components are an increase in extracellular pota ssium concentration, a decrease in extracellular calcium concentration and the constant speed diffusion of the ions along the extracellular space of the stripe of Gennari within the primary visual cortex explai ns the observations.