Photoreceptor Damage: Causes and Possibilities Over 10,000,000 people worldwide suffer from some sort of blindness or disability due to photoreceptor damage. These effects can be caused by a number of diseases, including retinitis pigmentosa, macular degeneration and tumors. These diseases range in severity, from being a mere hindrance to completely blinding the individual. Until recently, those affected were left without hope of a cure or even a treatment that would improve their vision in any way. But in recent years several groups of scientists have been working on a partial cure in the form of neuroprostheses, artificial devices that are inserted into the eye behind or over the damaged retinal area. These photoreceptive chips, in theory, should also provide information to healthy neurons residing in the retina, replacing damaged photoreceptors. When we open our eyes, millions of tiny events occur that allow us to see. Our pupils automatically constrict based on the light level, the variable lens bends and adapts to the distance of what we are looking at, and our photoreceptors receive information based on the above factors. (This is extremely simplified, but it will suffice for now.) Photoreceptors are tiny, specialized neurons located in the retina at the back of the eye. There are two types of photoreceptors, rods and cones. Each follows the same principles: when light hits them, they respond with a chemical reaction using a substance known as rhodopsin. Once this reaction occurs, a chain of events sends this message to a number of sophisticated and specialized neurons, eventually reaching the brain and resulting in what we call vision. The rods (of which there are approximately one hundred million in each eye) are found mainly in the periphery of the eye. our field of vision. They are extremely sensitive to light and are often "tied" together at a lower level to allow for greater sensitivity. The rods do not see with good resolution and cannot distinguish colors. Cones (there are only about five million of them) are found primarily in the center of the visual field, a place called the fovea. The words you are reading now are processed by the cones in the fovea. They operate in brighter light conditions than rods and detect color (there are three types, each responding to a particular range of wavelengths). Cones do not collect their results and exist for resolution, not simple detection. The only drawback of the cone system is the amount of light saturation needed to stimulate them and send the signal to the brain.