The Eye and Vision — Page 77, Lesson 118

Now, let's talk about what happens after light passes through the pupil. It then travels through a clear structure called the lens. The lens is flexible, and its shape is changed by the muscles that surround it, which are called the ciliary muscles. These muscles allow the lens to make the final adjustments needed to focus the light precisely onto the fovea, which is the central point of your sharpest vision. This process of changing the lens's shape is known as accommodation. The power of accommodation—how well and how quickly your lens can change shape—can be affected by two key factors: the aging process and fatigue. When you are tired, your accommodation is diminished, which means your lens struggles to focus properly, resulting in blurred images. Let's look at how the lens changes shape for different distances. To focus clearly on a near object, the lens is thickened, becoming more rounded. To focus on a distant point, the lens is flattened, becoming thinner. Now, an important point: the lens projects the image onto the retina, but it does so inverted and reversed—upside down and back to front. However, your brain perceives the object in the upright position because it has learned to consider that inverted image as normal. Your brain corrects the orientation for you automatically. Let's move on to the retina. The retina is a light-sensitive screen that lines the inside of the eyeball. On this screen are light-sensitive cells. When light falls on them, they generate a small electrical charge. That charge is then passed to the visual cortex of the brain by nerve fibres, which are called neurones. These neurones combine to form the optic nerve. The optic nerve enters the back of the eyeball, along with the small blood vessels needed to bring oxygen to the cells of the eye. The light-sensitive cell receptors of the retina are of two types: rods and cones. The very centre of the retina is called the fovea, and the receptors in this area are all cones. As you move outwards from the fovea, the cones become less dense and are gradually replaced by rods, so that in the periphery—the outer edges of your vision—there are no cones at all. We have specific terms for the type of vision each receptor provides. Vision through the functioning of the rods is called scotopic vision, whereas vision through the operation of the cones is known as photopic vision. When both the rods and cones are in operation together, that is called mesopic vision. Let's look at the cones in detail. The cones are used for direct vision in good light and are colour sensitive. Each cone has its own dedicated neurone connecting it to the brain, and this one-to-one connection allows them to detect very fine detail. The human eye is capable of distinguishing approximately 1000 different shades of colour. Now for the rods. The maximum density of rods is found about 10 degrees from the fovea—so, off to the side of your central vision. Unlike cones, several rods are connected to the brain by a single neurone. The rods can only detect black and white, but they are much more sensitive at lower light levels. As light decreases, the sensing task is passed over from the cones to the rods. This means that in poor light levels, we see only in black or white, or varying shades of grey. Rods are responsible for our peripheral vision. This has a practical implication for you as a pilot: at night time, with a dimly lit flight deck, the colour of your instruments must be bright enough for cone vision to be used. Rods are also sensitive to movement; the movement of an object to the side of us is quickly picked up by the rods. Finally, it's important to understand that rods and cones are the nerve endings of the optic nerve. As an extension of the brain, they are very much affected by a shortage of oxygen, an excess of alcohol, or by drugs or medication. These factors can directly degrade your visual performance.