Wonders of Science: The mechanics of sight, Part II

by Eric Rasmussen

“They say seeing is believing, but the opposite is true. Believing is seeing.”

— Errol Morris

The sense of sight is a marvel of biological evolution. This unique property gives those who possess it the ability to look upon the brilliance of nature with its many shapes, vivid colors and contrasting textures. So how does our vision work? What biochemical processes allow for this unique sense?

Our eyes are composed of a swath of different cells, all responsible for various functions. Among these are photoreceptor cells responsible for converting light into visual stimuli our brain can interpret. Three known types of photoreceptor cells function in the eyes of mammals: cones, rods and photosensitive retinal ganglion cells.

Cone cells

In addition to perceiving color, cone cells are unique in that they cluster in the central fovea of our eyes. Here light is most strongly focused, so cone cells are capable of high resolution of small objects. The next time you are super hungry and staring at a brightly-colored box of McDonald’s French fries, you are utilizing your cones.

Rod cells

Unlike cone cells that require large amounts of light to start a signal, rod cells are extremely sensitive. A single photon, the subatomic particle that makes up light, can trigger rods to action.

Because rod cells are so sensitive to light, our night vision is mediated through these cells. However, rod cells are located along the periphery of our retinas, so rod cells resolve objects best when not looking directly at them.

To test this phenomenon, just look directly at a faint celestial object in the night sky. Then slightly avert your gaze.

Each of us may notice the object looks brighter. Why? Because we are now focusing on the object with our low-light sensitive rod cells.

Photosensitive retinal ganglion cells 

These cells, intimately linked with our bodies’ internal clocks, are better known as our circadian clocks. They initiate biochemical pathways in relation to waking up and going to sleep. Studies suggest when these cells detect higher intensity blue light, they signal the body of daylight hours. To prevent users from suffering sleeplessness, phone manufacturers sometimes offer a feature to turn off the blue light emitted from their mobile devices. In this way, our cell phones won’t interfere with our natural biorhythms.

Biochemical process

Although differences exist in the sizes, shapes, and locations of photoreceptor cells in the retina, all three engage in similar biochemical processes: They convert light to sight. The process involves a photosensitive pigment called retinal, a form of vitamin A bound to specific proteins called opsins.

Opsin molecules are highly photosensitive and react strongly to light. When a person looks at a light source, photons stream through the eye lens, and opsin molecules absorb them. Light energy absorption causes a physical change in the structural shape of the opsin molecule, which induces an electrical current to run from eyeball to brain along the optic nerve. Our brains then register these electrical signals into a mental image of the outside world. The body then expends energy to reform the structure of the opsin molecule, so as to make it ready to function again. But this process takes time to complete, which is why we must wait up to 30 minutes to have full night vision after seeing a bright light!

Another wonder of science.

Next month, we’ll explore the evolution of color vision and shed some light (pun intended) on the evolutionary history that connects us with other species across space and time!

Sources: http://10000birds.com/wp-content/uploads/Nictitating-membrane-opening-on-a-Great-Egret.jpg & http://i42.tinypic.com/1z2nnrd.jpg

Eric Rasmussen, BS, M.Ed., obtained his bachelor of science degree at the University of Colorado at Boulder. He majored in ecology and evolutionary biology, and now serves as a Learning Technology Coach at Erie High School and Erie Middle School in the St. Vrain Valley School District, CO. 

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