Our eyes contain millions of color-sensitive cells, called cones, which maximally respond to red, green, and blue light. With just these three types of color receptors, we can see the full rainbow of our world.
Animals with fewer types of cones cannot see the full visual spectrum. For example, dogs only have green and blue cones, so they can only see green, blue, and a little yellow.
Animals with more types of cones can see colors we can’t even imagine. For example, the mantis shrimp has 12 different types of cones!
Does a mantis shrimp see better than we do?
To answer this question, scientists tested the ability of mantis shrimp to distinguish between two different hues, or shades, of the same color. This spectral discrimination task is easy to do when the hues are really different but becomes harder when the hues are more similar. In their experiments, the scientists trained the shrimp to choose a certain hue over any other by giving them a food reward after making the correct choice. The scientists then adjusted the test hue until the shrimp couldn’t tell the difference between the two hues and its success rate was no better than chance at 50%.
Surprisingly, the mantis shrimp aren’t very good at this task! When the hues were within 20 nanometers (nm) of their trained wavelength, the shrimp could no longer tell them apart! To give you some perspective, we humans are able to tell the difference between hues that are only 1 to 5 nm apart!
Why are we able to discriminate colors so much better with fewer types of cones?
Neurons in our eyes compare the responses of different types of cones in an antagonist manner. This opponent process allows us to quickly record the differences between different cone responses. In mantis shrimp, the cone types work independently of each other, without complicated neural computations. Although this system does not allow for super accurate wavelength discriminations, it would allow mantis shrimp to quickly determine colors, and this may be extremely advantageous in their sink or swim environment.
Even though our eyes are structured very differently from those of the mantis shrimp, we are both able to experience the colorful world around us. Our different computational strategies to process color information may be an example of independent evolution. So, when it comes to color vision, more isn’t always better, but it remains to be discovered what mantis shrimp actually see and how the brain uses these visual signals.
How well can you do a color discrimination task?
Thoen, Hanne H., et al. “A different form of color vision in mantis shrimp.” Science 343.6169 (2014): 411-413.