Have you wondered why you look up when walking in the dark? There is an anatomic explanation for this behavior. To understand this issue better we will give a brief anatomy lesson.
How is the eye is like a camera?
The eye is like a spherical camera. The image above is an artistic version of an eye with the top half removed to reveal the inside anatomy. The eye works by allowing light to enter the eye through the clear cornea. The light then passes through the pupil, the circular opening in the iris. The rays of light are focused by the lens, which lies behind the iris. Light then travels through the clear vitreous gel that fills the eye. The focused beam of light finally strikes the retina, which is like the film, or CCD array, in the back of a camera. The retina contains receptors (photoreceptors) that detect light and then send a tiny electrical signal to the brain for interpretation.
What is the difference between the rod and cone receptors?
There are two main types of retinal photoreceptors used for vision: cones and rods. The cones work best in bright illumination and detect color. The rods work best in dim illumination and do not detect color. The rods are the photoreceptors used to navigate at night in dim light. As it turns out, the retinal photoreceptors are not distributed across the retina evenly. Most of the cones are located in the macula…the center of the retina. This is the area of the retina that humans use to see sharp details with good color perception during the daytime. When you look directly at an object, light reflected from its surface is directed onto the macula.
Where in the retina are the rods and cones located?
The rods are located in the retina around, but not in the macula (the center of the retina). Therefore, your best night vision is not provided by the central retina. Indeed, many notice they see stars in the night sky the best if they look slightly off center from the area of interest. Furthermore, the concentration of rods in the retina is greater above the macula than below the macula (Curcio, 1990). Consequently, the best vision for walking at night is when the eyes look slightly up. In this position, dim light from the path ahead falls onto the retina above the macula, where the rods are able to process the image much better than the cone-rich macula.
Why did the human eye evolve this way?
Presumably, evolutionary pressure helped create this anatomical situation. Even during the daytime, the amount of light entering the eye varies with gaze direction. When looking straight ahead on a sunny day, more light enters the eye from above than from below the midline. Light reflected from the ground is dimmer than light entering the eye from the sun above. So even in the daytime, vision is aided by having more rod photoreceptors in the top half of the retina than from the bottom half. Remember, light from below the eye, strikes the top part of the retina and light from above the eye strikes the bottom part of the retina. Thus, due to the optical qualities of light and the anatomy of the eye, you may find yourself looking upward when walking around in the dark.
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