Early stages of glaucoma do not cause symptoms, but as the disease advances, we must be aware of our limited visual field which most patients do not perceive. The cerebral cortex has a coping mechanism that fills in voids (scotomas) in the visual field with information from surrounding areas. This "filling" stops the patient from seeing visual field loss areas as black spots, fooling perception. When the scotoma or void is too large to be filled in by this coping mechanism, the patient then becomes aware of the loss and perceives it as a sudden deterioration, despite the fact that glaucoma is a slow and progressive disease.