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3j - Visual Pathways

Posted by Helen St Clair Tracy in Level 3 - Introduction to the Visual Brain
Published: 07/03/2019, 12:13am | Updated: 15/03/2019, 6:23am

Video Link: https://vimeo.com/323532021

The visual process starts with what you are looking at, in the world outside of your mind.

The only world any of us are able to see is the world our brain creates - our inside world - but it should be a reasonably good match with our surroundings - our outside world. The real outside world does of course exist, and that is the starting point of the visual processes.

What you are looking at, in the diagram below a house and some trees, is first processed through the eye, working like a digital camera

Eye Visual Pathways

Looking more closely at the eye (diagram below)

Credit Gordon N Dutton, Vision and the Brain, AFB press 2015

The visual information passes through the clear transparent part of the front of the eye called the cornea. The cornea bends the incoming light, and directs it through your pupil to a lens just behind your pupil. This lens focuses the light further, onto the back of your eye, to create a picture on the lining at the back (the retina).

The retina converts the picture into electrical signals, and sends this visual information on its journey...

First, the picture made of electrical signals travels along the optic nerves (see image below), to a junction point called the optic chiasm, where it is split into two optic tracts, one taking the information about the right side of the scene (from both eyes) to the left side of your brain, the other, taking visual information about the left side of the scene (from both eyes) to the right side of the brain.

Credit Gordon N Dutton, Vision and the Brain, AFB press 2015

At the end of the optic tract, on both sides, are two relay stations inside the brain, called the lateral geniculate bodies, and here the visual information is relayed into the optic radiations, which are the bundles of nerve fibres taking the picture to the back of the brain, called the occipital lobes (see lesson 1b).

The eyes and their optic nerves are considered to be ocular (of the eye), as opposed to cerebral (of the brain) - than means that issues in these areas are connected with the eyes and eye pathways, rather than brain and brain visual pathways. They are all, of course, connected.

The occipital lobes, start to assemble the basic parts the image of what you are looking at, to construct the structure of the picture we create in our minds, as a copy of our surroundings and what we are looking at.

Brain Visual Pathways

We introduced the four different lobes of the brain in the lessons in Level 1, and in these lessons have explained several different visual processes, and the relevant locations in the brain where this happens.

In lesson 3i we explained that visual information in the primary visual cortex (lesson 3b) travels to two main destinations.

  • To the posterior parietal lobes, where the visual attention map (lesson 3g) and depth map of the scene in front of us (lesson 3f) is made. This is so you can know where things are as you and move amongst them and...
  • To the temporal lobes, for recognition, so you know what they are.

Some call these the what and the where pathways.

The where pathway, is also called the dorsal stream.
The what pathway is also called the ventral stream.

Dorsal Stream / Where Pathway

So the dorsal stream takes visual information from the occipital lobes, to the posterior parietal lobes, and adds the depth map (lesson 3f) and visual attention map (lesson 3g) to inform you where, what you are looking at, is. This is why it is sometimes called the Where Pathway or the Where Is It? Pathway

Ventral Stream / What Pathway

The ventral stream takes visual information from the occipital lobes to the temporal lobes, where what you are looking at can be matched with an existing memory, like using Google to look things up, so you recognise what you are looking at, or it can be learned for future recognition. This is why it is sometimes called the What Pathway or the What Is It? Pathway.

The brain has an enormous number of pathways, the ventral and dorsal streams are just two of the key visual pathways.

Checklist:

Before you move onto the next lesson, please check:

  • You understand the difference between the visual pathways of the eyes and brain.
  • You know where the dorsal and ventral streams are.
  • You understand the difference between the What Is It? And Where Is It? Pathways.

Next lesson: Level 3k An Introduction to the Visual Brain - Frontal Vision

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