• Dr. Timothy Smith

More Than Meets the Eye: Sight in Humans and Other Animals

Mantis Shrimp

Pictured Above: Mantis Shrimp

Photo Source: Flickr

Human eyes do remarkable things, but the eyes of other animals show us that there is much more to see of the world than we can with our own. Eyes form remarkable sensory organs that take in electromagnetic radiation (which come in the form of waves) and convert that energy into nerve signals that an organism can interpret as light, colors, and objects. Different from ears that pick up physical vibrations through the air or the nose that detects the presence of chemicals in the air, eyes in humans serve to only pick up light in what is referred to as the visible spectrum—electromagnetic energy waves in a certain range, specifically 400 to 700 nanometers. The visible range covers the colors of the rainbow from violet and blue in the shorter wavelengths to red in long wavelengths.

Eyes work by taking light through photosensitive cells in the back of the eye that convert photons of light into nerve impulses that get processed by the brain to determine shapes, colors, and distances. In other words, what we are seeing now is the brain’s interpretation of the light we took in. The human eye contains two types of photo-sensing cells called rods and cones; the rod cells respond to light over a broader range than cone cells and produce the black and white view of the world. Rod cells with high sensitivity to low light help us to see at night and in dim conditions. On the other hand, cone cells react to tighter ranges of light, which provide our color view of the world. Human eyes have three types of cone cells that detect light in the blue, green, and red spectrum. These cones allow us to see the many colors of the rainbow.

Human eyes, although remarkable in many ways, appear quite limited compared to other animals' eyes in the animal kingdom. Many insects such as bees and butterflies, for example, see ultraviolet light, a light vibrating too fast for the human eye. These insects use the ultraviolet spectrum to see markings on flowers that serve as runway guides to direct them to the nectar in the flower. Remember, the human eye has three different cone cells to help us see color. Dogs have only two types of cone cells that respond to blue or yellow light.

At the other end of the spectrum, the mantis shrimp has truly remarkable eyes with sixteen different photoreceptor cells that cover the range from ultraviolet to red. The mantis shrimp is named for its appendages which resemble the attacking legs of the praying mantis, and it lives in the western Pacific Ocean. Researchers, Thomas Cronin, and others, in their paper titled "Filtering and Polychromatic Vision in Mantis Shrimps: Themes in Visible and Ultraviolet Vision," described the visual system of the mantis shrimp as the most complex eyes in the animal kingdom. If multiple color receptors were not enough, researchers at Queensland Brain Institute in Australia found that the mantis shrimp also has photodetectors that can see polarized and circular light as well!

Science discovered that light moves actually as a wave of electromagnetic energy—a tiny wave, but a wave nonetheless with peaks and troughs just like a wave on a lake or the ocean. The frequency of electromagnetic waves determines their place in the electromagnetic spectrum from x-rays at the high end down through ultraviolet, the visible spectrum with all the colors of the rainbow to longer waves such as infrared and radio waves.

Unlike a wave on water, light waves can oscillate multiple directions at a right angle to the course of the light beam's travel. As a light wave travels from its source, it vibrates both up and down and side to side. Such light waves can get changed when they bounce off surfaces such as water or when they pass through substances like glass, plastic, or crystal. Polarizing sunglasses help to reduce glare by filtering out everything but the light oscillating up and down. All the other light oscillating horizontally, which is what causes glare, gets filtered out.

Circular light refers to light beams that, instead of oscillating up and down along the path of travel, instead swings around in a circle. You can imagine circular light traveling like a corkscrew through the air instead of an arrow. People cannot see circular light without the use of sophisticated instruments, but the mantis shrimp can. Researchers at the University of Queensland Brain Institute in Australia demonstrated recently that parts of the mantis shrimp shell create circular light, and the shrimp use their ability to see circular light to avoid hiding spots in the coral that already has a mantis shrimp inside. (qbi.uq.edu.au/) The mantis shrimp has devastating appendages that can strike prey with the impact of .22 caliber bullet. Avoiding conflict with another mantis shrimp makes perfect sense.

The visual system in animals relies on the remarkable photoreceptor system found in the eyes of a wide variety of species, from mammals to insects. Eyes take in electromagnetic energy and with photosensitive cells and convert the light energy into nerve signals that the animal can use to make sense of its environment and drive actions from like hunting, mating, fighting, and defense. The human eye contains rods and cones that work to help people see color in the day and to see black and white at night. Researchers have found that the human visual system, although remarkable, does not nearly reflect the variety and complexity of optical systems in some other animals. Many insects can see in the ultraviolet range, and the mantis shrimp has the most elaborate vision system in nature to date. Researchers continue to discover remarkable aspects of the mantis shrimp vision system like the finding that the mantis shrimp sees polarized and circular light. It is hard to imagine precisely what the mantis shrimp sees, but we can use the variety of photoreception in the mantis shrimp's eye to fuel our imagination of what we still do not yet see in the world around us.

Dr. Timothy Smith

Dr. Smith’s career in scientific and information research spans the areas of bioinformatics, artificial intelligence, toxicology, and chemistry. He has published a number of peer-reviewed scientific papers. He has worked over the past seventeen years developing advanced analytics, machine learning, and knowledge management tools to enable research and support high level decision making. Tim completed his Ph.D. in Toxicology at Cornell University and a Bachelor of Science in chemistry from the University of Washington.

You can buy his book on Amazon in paperback here and in kindle format here.

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