11 - Visual Pathway

Visual Pathways - This shows how information from the upper or lower visual fields of both eyes is conveyed by different segments of the geniculocalcarine tract.

*The upper part of the visual field strikes the lower part of the retina and so that information is carried by the lower part of the optic nerve and optic tract.* - This part stays beneath the part of the optic tract that carries the signals from the top part of the retina, which corresponds to the lower part of the visual field.

Visual Pathway Defects - A very small lesion in the central part of the optic nerve of the right eye

- A very small lesion in the central part of the optic nerve of the right eye (position 1) will interrupt vision that would come from that part of the retina.

Visual Pathway Defects - completely sever or somehow damage the optic tract

- completely sever or somehow damage the optic tract (position 4) between the optic chiasm and the lateral geniculate nucleus on the right side... → That would affect all vision that comes from the right side of the retina, which corresponds to the left visual field in both eyes.

Visual Pathway Defects - cut entire R optic nerve

- cut the entire optic nerve (position 2) it would interrupt all vision from the right eye. - You can match the symptoms to the defects yourself for the remaining.

Visual Pathway - starting with the image

- image is transduced within the photoreceptors and encoded within the ganglion cells. - travels down the axons of the optic nerve (cranial nerve 2) and reaches the optic chiasm. - then the information branches to the L & R lateral geniculate nucleus ( also some of it travels to superior colliculi in the pretectal area.) - From there optic radiations travel the geniculocalcarine tract (one part of which is Meyer's loop and the optical radiations), and the information enters the visual cortex in the occipital lobe.

LGN: *Magnocellular layers*

1-2 input from large retinal ganglion cells larger receptive field detection of motion and location (bigger picture)

Lesion on the right optic nerve would lead to:

1. Right (ipsilateral) anoxia (blindness)

Lesion on the optic chiasm would lead to:

2. Bitemporal hemianopia

LGN: *Parvocellular layers *

3-6 input from small retinal ganglion cells smaller receptive field detection of color and form (finer details)

Lesion on the right optic tract would lead to:

3. Left (contralateral) homonymous hemianopia

Lesion on the right Meyer loop would lead to:

4. Left (contralateral) superior quadrantanopia Meyers loop = upper fibers

Lesion on the right optic radiation would lead to:

5. Left (contralateral) inferior quadrantanopia Lower fibers

A right PCA infarct would lead to:

6. Left (contralateral) hemianopia with macular sparing

Left macular degeneration would lead to

7. Left central scotoma

Visual Pathway Defects - A lesion that only interrupts part of the optic tract (R Meyer's loop)

A lesion that only interrupts part of the optic tract (position 5) would only cause a partial defect in vision. → In this case we have interrupted Meyer's loop and that is the lowest part of the optic tract. That corresponds to the bottom-most fibers coming from the retina, which corresponds to the top most part of the image.

Visual Field Defects

A patient might be blind altogether in the left eye (A). The patient might be blind only in the right visual field of the left eye (B). Both eyes may be experiencing defect in half their vision, but the defect is in the opposite half of the visual field in each eye. *Bitemporal Hemianopia* (C): can't see in the temporal half of the visual field of each eye. *Contralateral homonymous hemianopia (D)* - In this case both right halves of the visual field are affected in both eyes. *Contralateral homonymous superior quandrantanopia* (E) - The defect is in the upper right quadrant of both visual fields. *All of these defects can be explained by lesions at various points in the optical pathways.*

*sectoranopia*

Loss of vision in a sector of the visual field.

