CNS Unit 8 Visual System Flashcards
How does light enter the eye?
What happens when the image is formed?
The eye enters the eye through the lens to form image on the retina.
This is inverted and reveresed
Where does the Superior and Inferior Visual Fields project?
- The Superior visual field projects onto the lower retina
- The Inferior Visual fields projects onto the upper retina
If someone was staning in front of you, the top half of the their body would project to the lower retina; and the bottom half would project to the upper retina
Where do the Right and Left Visual Fields project?
- The Right Visual field will project to the left side of the retina of each eye
- The Left Visual field will project to the Right side of the retina of each eye
If someone was standing in front of you, the left side of their body would project to the Right Retina of both eyes; and the right side of their body would project to the Left Retina of both eyes.
When looking at a Normal Visual Field, where do they normally track?
They extned more to the periphery than medially (toward nose); It extends more inferiorly than superiorly
What is the Fovea?
The Central fixation point for each eye
- Its the area of retina with highest visual acuity
- Its surrounded by the Macula, this also has a high visual acuity
- It projects to the occipital pole
What is the Optic Disk formed by?
The optic disk is formed by axons leaving the retina they enter the Optic nerve
Are there any photoreceptors over the Optic Disc?
There are no Photorecpetors over ther Optic Disk, there is also a small blind spot located approximately 15° laterally and slightly inferior to the central fixation point of the eye
We do not see any functional deficits when both eyes are used, when one eye is used, the visual system fills in the blind spot
What are the 3 layers of the Retina?
1st: The outermost layer contains the photoreceptors.
2nd: The Middle Layer or Bipolar Layer
3rd: Innermost Layer
With the 1st layer of the Retina, What does this layer contain?
This layer contains our photoreceptors which respond to light and create synapses onto bipolar cells
We have 2 types of Photorecpetors:
- Rods: These are specialized receptors for Peripheral vision and function in dim light (outnumber cones 2-:1).
Poor spatial and temporal resolution for visual stimuli - Cones: Specialized recpeptors for color vision and acuity. High Spatial and temporal resolution; Highly represented in the fovea.
Function optimally in bright light
What is the Middle/Bipolar Layer in charge of?
This layer receive input from the photorecptors; then send the input to the ganglion cell layer
With the 3rd layer of the Retina, What does this layer contain?
What are the different types and what do they do?
This is our Innermost layer that contains our Ganglion Cells
We have 2 types of Ganglion cells
- Parasol Cells: Repond to gross stimulus features and movement (Large cells bodies and very large diameter fibers)
They are going to project to layers of the Lateral Geniculate Nucleus (LGN) of the thalamus, known as the Magnocellular layers. - Midget Cells: Respond to very fine visual details and colors; They have small cells bodies and small diameter fibers.
They are going to project to the Parvocellular Layers of the LGN of the thalamus
How many layers are there in the LGN of the Thalamus?
There are 6 layers
- Parvocellular Layers (P pathway): Layers 6-3
- Magnocellular Layers (M pathway: Layers 2-1
Remember Parasol Cells project to Magnocellular layers; Midget Cells project to Parvocellular layers (these are from the Ganglion Layer of Retina)
The Magnocellular Layers are receiving Input from the Parasol cells
The Parvocellular Layers are getting input the Midget cells
(The output for both layers is the Primary Visual Cortex)
Where does the Optic Nerve Receive input from and where does it exit?
From the Retinal Ganglion Cell Axons and they exit in the Optic Canal
The entire visual field from the right eye is carried in the right optic nerve; and the entire visual field from the left eye is carried in the left optic nerve
What is the role of the Optic Chiasm?
What would happen if there was injury to the Optic Chiasm?
In the Optic Chiasm we see that fibers cross; The fibers that cross are the Medial/Nasal fibers
- The Nasal Fibers are responsible for the Lateral/Temporal part of our visual field
If there was an injury to the Optic Chiasm we would have Bilateral Hemianopsia, AKA Tunnel Vision.
What is the role of the Optic Tracts?
What would happen if there was damage to one of the Optic Tracts?
The Optic Tracts carries information from the Ipsilateral hemiretina of each eye
R Optic Tract = Left Visual Field
L Optic Tract = Right Visual Field
*For ex. The R Optic Tract is carrying inforation from the R side of the Retina in each eye
*
- If there was damage to one of the Optic Tracts, we would lose vision to the Contralateral Visual Field. This is called Contralateral Homonymous Hemianopsia.
Where do the Optic Tracts synapse?
