Senses Primer Part 1: Vision

Your vision is by far your most important sense. Humans are visual creatures, and many of our day-to-day functions rely on our ability to see. With vision being such an important trait of ours, I thought it would be interesting to dissect the mechanics of vision a little.


Vision is the ability to perceive light, both intensity and color. That’s pretty much it. Your body and brain has the ability to take light, and turn it into subjective experiences. That’s pretty cool too.


Your eyes are where light enters your body to be interpreted. The shape of your eyes is similar to a lens (there is literally a part of your eye called the lens). Light enters your eyes, and the lens concentrates all the light onto the back of the eye. The back of the eye has a bunch of light sensors that pick up this concentration of light.


These specific sensors are called rods and cones, which are cells. Rods detect the intensity of light (light vs. dark), and cones detect color. Rods have high sensitivity. A single photon of light is detectable by a rod cell. On the other hand, cones have high resolution. Cones detect small differences in color. 



This image can be a little hard to understand without background knowledge, but here goes.


Rods detect intensity using rhodopsins. When rhodopsin is activated by light, it activates another protein called transducin. Transducin goes over to a protein called phosphodiesterase (PDE), which makes turns cGMP into GMP. cGMP is used to open sodium channels, which are receptors that let sodium into the rods. When phosphodiesterase turns cGMP into GMP, less cGMPs exist to keep sodium channels open. When sodium channels close, rods send a signal to your brain that tells your brain “light is shining on me”.


Cones detect color using photopsins, using a similar concept. Photopsins are a special type of protein that activate when a specific type of color is shined on them. S-photopsins activate for blue, M green, and L red. 


Combining these two mechanisms, you can get a decent picture of what you’re looking at in the outside world, but that’s not the complete story for vision. When you see, you focus on certain things, and selective items capture your attention. Rods and cones send light patterns to the visual processing part of your brain, for further processing. This processing will tell you what to pay attention to, and alter your general perception of the world.


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