Why does a migraine scotoma grow as it moves toward the edge of vision? · Grey Matter

The visual cortex gives the centre of gaze far more surface than the periphery, so each millimetre of cortex near the map of the centre covers a tiny angle of the visual field and each millimetre near the map of the periphery covers a large one.

Why does a migraine scotoma grow as it moves toward the edge of vision?

The visual cortex gives the centre of gaze far more surface than the periphery, so each millimetre of cortex near the map of the centre covers a tiny angle of the visual field and each millimetre near the map of the periphery covers a large one. A wave of spreading depression crossing the cortex at a steady few millimetres per minute therefore appears to speed up and widen as it moves outward through the visual field. The usual pattern, a small flickering spot near the centre that grows into a wide arc drifting to the edge over 5 to 20 minutes, is what a constant-speed wave on that distorted map predicts.