it has a small area that is clear which is called the macula. it is the area of greatest visual acuity. so if i turn my head towards you, the macula analyzes your features. unfortunately, the cells are sensitive to damage. so this is a really serious problem. we have no treatment for that, even though this is a point of great visual acuity. blindness is a range of conditions. it ranges from complete blindness to partial blindness. complete blindness means you do not see images, you do not see figures were people. you can tell the difference between day and night. partial blindness varies from tunnel vision, cloudy vision, night blindness. now, you can have difficulty with vision from two sources, one is if you have a damage to the visual pathway that goes from the retina into the brain, and carla will discuss that, we are not going to focus on that. we are going to limit ourselves to disorders of the retina. as you indicated, there are close to 300 million people worldwide that have various degrees of impairment and they fall into two categories

the other part of the retina outside of the macula called the peripheral retina. this is very important, too, because it's the part of the retina that lets us see in the dark. so this is low-light vision. actually, you can appreciate this part of the retina, too, because if you've ever looked at a starry night, if you've noticed if you don't point your eye directly at the star but you look a little off kilter, the star gets brighter, and that's because you're using the edges of your retina, the peripheral retina, which has very high sensitivity for light but isn't as good for high resolution images. so we have these two aspects of the retina. now, the last thing i want to talk about is what are the types of neurons in the retina. so we can look at a blowup of this macular region. there are nerve cells -- specialized nerve cells in the retina that are capable of converting light energy to a neural signal, and those are called the photoreceptors. there are two kinds of photoreceptors. there are the rods and there are the cones. the cones are the photoreceptors that