Visual Processing and Pathways

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  • 0:01 Seeing
  • 0:55 Cortical Visual Pathways
  • 3:02 Noncortical Pathways
  • 3:42 Parallel Processing
  • 4:45 Lesson Summary
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Lesson Transcript
Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

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.'

Noncortical Pathways

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.

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