Visual aura symptoms are blamed on changes in the brain that slowly spread from one area of the brain to another. This change is thought to cause a series of irregular brain activity including abnormal blood flow.
These disturbances, called cortical spreading depression, impacts the occipital lobe — the part of the brain responsible for processing vision. Disturbances in the occipital lobe are believed to cause what are sometimes called illusions or hallucinations related to visual aura.
Different people report seeing different things when they experience visual aura. Some have described it as looking like the television snow when the reception is fuzzy. Others report seeing certain geometric shapes or patterns or simply a halo of light.