Take a moment to look at everything around you. It’s likely that you see a multitude of colors. You can perceive shadows, depth, and three dimensions. It feels like you can do all this the second you open your eyes. But how can you do this? Where is vision processed in the brain?
To answer this question, it's important to go through the entire process of seeing, from eye to brain. We often take our vision for granted. By knowing how our vision is processed, we may have a greater appreciation for what our eyes and brains can do.
Where Is Vision Processed in the Brain?
Vision is processed in many areas throughout the eye and brain. The retina first takes in sensory information and converts it into neural signals. Those signals are handed off to the optic nerve, which hands them off to the thalamus and visual cortex.
Of course, this is just the short answer to this question. Vision is very complicated. A majority of our sensory receptors are actually located in our tiny eyeballs! So let's go through each of these steps, one by one, and keep it simple. A lot happens when we open our eyes and look at the world around us!
All About the Eyes
When we think about vision processing, we often point to the retina. But by the time that sensory information reaches the retina, it has already gone through many layers of the eye!
The front layer of the eye is called the cornea. (This is completely clear, so we don't see it in front of other parts of the eye like the pupil.) The cornea is dome-shaped and bends light so that our eyes can focus properly.
Light enters the eyes through the pupils. You might have noticed that your pupils dilate or shrink depending on how light or dark it is around you. That is controlled by the iris, which is the part of the eye that gives you your eye color!
The last place where light passes through before the retina is the lens. The lens is on the inside of the eye, but it works alongside the cornea to bend light and prepare it for the retina.
What Is the Retina?
The retina is a layer of tissue cells that line the back of the eye. It absorbs the light that is bent by the other parts of the eye and converts them into electrochemical signals. Those signals move through many layers of the retina before they reach the optic nerve in the brain.
For now, we will just focus on three different layers of the retina.
Photoreceptor Cells (Layer One)
Photoreceptor cells take electromagnetic waves (light) and convert them to electrical signals. There are millions of these cells in the eye, and they are either "rods" or "cones." Rods and cones both have different jobs when it comes to perceiving light.
Rods are responsible for picking up on things like shadows and general shapes. They also take in information from our peripheral vision.
Cones are responsible for perceiving colors and the finer details of what we are seeing. Without cones, we would only see in black, white, and gray!
Interneurons (Layer Two)
These neurons are also known as "bipolar neurons." This layer of cells passes the signals, or information about light, from the first layer to the innermost layer of ganglion neurons.
Ganglion Cells (Layer Three)
This last layer of cells forms the connection between the eye and the optic nerve.
What Is The Optic Nerve?
The optic nerve, also known as the second cranial nerve or cranial nerve II (CNII), is a bundle of over one million nerve fibers. Although the optic nerve extends outside of what we know as "the brain," it is part of the brain and the central nervous system. It's a unique set of nerves that travels from the eyeball to other parts of the brain.
The optic nerve carries information to multiple parts of the brain: the brainstem, thalamus, and visual cortex. These areas have different responsibilities when it comes to perception.
The brain stem works not to flesh out the deeper meaning of what we are seeing, but to ensure the eyes continue to work. Within the brain stem is a pretectum, a group of cells that aids the iris in controlling the pupil. When we turn on the lights in the morning or see someone's high beams on the road, the pretectum is at work.
Another part of the brainstem that aids our vision is the superior colliculus. Experts still have quite a few questions about what the superior colliculus does. It's safe to say that this area receives various types of sensory information to help us respond to visual stimuli. Let's say you hear a bird making a loud noise in the sky. As you turn your head to see the bird, you watch it fly away. The sensory colliculus is responsible for helping you keep an eye on this moving object. It may also be the area of the brain that tells you to look at that noisy bird in the first place!
Our superior colliculus relies on information from the visual cortex. But before the electrical signals from the optic nerve can reach the visual cortex, they go through the thalamus. The thalamus has a lot of work to do. Not only does it send information from the retina and optic nerve to the visual cortex, but it also receives information from other senses! It relays information about sight, sound, taste, and touch to the cerebral cortex. The thalamus also plays a role in some of the ways we respond to sensory information: motor activity, emotion, memory, arousal, etc.
Electric signals concerning vision specifically go to the lateral geniculate nucleus (LGN.) The LGN has its own multiple layers of cells that organize information. The small parvocellular layers continue to process color and structure. The larger magnocellular layers process motion and contrast.
Despite everything that happens in the thalamus, this part of the brain is mostly a stop on the way to the visual cortex.
The visual cortex is a part of the cerebral cortex, which is located in the occipital lobe. Visual information travels far - the visual cortex is way in the back of the brain. One interesting thing to note about the visual cortex is that it appears in both hemispheres of the brain, but visual information doesn't travel in the way you might expect. . The left visual cortex receives information from the right eye and visual field. The right visual cortex receives information from the left eye and visual field.
The first place where visual information goes in the visual cortex is the primary visual cortex. I won't go too far into the different parts of the visual cortex now, but there are six different parts that neuroscientists have identified. The primary visual cortex transmits information to the other parts through two pathways: the ventral stream and the dorsal stream.
Scientists have been able to learn quite a bit about where visual information goes, but there are still many unanswered questions! Vision is arguably the most complicated sense that we perceive, so be grateful that you have it!