How do we see? Seeing is actually a very complex process, and it's really amazing that the brain can process visual data so well. In this lesson, we'll explore the visual pathways that allow for seeing and check out a few basic visual processes.
Do you see what I see? No, of course not. I'm sitting in my office looking out the window and you are…somewhere else, presumably looking at this video. But, we are both looking at something. The fact that our eyes and brains can work together to create the sensation we call seeing is actually pretty incredible. We call the part of the central nervous system responsible for processing visual information the visual system, and it's got a big job. Light hits objects, and waves of different lengths bounce off those objects into our eyes and are interpreted by the brain as color, depth, space, and all of the other things we need to know to interact with the world around us. That's visual processing at its most basic. Want to know more? Well, just wait and see.
Cortical Visual Pathways
Let's break this visual system down. The conscious perception of seeing is a result of the cortical visual pathways, the flow of visual information from the eyes to the primary visual cortex to other processing centers in the brain. That's the simple version. Here's how this works. Light waves enter the eye through the retina and strike cells designed to process light called photoreceptors. There are two kinds of these. Rods are on the sides of the retina and help distinguish visual information in low-light environments. Cones are in the center of the retina and process color. There are three sizes of cones, each of which can either process light waves for either red, green, or blue. In total, there are over 100 million photoreceptors in the human eye.
Once light has been processed by photoreceptors, it is passed onto ganglion cells that are designed to process light information into an electrical signal. That signal is passed onto the optic nerve, the primary set of cells that transmit electrical visual data to the brain. Now at one point, called the optic chiasm, the optic nerves from each eye cross. This is important. You get visual data from two eyes, but you don't see in doubles. The optic chiasm superimposes information from both eyes together, so that you only see one image. From here, about 90% of the visual data is sent to the lateral geniculate nucleus, a relay center located in the thalamus for visual information, and from there reaches the primary visual cortex, the main area of the brain that interprets visual data. This is where your mind starts to recognize that the eyes are seeing, and then from there, the information is dispersed to various areas to create a reaction, like, 'Hey, that's pretty' or 'Oh no, that's coming right at me.'
So, that's basically how we see. But what about that 10% of visual data that doesn't make it to the primary visual cortex? That information leaves the optic nerve via subcortical pathways. This information is responsible for subconscious reactions, things like pupil constriction, reflexive reactions to visual stimuli, and the circadian rhythms, or biological clock. Instead of passing through the thalamus, this information goes to the superior colliculus, which is a very important-sounding name for the part of the brain that processes this visual data to produce an automatic response.
When we look at all of this, it's really pretty amazing that our minds can process any visual data, but the truth is that we are generally processing multiple things at the same time. The brain's ability to simultaneously process various information of different qualities is called parallel processing. Basically, it's multi-tasking. In terms of vision, visual data is received as four different components: color, motion, shape, and depth. The brain must unite all four into a single image, which is then compared to memories for reference and processed as sight. This is just one of several visual processes your mind constantly performs with that visual data. It also extracts details that may be more relevant than others, a process called feature detection, can fill in visual gaps by inferring patterns and distinguish real from implied motions and shapes. That's a lot to handle, but the visual system makes it look easy.
The visual system, the part of the central nervous system responsible for processing visual information, is what allows us to see, and it does a good job. The vast majority of visual information is processed through cortical visual pathways. This flow of visual information from the eyes to the primary visual cortex to other processing centers in the brain allows us to be consciously aware of the world around us. It starts with light that enters the retina of each eye that is filtered by photoreceptors and passed into ganglion cells that turn the light waves into an electrical signal. This signal travels up the optic nerve and across the optic chiasm, where optic nerves from each eye cross and superimpose two sets of visual data into one, before passing through the lateral geniculate nucleus and onto the primary visual cortex where it is processed. Some data, however, leaves the optic nerve along subcortical pathways that transmit information to the superior colliculus. Not only can the brain process this, it can actually simultaneously process various information of color, motion, depth, and shape called parallel processing. This is just one visual process of the brain. What else can it do? Well, just open your eyes and see for yourself.