The eye is a fragile and mostly hollow globe lined with neuronal cells that are constantly firing and sending information to the brain. Every millisecond, your eyes take in thousands of colors and shapes without getting very tired. They constantly send signals with information on these colors to your brain. Your brain is then able to come to conclusions based on this information. These visual conclusions could include things like, my shirt is missing a button, or that truck has eight wheels.
Our brain comes to these conclusions all the time. We are consciously aware of what is happening around us when we can see it. But why is that? How are we able to see shapes and colors all the time? What biological mechanisms make the sights we see in our everyday lives possible?
It all happens in the retina of the eyeball which is lined with neurons called photoreceptors. There are two kinds of photoreceptors: rods and cones. Rods are responsible for taking in information on size, shape, and brightness by absorbing black and white wavelengths of light reflected at them. Cones are responsible for taking in information on colors by absorbing blue, green, or red wavelengths of light reflected at them. There are three types of cones: L-cones, S-cones, and M-cones, which respectively absorb red, blue, and green wavelengths exclusively.
Rods and cones are constructed similarly but have many differences. One of the most notable differences lies in the disks of the photoreceptor cells. These disks extend on the opposite side of the cell's synaptic endings and contain proteins that absorb the light reflected at our eyes. The proteins photopigments in a rod's disks are called rhodopsins and the proteins in a cone's disks are called photopsins.
Attached to each of the rhodopsin proteins in rods is a molecule called 11-cis Retinal, which is a form of Vitamin A2. When light hits the rhodopsin protein, it triggers a chemical reaction in the 11-cis Retinal molecule. This reaction, known as an isomerization reaction, does not change the number of atoms in the molecule but instead it changes the structure and arrangement of the atoms in the molecule. One good example of isomerization is converting glucose into fructose.
This isomerization reaction converts 11-cis Retinal to normal Vitamin A2, which is also called all-trans Retinal. This transformation triggers a nerve impulse which is then interpreted and perceived by the brain as brightness. After this event, the Vitamin A2 molecule is released by its attached rhodopsin protein and is turned into Vitamin A, also known as all-trans Retinol, when 2 hydrogen atoms are added to it. This Vitamin A is then isomerized back into 11-cis Retinal. It then reattaches itself to the rhodopsin protein and the cycle repeats over and over.
Like rhodopsins, photopsins have attached molecules of retinal. The difference lies in the opsin part of the protein. Because the opsins of photopsin and rhodopsin are different by a few amino acids, photopsins can differentiate between different wavelengths of light unlike rhodopsins which only detect brightness. Each photopsin variant (L, S, and M) is programmed to send signals to the brain upon detecting their corresponding color
Some organisms, and a few people, possess a fourth cone type called the R-cone. This ability is known as tetrachromacy. People who are tetrachromatic can see up to 100 million more colors than the average person. Additionally, the range in which these colors exist could be near or far from the spectrum that most of us are familiar with. For example, if you looked at a picture of some scenery and felt it was dull compared to the real thing, you may be able to see a spectrum of colors quite far from the rainbow spectrum that we associate with our standard colors. In contrast, if this wasn't the case or if it was more subtle, then extensive testing would need to be done to prove whether you're a tetrachromat.