2. Basic Visual Pathways

BASIC VISUAL PATHWAYS

A. The pathway:

Vision is generated by photoreceptors in the retina, a layer of cells at the back of the eye. The information leaves the eye by way of the optic nerve, and there is a partial crossing of axons at the optic chiasm. After the chiasm, the axons are called the optic tract. The optic tract wraps around the midbrain to get to the lateral geniculate nucleus (LGN), where all the axons must synapse. From there, the LGN axons fan out through the deep white matter of the brain as the optic radiations, which will ultimately travel to primary visual cortex, at the back of the brain.

B. Visual fields:

Information about the world enters both eyes with a great deal of overlap. Try closing one eye, and you will find that your range of vision in the remaining eye is mainly limited by your nose. The image projected onto your retina can be cut down the middle, with the fovea defining the center. Now you have essentially two halves of the retina, a left half and a right half. Generally, the halves are referred to as a temporal half (next to your temple) and a nasal half (next to your nose).

Visual images are inverted as they pass through the lens. Therefore, in your right eye, the nasal retina sees the right half of the world, while the temporal retina sees the left half of the world. Notice also that the right nasal retina and the left temporal retina see pretty much the same thing. If you drew a line through the world at your nose, they would see everything to the right of that line. That field of view is called the right hemifield.
So, what you see is divided into right and left hemifields. Each eye gets information from both hemifields. For every object that you can see, both eyes are actually seeing it - this is crucial for depth perception - but the image will be falling on one nasal retina and one temporal retina.

Why bother to divide the retinas at all? Recall that the brain works on a crossed wires system. The left half of the brain controls the right side of the body, and vice versa. Therefore the left half of the brain is only interested in visual input from the right side of the world. To insure that the brain doesn't get extraneous information, the fibers from the retina sort themselves out to separate right hemifield from left hemifield. Specifically, fibers from the nasal retinas cross over at the optic chiasm - whereas the temporal retinas, already positioned to see the opposite side of the world, do not cross. Here is what it looks like:

The practical consequences of this crossing are that damaging the visual system before the chiasm will affect one eye, both hemifields - analogous to closing one eye. Damaging the pathway after the chiasm, though, will damage parts of both eyes, and only one hemifield. There is no easy way to imagine what this would look like. Your field of view would be only 90°, from straight ahead to one side.

C. Lesions

The easiest way to demonstrate to yourself the consequences of lesions is to strike through a pathway, follow the fibers back to the retina, and see what was affected. Notice that there are lines and numbers drawn on the visual field diagram. For each "cut", determine what parts of the patient's visual field will be affected. The way to record a loss of visual field is with two circles, called "perimetry charts" as below. You can think of these circles as a pair of goggles that the patient is looking through, and you blacken those parts of the goggles where vision is lost. This is done separately for each eye, and drawn from the patient's perspective - the right circle represents the right eye. For example:

Now, try to draw visual field diagrams for lesions 1-3 in the diagram above.
Scroll down for the answers.
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Lesion 1: This is analogous to losing an eye. One eye is completely blacked out.
Lesion 2: Here you have only cut inputs from the nasal retinas, so you would lose peripheral vision on both sides. This can be caused by a pituitary tumor (the pituitary lies just under the optic chiasm).
Lesion 3: This lesion represents the loss of the left hemifield. Both eyes will be blind to anything on the left side of the world (assuming the eyes are pointed straight ahead).

What about the last 3 lesions? To figure these out, you need to know about Meyer's loop and the optic radiations. The optic radiations follow a very wide three dimensional arc. Here is how the radiations are conventionally drawn, and how they look from the side:

You can see that the longer loop actually dives into the temporal lobe before it heads back to the occipital lobe. This group of fibers is called Meyer's loop. Recall that, since the lens inverts all images, the lower half of the retina sees the upper half of the world. This orientation is preserved through the pathway, so that the lower optic radiations, or Meyer's loop, are carrying information from the upper visual world.

Now try to work through lesions 4-6.
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Lesion 4: Meyer's loop has been cut, so vision will be lost in the upper visual world, but only in the left hemifield.
Lesion 5: Here the parietal portion of the optic radiations were cut, so you would affect the lower visual world on one side.
Lesion 6: At first this seems to be a straightforward loss of one hemifield. However, a curious phenomenon results when cortex itself is lesioned.
Vision at the fovea is spared, perhaps because there is such a large representation of the fovea in the cortex, or perhaps due to overlapping blood supply. The loss of vision is not a complete hemifield, then, but a notched hemifield. This phenomenon is called macular sparing.

D. Anatomy

You should be able to follow the visual pathway through coronal or horizontal sections. In a coronal series, the most rostral thing you will see is the optic chiasm. The optic tracts will diverge and sneak up laterally around the cerebral peduncles before diving into the LGN. Don't confuse the optic chiasm with the anterior commissure; the chiasm will always hang down from the base of the brain, while the commissure will be embedded in tissue.

In horizontal sections you can see the optic radiations clearly, and you can identify the general vicinity of visual cortex. First find the calcarine sulcus on the medial surface of the occipital lobe. Primary visual cortex, or V1, is buried within this sulcus. In a fortuitous section, you may be able to see a fine white stripe running within the grey matter inside the sulcus. This stripe marks V1, and gives it a third name, striate cortex.