Most glaucoma patients are unaware of their visual problem until a late stage of the disease, probably because their visual field defects are being filled-in by the brain. According to new insights into how our brain works, plasticity of the visual cortex allows the brain to reorganize after damage to the retina or the visual pathways. This plasticity involves activation of existing but normally ineffective synapses, in a highly organized system of long-range horizontal connections in the visual cortex. Normal cells in the cortex surrounding the lesion thus take control of the deprived ones. As a result, glaucomatous field defects are probably concealed in the colors and patterns of the surround, up to the point where visual input has decreased to such an extent that the brain is not able anymore to compose a plausible image.