The Optic Tracts wrap around the midbrain, and synapse on the Lateral Geniculate Nucleus of the Thalamus
If we are thinking about the Right Optic Tract, they are carrying information from our Left visual field. The Right optic tracts are going to then synapse on the Right LGN.
Right LGN will have information about the Left Visual Field
What is the Extrageniculate Pathways?
This is when a minority of fibers in the optic tract bypass the LGN to enter the Superior Colliculus and Pre-tectal areas; they form the Extrageniculate visual pathways.
The pretectal areas are involved in the pupillary light reflex.
Both the Superior Colliculus and Pretectal areas are involved in directing visual attention and eye movements toward a visual stimulus. This happens through projections to the brainstem and the lateal parietal cortex as well as our frontal eye fields.
What are Optic Radiations?
What happens if there are lesions of Optic Radiations?
When axons leaving the LGN project to the Primary Visual Cortex (PVC); These axons fan out over a large area as they project back to the PVC. This is what forms the optic radiations.
- They are simular to optic tracts, becuase Ipsilateral optic radiations carry information from the contralateal visual fields
All fibers of Optic Radiations pass through parietal and temporal lobes
Lesions of Optic Radiations would cause Homonymous Contralateral visual field loss.
What are the 2 fibers with Optic Radiations?
- The Inferior Fibers are called Meyer’s Loop or Inferior Optic Radiation.
- Superior Loop/Superior Optic Radiations
What is the pathway of the Inferior Optic Radiation (Meyer’s Loop), and what is it’s function?
They pass through the temporal lobe as they leave the LGN and they arc backwards towards the Primary Visual Cortex in the occipital lobe. They go down to the Lower Bank of the Calcarine Fissure.
- It carryies information from our Inferior Retina and therefore they’re supplying information about our Superior visual field.
For ex. if someone is standing to my left, we would see their top half of their body, which is being carried out on the Right Optic Radiation, this then goes to the inferior part of the right retina in both eyes, to the R Optic tract, to the Right LGN and to the inferior Optic radiation
What would happen if we had a lesion to the tempoal lobe that damaged Meyer’s Loop?
We would get Contralateral Superior Quadrantanopia; “Pie to the sky”
What would happen if we had a lesion to the parietal lobe that caused damage to the Superior Optic Radiation?
We would get Contralateral Inferior Quadrantonopia; “Pie on the floor”
The Primary Visual Cortex is Retinotopically Organized, what does this mean?
This means what we see in our retina goes to very specific parts of the occipital cortex.
- The Fovea is going to occupy about 50% of the primary visual cortex; The Fovea and Macula are represented in the occiptial pole and both have so much detailed information in them, so they need a large part of the brain to be able to process the information
- The more peripheral regions of retinas are represented more anteriorly along the calcarine fissue (Labled 2 and 3 in the picture); they get the other 50% looking more at gross detail so they dont need such a big spot in the brain
Parallel Channels for Analyzing Motion, Form, and Color
If we are looking at Motion/Spatial Analysis what is the Retinal Ganglion cell assocaited and what is the pathway it takes to get to the Higher-order Visual association cortex?
Starting from the Retinal Ganglion cells to Thalamus (LGN) to Primary Visual to Visual Association to Higher-order Visual Association
- Retinal Ganglion Cell: Parasol Cells
to - Thalamus LGN: Magnocellular Layers
to - Primary Visual Cortex (BA 17): Goes to Layer 4C alpha then layer 4B
to - Visual Assocation Cortex (BA 18): Thick Strip OR it goes straight to Higher-Order Visual Association Cortex: Dorsolateral parieto-occipital Cortex
If it did not go stright to the Higher-order Visual Cortex and went to the Visual Association, it will then go to… - Higher-Order Visual Association Cortex: Dorsolateral parieto-occipital Cortex
Parallel Channels for Analyzing Motion, Form, and Color
If we are looking at Form what is the Retinal Ganglion cell assocaited and what is the pathway it takes to get to the Higher-order Visual association cortex?
Starting from the Retinal Ganglion cells to Thalamus (LGN) to Primary Visual to Visual Association to Higher-order Visual Association
- Retinal Ganglion Cell: Midget Cells
to - Thalamus LGN: Parvocellular Layers
to - Primary Visual Cortex (BA 17): Goes to Layer 4C Beta then layer 2,3 Interblobs
to - Visual Assocation Cortex (BA 18): Pale Strip
to - Higher-Order Visual Association Cortex: Inferior Occipitotemporal Cortex
Parallel Channels for Analyzing Motion, Form, and Color
If we are looking at Color what is the Retinal Ganglion cell assocaited and what is the pathway it takes to get to the Higher-order Visual association cortex?
