If you’re reading this article, you can thank the crafty and tireless work of evolution. While we may take vision for granted as a simple and straightforward process, how we see is not so simple at all.
How we see
In normal conditions, light first passes through the clear, dome-shaped front portion of our eye (cornea). The cornea bends the light and helps the eye focus. From the cornea, light enters through the pupil, the dark opening in the center of the eye. The amount of light going through the pupil is regulated by the colored part of the eye called the iris. The iris, described as a muscular “curtain,” constricts or dilates to regulate the size of the “window,” which light passes through.
Next, the lens—a clear inner portion of the eye—works with the cornea to focus light on a tiny spot at the back of eye called the retina. This light-sensitive tissue layer contains photoreceptors called rods that sense the light and convert it to electrical signals. For more detailed vision, we have an even smaller area at the back of the eye called the fovea. The fovea is densely packed with light-sensing cells called cones. This area gives us the sharp central vision needed for reading, driving and recognizing high visual detail.
Finally, these electrical signals travel through the optic nerve to the brain to give us sight.
Seeing in Color
People have trichromatic color vision rooted in the red, blue and green primary colors. Our ability to see color may have evolved to help primates discriminate red colorful fruit and young leaves from other vegetation that was not as nutritious or beneficial to survival. Other hypotheses suggest that being able to see skin flushing may have been one way that primates could detect the mood of others, which was an evolutionary advantage.
We sense color because our retinal cone cells contain different photosensitive pigments that are sensitive to certain wavelengths of light. When light hits each of these pigments, the brain compares signals from each photoreceptor to determine the intensity and color of the light we are receiving. On average, humans can distinguish around 10 million different colors. Visit the paint section at your local hardware store as a perfect example.
About 3% of the world’s population has some form of vision impairment, and over 39 million people are considered blind. But the outlook may not be so hazy. A recent review published in Physiological Reviews has generated hope for people who are visually impaired. New developments in gene therapies have the potential to restore lost or impaired vision in millions of people.
Vision is a complex aspect of life. Now that you know the underlying wonders of how sight works, perhaps you’ll see the world through a whole new lens.
Brady Holmer is a PhD student in exercise physiology at the University of Florida. His lab focuses on cardiovascular physiology; mainly how exercise can play a role in health, disease and aging. Holmer hosts a podcast called “Science & Chill,” where he sits down with scientists in the fields of physiology, biology, health and nutrition to discuss their work.