Vision is made possible by light entering the eye through a set of lenses, first the cornea and then the crystalline lens, which focuses it on a multilayer tissue at the back of the eye, the retina. Photosensitive cells in the retina (photoreceptors) contain photopigment (a pigment that chemically reacts to light). These photoreceptors absorb the light and generate an electrical impulse that is transmitted to the brain, specifically to the lateral geniculate nucleus, located in the thalamus, which then relays to the occipital lobe of the brain’s cortex. Further processing occurs in other cortical modules, both in the temporal and the parietal [Page 2302]lobes. Any element in this complex system may be subject to disorder, which will affect the quality of vision. In this ...