The visual sensory system enables you to be aware of color, light level, contrast, motion and other visual stimuli. If you “saw it” - your visual system was responsible for that.

How does the Visual Sensory System work?

Let’s catch a ride on a light wave and follow it through your visual sensory system.

You want to go visit your friend who recently moved to West 54th St, where you’ve never been before. You have a city map with all the public transportation lines marked on it. How is all that information going to go from the map to your brain and result in a plan of action?

The Eye

The entrance to your visual system is through your eye. A light wave reflects off the words ”West 54th St,” enters your eye through the pupil and is focused by the lens right in back of your pupil so that it perfectly hits the retina at the back of your eyeball. (If your lens doesn’t perfectly focus on its own, you’re likely wearing an external lens - e.g. glasses - to help it out.)

Your retina is lined with two different types of photoreceptor cells, meaning they’re sensitive to and react to light: cones and rods.

Cones are the cells that are sensitive to colors, and you have about 6 million of them in each eye. There are red-, green- and blue-sensitive cones. All colors you see (and we see a wide range of colors) are due to the percentages of red, green and blue light in the incoming lightwave, as well as different levels of brightness (the amount of reflected light) and saturation (how much white light is in the makeup of the color).

The words “West 54th St” on your map actually reflected NO light, so your photoreceptor cells perceived it as black. The Q subway line running alongside 54th St, however, is colored purple. Both red and blue cones were activated, and passed that signal along.

If you were trying to read this map at night or in any other poor lighting condition, your cones wouldn’t be of much use. They need lots of light to work. Instead, the primary photoreceptor at work would be your rods - all 120 million per eye.

Rods are super-sensitive to light brightness and contrast - but not color. They do adjust more slowly than cones, so you may have to wait for a few minutes after you walk out of a building onto a poorly lit street in order to read your map. Once they do adjust, your rods will pick up all the brightness contrasts between different elements of your map. The black letters “West 54th St” will show up perfectly on the white background. That Q subway line will now look grey, because your cones can’t pick up enough of the color coming off it, and you might end up confusing it with the O subway line that is green, but with the same level of brightness. Good thing they labeled it.

Once your cones, rods or a combination of both pick up the incoming light wave, they send a chemical signal to the next layer of cells in your retina, the bi-polar nerves, which pick up even more details about the light signals relating to contrast between adjacent colors or levels of brightness (that’s how you can tell where an object or part of it ends, or where a shadow falls).

The bi-polar layer sends its signal to the NEXT layer of cells in your retina, your retinal ganglion cells. The ganglion cells are the ones that send the signal to the optic nerve, on its way to the brain.

The Brain

You have two eyes, and one optic nerve leading from each eye into the brain. Before the information goes anywhere to get processed (so you can actually “see” something), there is going to be a meeting of the optic nerves in what’s called the optic chiasm.

Some of the signals from your right eye cross over, join the signals from your left eye, and go on their merry way to the left side of your brain. Some of the signals from your left eye cross over, join the signals from your right eye and go happily on to the right side of your brain. This is important for accurate depth perception.

The first stop for processing of visual information is the lateral geniculate nucleus (LGN), in the center of the brain. The LGN separates the incoming information into color and fine structure, which are processed by small cells known as the parvocellular layer, and contrast and motion, which are simultaneously processed by large cells known as the magnocellular layer.

As you ride the bus on the way to West 54th St and the street signs flash past you, it’s the magnocellular layer that will process the contrast and motion signals you need to pick out “West 54th St” when it’s moving through your visual field at a fast clip. Your parvocellular layer doesn’t work well when the object in your field of vision is moving, so you’ll have to wait until you get off the bus for your parvocellular layer to process the dark blue color used for the words “West 54th St” on the sign.

The LGN cells send their information to the primary visual cortex (V1). The cells in V1 are primarily responsible for processing and interpreting where objects are in space. To aid this, V1 cells are organized in a way to perfectly parallel the information as perceived by the retina.

After you get off the bus, you have to get to the other side of the street. You look both ways and note any vehicles. Taking into account how far away they are, what direction they are moving in and how fast (the parietal lobe of the brain helps with processing the motion component as well), you can make a safe decision about when to cross the street.

The secondary visual cortex (V2) is primarily responsible for processing color and for color constancy (identifying colors as the same general color, even when under different illuminations).

Your friend told you that her house on West 54th Street was the one with an orange car parked out front. V2 enables you to identify her orange car as orange, whether it’s on a bright, sunny day, an overcast day, or at twilight.

Visual information is also routed to V3 and V4 for help with color and form perception, and to the inferior temporal lobe for face and object recognition.

When your friend comes out of her house to greet you, she has a new haircut and has given up glasses for contact lens. Despite the change in identifying details, you’re able to recognize her from the whole picture and compensate for the “missing” information.

“Hi!”  you say. “It’s good to see you!”

What happens when it doesn’t work right?

In all the examples of visual information processing above, you saw several examples of where a malfunction in that part would present a big problem in perception, understanding and action.

Disorders in the eye section of the visual system can cause partial or total blindness or color vision deficiencies (color blindness).

Disorders in the brain section of the visual system can cause difficulties:

• differentiating between two similar figures (like the O and the Q subway lines)
• distinguishing an object from its background (picking out the information you need from a map full of words, streets, subway lines and more)
• telling where objects are in space in relation to each other or to you (making it altogether hard to understand a map and how to use it)
• identifying an object or person when only parts are visible (making it very hard to realize that the woman with the new haircut and no glasses is your childhood friend)
• using information from the eyes to direct physical motion (you’re going to have a tricky time trying to fold that map neatly)

See here for more details on Visual Processing Disorder, how it manifests and what to do about it.