Scotoma · Grey Matter

A scotoma is a patch of the visual field in which vision is lost or distorted while the rest of the field stays normal, and in migraine it is the visible footprint of a wave crossing the visual cortex.

Scotoma. A scotoma is a patch of the visual field in which vision is lost or distorted while the rest of the field stays normal, and in migraine it is the visible footprint of a wave crossing the visual cortex.

The word covers any such patch, from damage to the retina or the optic nerve to a lesion in the cortex. In migraine aura the scotoma is scintillating: it usually begins as a small flickering spot near the centre of gaze, then expands into an arc with a shimmering zigzag edge (the fortification spectrum, from its likeness to the walls of a star fort seen from above) that moves outward over 5 to 20 minutes, leaving a blind or blurred zone inside it, and the whole event usually ends within an hour. The scotoma is cortical, so it sits in the same place for both eyes, and closing one eye does not make it go away.

The edge and the hole are two phases of one wave. The bright edge matches the front of cortical spreading depression, where neurons are briefly excited; the dark interior matches the depressed tissue behind it, which cannot respond for minutes.

It grows as it moves outward. The visual cortex gives the centre of gaze far more area than the periphery, so a wave advancing at a steady few millimetres per minute across the cortex covers ever larger angles of the visual field as it reaches the map of the periphery.

Negative and positive scotomas differ. A negative scotoma is simply missing vision; a positive one adds light, shimmer or colour. Migraine aura usually shows both, a positive edge around a negative centre.

Questions: 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.