Vision
Of all the senses, vision is the most important for safe flight. Most of the things perceived while flying are visual or heavily supplemented by vision. As remarkable and vital as it is, vision is subject to limitations. Two limitations are illusions and blind spots. The more a pilot understands about the eyes and how they function, the easier it is to use vision effectively and correct for potential problems.
The eye functions like a camera. Its structure includes an aperture, a lens, a mechanism for focusing, and a surface for registering images. Light enters through the cornea at the front of the eyeball, travels through the lens, and falls on the retina. The retina contains light sensitive cells that convert light energy into electrical impulses that travel through nerves to the brain. The brain interprets the electrical signals to form images. There are two kinds of light-sensitive cells in the eyes: rods and cones.
The cones are responsible for all color vision, from appreciating a glorious sunset to discerning the subtle shades in a fine painting. Cones are present throughout the retina, but are concentrated toward the center of the field of vision at the back of the retina. There is a small pit, called the fovea, where almost all the light sensing cells are cones. This is the area where most “looking” occurs (the center of the visual field where detail, color sensitivity, and resolution are highest).
While the cones and their associated nerves are well suited to detecting fine detail and color in high light levels, the rods are better able to detect movement and provide vision in dim light. The rods are unable to discern color but are very sensitive at low light levels. The trouble with rods is that a large amount of light overwhelms them, and they take a long time to “reset” and readjust to a dim light environment. There are so many cones in the fovea that the very center of the visual field contains virtually no rods at all. In low light conditions the middle of the visual field is not very sensitive, but farther from the fovea, the rods are more numerous and provide the major portion of night vision.
The area where the optic nerve enters the eyeball has no rods or cones, leaving a blind spot in the field of vision. If an image falls on the "blind spot" the brain does not sense the image. Normally, each eye compensates for the other’s blind spot. Cover the right eye and hold a page at arm’s length. Focus the left eye on the X on the right side of the windshield and notice what happens to the airplane while slowly bringing the page closer to the eye.
The importance of the blind spot and the physiological nature of the brain creates a dangerous potential for collisions between aircraft.
If a plane's image is focused on your you blind spot, and you don't effect a proper scan for aircraft in your vicinity, you will not see the aircraft you are about to collide with. the reason is easy. There aren't any nerve receptors for the image to stimulate.
If the image is not located on the area of the blind spot then another phenomenon of vision, and the nervous system, is the brain slowly "adjusts down" its reception of the image in the brain.
Think about eating a nice, pungent yellow onion. For the first few minutes the smell is overpowering. After a short period of time, you hardly notice the smell received from the onion by the olfactory sensors in your nasal epithelium. The first rush of impulses to the brain are fully analyzed as a strong onion odor. In a few minutes the brain decreases its interpretation of the impulses. You don't have the same harsh onion smell you first received.
Your vision reacts the same way. You receive the visual signals of a plane spotted in your field of vision but the brain reacts like it was never there. Not a good situation for collision avoidance.
I will continue to write about selected areas of sensory input to the brain for more interpretation.
Enjoy