Hypercolumns of the Visual Cortex

Ocular Dominance Columns - Stereopsis (depth perception)with some help from corpus callosum Orientation Columns Blobs - color vision

Visual Pathways - how information from one eye is ultimately distributed over the visual cortex of both hemispheres, depending on left or right visual field. Note how the foveal region accounts for a disproportionate cross-section of the nerve, tract, radiations and cortex. ON = optic nerve OC = optic chiasm OT = optic tract

T - temporal, N-nasal -peripheral part and a central part, and the central part is what will be projecting on the fovea. - *Image is flipped onto the retina and the information is carried by the optic nerve* - see on the x-section of the optic nerve that the blue T is on the upper R portion of the optic nerve, and that corresponds with the lower L hand part of the image. *At the optic tract after the optic chiasm, only information from the R visual field is carried in the L part of the optic tract.*

Patient came in complaining that he was unable to see out of his right eye. - And so upon testing it was revealed that he could see out of both eyes, but it was actually the right half of both eyes that was missing vision.

The image above shows the corresponding infarct that was found to cause his symptoms. - It was a left hemisphere problem that led to a problem in the right visual field of both eyes.

Visual Pathways - information from both eyes is ultimately distributed over the visual cortex of both hemispheres, according to visual field.

The signals from the medial retina cross over. - the right eye's right medial retina crosses over to the left optic tract and the left eye medial retina crosses to the right optic tract. - The information in the left part of the optic tract is from the right part of the visual field.

Retinotopic mapping - pine tree example

There is an image of a pine tree, and when that image is projected on the retina it's inverted. - To a certain approximation, there will be a pine tree shaped region of the visual cortex that is stimulated in response to this image.

A helpful way to think about where the defect is:

Think about a giant eyeball and if you were to shoot into it. Where would the exit wound be? - For example if you were to shoot at the right, then the exit wound would be at the left. - And the left hand part of the retina and everything connected to it become potential suspects in terms to where the lesion is. Another example: - Shoot from the left, the exit wound would be on the right. - If the defect is on the upper left part of the visual field (position 5), then you shoot from the upper left and it exits at the lower right. - And that means the lesion is on the lower right part of the optic tract.

Visual Field Test

Various lesions in the neural pathways of the visual system affect the sense of the image. A visual field test can be performed - A dot of light is shown within the interior surface of the hemisphere and the patient's head is held fixed in the device. - The patient's head is held steady while the point of light is shown around the hemisphere. A map of the patient's visual system is performed and this is called *Goldmann perimetry.*

*scotoma*

an area of partial alteration in the field of vision consisting of a partially diminished or entirely degenerated visual acuity that is surrounded by a field of normal - or relatively well-preserved - vision.

*hemianopia*

defect in one hemisphere of vision

*quadrantanopia*

defect in one quadrant of vision

Retinotopic mapping

information from each region of the retina is kept separate from the information coming from other parts. - travels in parallel fibers through the visual system, and eventually ending in the primary visual cortex. [*This scheme is called retinotopic*] - each portion of the visual cortex has a corresponding region on the retina.

*bitemporal*

loss of visual in both temporal fields

*heteronymous*

oppposite sides of both eyes (bitemporal, binasal)

Visual pathway - gross diagram

optic nerve comes to the optic chiasm, and becomes the optic tract. Then it synapses on the lateral geniculate body. And that in turn sends projections out via optic radiations. - can see Meyer's loop, one specific part of the optic tract traveling from the lateral geniculate nucleus out to the primary visual cortex.

Lateral Geniculate Nucleus (LGN)

receives visual data from both eyes Information coming in through the left visual field strikes the right part of the retina. - From the left eye that information crosses over at the optic chiasm, and it is joined by information from the same part of the visual field coming from the right eye. - It then makes several projections onto the lateral geniculate nucleus (organized into layers) - *Magnocellular layers (1-2) * - *Parvocellular layers (3-6)*

*homonymous*

same side of both eyes (both L visual fields)

The dark part of the visual field diagram corresponds to

the defect in vision, then match back to the defect in the visual pathways that are inverted with respect to left and right and up and down.

Tips to remember about visual cortex and visual field

the visual cortex has a left and right, up and down organization that matches the organization of the image; it is just flipped over. - The organization of the visual field and visual cortex are similar, but just inverted.