The Visual System (Part 1)

I am a visual learner, and I learn well from pictures.

In particular, I learn well from picture stories, which are sequences of pictures and words in an alternating pattern:

Picture.
Text.

Picture.
Text.

Picture.
Text.

Recently, I have been learning about visual perception, and I would like to share what I learned in the way that I like to learn things. This is a general overview of the visual system, as shown and told in a picture story about the structures and functions of the eyes and the brain with which we see.

Naturally, it starts with a cat…

…a flat cat.

The flat cat is a plane through the brain, and we are looking at the brain plane from the top down to see the basic layout of the visual system.

In the visual system, seeing starts with the eyes.

The eyes collect and focus the light of the world as an image on the retina, a layer of photosensitive tissue at the back of the eye that serves the same function as the image sensor of a digital camera.

On the retina, the focused image is upside-down and left-right reversed, so the right side of the world is seen with the left sides of both eyes, and the left side of the world is seen with the right sides of both eyes.

In the retina, millions of photoreceptor cells absorb the light of the image, convert it into electrical signals, and transmit the signals to nerve cells that relay the visual data into the brain. Along the nerve fibers of nerve cells, the data travel out of the eyes, into the brain, and from the front of the brain to the back of the brain.

The journey starts with the optic nerve, the bundle of fibers that initially exits the eye.

At this stage, the fibers from the left and right sides of each eye travel together, and so do the data from the left and right sides of the world.

At the optic chiasma, the fibers split, and so do the data from the two sides of the world.

The fibers from the left side of the left eye go to the left side of the brain, the uncrossed fibers carrying the data from the right side of the world.

The fibers from the right side of the right eye go to the right side of the brain, the uncrossed fibers carrying the data from the left side of the world.

The fibers from the right side of the left eye cross over to the right side of the brain, the crossed fibers carrying the data from the left side of the world.

The fibers from the left side of the right eye cross over to the left side of the brain, the crossed fibers carrying the data from the right side of the world.

As a result, the fibers from the left sides of both eyes go to the left side of the brain, and the fibers from the right sides of both eyes go to the right side of the brain. Because the image on the retina is left-right reversed, the left side of the brain processses the right side of the world, and the right side of the brain processes the left side of the world.

This is the general pattern of left-right reversal with which the brain interacts with the body and the world.

Beyond the optic chiasma, the fibers travel through the optic tract, each set of fibers carrying data from one side of the world.

At the thalamus, the fibers terminate and transmit their signals to the cells of the lateral geniculate nucleus (LGN), a structure of the thalamus that is specialized for visual processing, as other structures of the thalamus are specialized for sensory processing in other sensory systems.

In the LGN, the crossed and uncrossed fibers terminate in different layers, where the data are processed in different ways on their way to the cerebral cortex, the big folded sheet of the big hoooman brain, and the part of our hoooman brains that makes us hooomans particularly smart.

Following processing, the cells of the LGN project their fibers to the back of the brain, as the optic radiation between the thalamus and the cortex.

In the back of the brain, the fibers terminate in the primary visual cortex (V1) of the occipital lobe, the region of the cerebral cortex that is specialized for visual processing.

Here, the cells of the cortex process the data that have traveled all the way through the brain, from the literal eyes at the front of the head to the figurative eyes in the back of the head, into an internal mental representation of the external physical world.

How this is done is mostly unknown, but it is hypothesized that the visual data are processed at higher and higher levels of complexity as they travel through V1 to the rest of the visual cortex, from V1 to V2 to V3 to V4 to V5, and the rest of the brain, from the occipital lobe to the temporal, parietal, and frontal lobes. In addition, connections from the top-down, from the temporal, parietal, and frontal lobes, feedback into the occipital lobe to affect visual processing in the visual cortex.

What a mess! A hot mess!

I googled the meaning of this expression just now, and it fits a bit, I wit.

In one hypothesis of visual processing, different pathways through the brain specialize in processing different kinds of information about the world. According to the Two Streams Hypothesis, visual data travels in two such streams beyond the occipital lobe.

In the ventral stream, data travels from the occipital lobe to the temporal lobe, where objects are recognized, categorized, and attended, the better to spot, identify, and desire the catnip in your field of view, if only you could get there to use and abuse it.

In the dorsal stream, data travels from the occipital lobe to the parietal lobe, where objects are placed in space, and movements to them coordinated, the better to pounce, paw, lick, and chew the catnip in the cabbage patch under the mango tree. Do these species of plants all grow together in the same place for you to pounce and paw in crazed catnipped eggsitement? Who knows? Who cares, as long as there is plenty of catnip to be had by all, purrr purrr purrr purrr purrr purrr purrr?

The dorsal stream is also called the “where” pathway for its role in spatial awareness, and the ventral stream is also called the “what” pathway for its role in object identification.

So there you have it, the whats and wheres of the visual system and its parts, rinsed and repeated in fifteen different versions of the same flat cat, and built up from the parts to the whole in a picture story that is an effective learning material for me, and perhaps you too.

In Part 2, I am going to blog about why I like to learn this way, from picture stories onscreen or onpaper, what that has to do with how I see, think, and learn in general, and how this kind of material can be used by all kinds of people to learn all kinds of things.

Meanwhile, here is a summary of the visual system for your visual system:

Is it burrrrrrrned into your brrrainzzz yet?

No worries, if it’s not. I’m sure that you’ll be seeing it again…and again…and again…purrr purrr purrr purrr purrr purrr purrr…

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2 thoughts on “The Visual System (Part 1)

  1. Pingback: The Visual System (Part 2) | Autistic And Awesome

  2. Pingback: When the Dorsal Stream is Shallow | The VisionHelp Blog

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