How does the Sense of Sight Work?

Michael Anissimov
Michael Anissimov

The experience of sight begins when photons from the world hit the lens of our eye, and get focused onto a small patch of photoreceptive cells on a part of the eye called the retina. These cells come in two types - rods and cones. Cones are for color detection, functioning well in bright light, and rods are more sensitive but also colorblind. Humans have about 125 million rod cells and 6 million cone cells. Some species have many more rods, especially those adapted to living at night. Some owls have night vision 100 times more acute than that the sight we are accustomed to.

The night vision of some owls is 100 times more acute than that of humans.
The night vision of some owls is 100 times more acute than that of humans.

Rods and cones perform a function called phototransduction, which simply means converting incoming light into electrical signals to be sent to the brain, making sight possible. All these cells contain photoreceptive proteins with various pigment molecules. In rods these are called rhodopsin. In cones, various pigments can be found, allowing the eye to distinguish between different colors. When light associated with the pigment impacts the photoreceptor cell, it sends a signal down the optic fiber, otherwise, it doesn't. Photoreceptor cells and the ability of sight are extremely old evolutionary innovations, dating back to the Cambrian period over 540 million years ago.

The experience of sight begins when photons hit the lens of our eye and get focused onto a small patch of photoreceptive cells on a part of the eye called the retina.
The experience of sight begins when photons hit the lens of our eye and get focused onto a small patch of photoreceptive cells on a part of the eye called the retina.

There are two notable structural characteristics of the human retina. The first is the fovea, a highly condensed area of photoreceptor cells located in the center of the retina. The cell density here is several times greater than on the periphery, explaining why when we look directly at something it is a lot clearer than looking at it through the corner of our eye.

Visual perception can be skewed by the contrast effect.
Visual perception can be skewed by the contrast effect.

The fovea is also responsible for the behavioral adaptations that provoke us to rapidly turn our heads and stare at something if it startles us. If the fovea didn't exist and photoreceptor density were uniform across the surface of the retina, we wouldn't need to do this - we'd only need to turn our head slightly so that the event at least fell within our field of vision. The foveal area is a relatively small portion of the visual field, about 10 degrees wide.

The second notable structural characteristic in the retina is our blind spot. This is where the optic fiber connects up to the back of the retina to get visual information, precluding the existence of photoreceptors in a small spot. Our brains automatically fill in our blind spots for us, but various visual exercises can prove that it's there.

Once light is converted into electrical impulses and sent down the optic fiber, it goes all the way to the back of the brain (after making a few stopovers), where the visual cortex is located. In the visual cortex, a hierarchy of detector cells isolates useful regularities in the visual data, discarding superfluous information. One layer of cells detects things like lines and curves.

A higher-up layer would detect regularities like motion and 3D shapes. The highest layer is where gestalts - overall symbols - appear, responsible for the conscious experience of sight under normal circumstances. The visual cortex is among the best understood of all brain areas, with a voluminous neuroscience literature.

The inability to identify different shades of red and green is a sign of color blindness.
The inability to identify different shades of red and green is a sign of color blindness.
Michael Anissimov
Michael Anissimov

Michael is a longtime wiseGEEK contributor who specializes in topics relating to paleontology, physics, biology, astronomy, chemistry, and futurism. In addition to being an avid blogger, Michael is particularly passionate about stem cell research, regenerative medicine, and life extension therapies. He has also worked for the Methuselah Foundation, the Singularity Institute for Artificial Intelligence, and the Lifeboat Foundation.

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Oh, one more quick note about multivitamins. They generally do not work as they are intended, which is part of the reason that the nutrition received naturally through one's diet is so much so emphasized these days, so it's no wonder that your son stopped taking one; it probably didn't help him.

But, in general, vitamins are something of a ruse for everyone, not just your son. In fact, not long ago I was reading in the Times about another recent study which served to disprove the effectiveness of vitamins. It showed how certain populations which take vitamins show no overall health benefits by doing so. Acknowledging the ineffectiveness of the "once-daily" muti-vitamin, Dr. Oz, for example, advises that your multi-vitamin be broken in half and taken at two different times of the day. Even Dr. Oz's good-intentioned claim however seems questionable however.

At any rate, every doctor will agree to the benefits of limiting dietary supplements to a minimum, for example only taking a vitamin d, vitamin b, or calcium supplement, and receiving the majority of ones nutrition through plain ol' food (preferably with limited processing). Just an additional thought after re-reading your post. Sorry for the long-windedness.


I'm not sure why exactly you posted the part about your son's bipolar disorder and his diet. For example, I'm really not sure if there could be a relationship between the development (changes) in his eyesight and his overall diet and development of bipolar disorder. All the same, (and I mean this only to help and not to condescend in any way, I promise. In fact I probably won't return to this post) you should re-emphasize the importance of vegetables to your son (even though he's a little old to be told what to do).

Most cases of bipolar disorder can be treated with a balanced diet, herbs, such as St. John's wort which is sometimes used to treat bipolar disorder, as well as the avoidance chemicals (like colognes and so forth) that might have affects on the hormone production in the body.

More than half of the chemicals put into colognes and perfumes, for instance, have not been tested for their effects on humans. In a similar vein of thought, the specific vitamins, minerals, and macro-nutrients that one receives through their diet on a daily basis can affect hormone regulation, and even neurotransmitter regulation, in the body. Theories on bipolar disorder almost universally admit that neurotransmitters (and by extension hormones) play some role, sometimes the majority of the role, in the disorder. Merely ingesting proteins or carbohydrates, for example, can affect the production of the neurotransmitters, dopamine and serotonin, respectively.

So, all in all, I don't know much about your son's situation, but perhaps something I've said here will help him in some way, that is to say, if any help is needed at all. Yet I won't condescend to say that mine or anyone's help is needed, again, I concede that I don't know much about his situation.

But I hope for all the best. Who knows? Perhaps my stumbling across this post will have helped you in some way. --Michael


My 22 year old sons eye color has changed from

dark blue, as an infant through junior high school, to pale grey/green. The iris color seems to be gradually fading all the time. The males in our family (including two grandfathers) usually retain a deep blue iris. He was diagnosed with bipolar disorder at 15 and has been on some heavy duty meds since. He eats very poorly with no vegies or fruit in his diet. He stopped taking vitamins a few years ago. His eyes are now a very pale light green. Any thoughts or similar experiences?

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