How does the visual information get from your eyes to your brain?
Visual information is carried from the retina by the retinal ganglion cells through the optic nerve. The optic nerve contains the axons of the ganglion cells and projects to the diencephalon. Upon arrival at the diencephalon, the visual information passes through the optic chiasm, runs through the optic tract, and heads to the thalamus (or the superior colliculus). All sensory information except from olfaction passes through the thalamus. From the thalamus, the information is brought to the occipital lobe.
Now let’s consider this with a little more detail…
Remember how the right side of the brain has information about the left half of the body? Well, this contralateral arrangement is maintained in the visual system as well. However, in the case of the visual system, the right side of the brain has the information from the left half of the visual field. For example, when you look right at the middle of this web page the things that are represented at the left, are in the left half of your visual field, while the things that are represented at the right, are in the right half of your visual field. Therefore, your visual representation of the left-things is in the right side of your brain, while your visual representation of the right-things is on the left side of your brain.
Now you have to try to understand how this separation of the visual world into halves occurs. All the information coming from the right eye (from both the left and right halves of your visual field) comes to the brain in the right optic nerve. Likewise, all the left eye information comes in the left optic nerve. The critical step occurs at the optic chiasm.
The retinal ganglion axons sort in the optic chiasm. I have tried to show this in this figure to the right. To understand it, imagine that you are the person in the picture and you are looking at the visual field that person is looking at.
The visual field is blue to the right and purple to the left. Can you get yourself to picture this? If so, you can figure out that the light from the left half (blue) of your visual field projects onto the right half of your retina (everything is projected upside-down and backward onto the retina). That’s why the right half of the retina (in each eye) is colored blue here.
Retinal ganglion cells from both the left and right halves of the retina project back to the brain from each eye in the optic nerves. However, once they reach the optic chiasm, they sort. All those retinal ganglion cell axons that are carrying information about the blue (left) half of the visual field go toward the right side of the brain in the right optic tract. Vice versa for the green half.
You don’t have to be able to work through all these details. It’s just hard to imagine how this sorting occurs without seeing a picture.
Once the visual information is running in the optic tracts, it goes to a region of the thalamus called the lateral geniculate nucleus (or LGN). From there, it projects to the occipital lobe. The distance from the LGN to the tip of the occipital lobe (where the primary visual cortex is found) is rather far, and the axons run in a loose, tract-like structure called the optic radiations.
Strokes can be so specific in location that the optic nerve, optic tract, LGN, optic radiation, or visual cortex can be singly affected. In addition, only a portion of each of these areas could be affected. In order to understand visual deficits after stroke, one needs to really work through the paths listed above. I will ask you two questions in your assignment for this week about