Starting from the Retinal Ganglion cells to Thalamus (LGN) to Primary Visual to Visual Association to Higher-order Visual Association
- Retinal Ganglion Cell: Midget Cells
to - Thalamus LGN: Parvocellular and Interlaminar regions
to - Primary Visual Cortex (BA 17): Goes to Layer 4C Beta then layer 2,3 blobs
to - Visual Assocation Cortex (BA 18): Thin Strip
to - Higher-Order Visual Association Cortex: Inferior Occipitotemporal Cortex
Parallel Channels for Analyzing Motion, Form, and Color
Where do Dorsal Pathways project?
The Dorsal Pathways project to Parieto-occipital Assocaition Cortex
This answers the question of where, this helps us analze motion as well as spatial relationships between objects
Parallel Channels for Analyzing Motion, Form, and Color
Where do Ventral Pathways project?
Ventral pathways project to occipitotemploral association cortex
This answers the question of what, “What are we looking at?”. It helps us identify colors, faces, latters and other visual stimuli
When assessing for visual disturbances, how do you test for Visual Acuity?
This is often reported with the Snellen Eye Chart of 20/x.
- “X” is distance at which a normal individual can see the smallest line of the eye chart seen at 20ft
Visual field defects usually DO NOT affect visual acuity
For example:
- If I have really poor vision, the line I can read at 20 feet, somebody with normal vision can read at 200 feet; My Visual acuity will then be 20/200
- If I have really good vision, the line I can read at 20 feet, somebody with normal vision can read at 15 feet; My Visual acuity will then be 20/15
What may cause Formed Visual Hallucinations?
- It can be caused from toxic or metabolic disturbances, maybe an overdose in drugs.
- It can be caused by a withdrawal from alcohol or sedatives
- Sometimes there are focal seizures in that ventral stream that can cause this
- Sometimes people will have complex migraines of other neurodegenerative conditions or psychiatric disorders
What type of Lesion is this? Where would this lesion be?
What may cause this?
This is a Monocular Visual Loss
- Lesion will be in the Optic Nerve or B
Can be caused by:
- Glaucoma
- Optic Neuritis
- Neuropathy of the optic nerve
- Elevated Intercranial Pressure
- Tumor
- Any trauma
What type of lesion is this? Where would this lesion be?
What may cause this?
This is a Scotoma
- This is a lesion of the Retina or A
Can be caused by:
- Retinal Infacts
- Hemorrhage
- Degeneration
- Infection
What type of lesion is this? Where would this lesion be?
What may cause this?
Bi-temporal Hemianopsia
- This is a lesion of the Optic Chiasm or C
Lose vision of lateral (temporal) visual fields
Can be caused by:
- Pituitary Adenoma
- Hypothalamus Glioma
- Some other miscellaneous tumors that may press on optic chiasm
What type of lesion is this? Where would this lesion be?
What may cause this?
Contralateral Homonymous Hemianopsia
- Lesion can be in Optic Tract (D), Lateral Geniculate Nucleus (F), the Entire Optic Radiation (G) , or the Entire Primary Visual Cortex (H)
Can be cause by:
- Tumors
- An infarct of several arteries
- Demyelination to either the tracts or the radiations
What type of lesion is this? Where would this lesion be?
What may cause this?
Contralateral Inferior Quadrantanopia
“Pie on the Floor Deficit”
- Lesion to Superior Optic Radiation (F) or to the Upper Bank of the Calcarine Fissure (I)
Can be caused by:
- Infarcts (Mainly the superior division of MCA or the PCA)
-
What type of lesion is this? Where would this lesion be?
What may cause this?
Contralateral Superior Quadrantanopia
“Pie in the Sky Deficit”
- Lesion to the Inferior Optic Radiation/Meyers Loop (E) or the Lower Bank of the Calcarine Fissure (J)
Localization fo Visual Field Defects
What is Macualr Sparing?
This is a partial lesion of the visual pathway
This can result in the central visual field to be sparred
This usually refers to cortical lesions
- Contralateral flow from MCA or PCA to visual cortex
Localization of Visual Field Defects
What would happen if there was a partial lesion to the Primary Visual Cortex?
What lesion would cause this?
- A partial lesion to the Primary Visual Cortex, specifically to the occipital pole, would primary involve the are represented by Fovea and Macula
- This may result in visual loss in the center of the visual field which would be called a Central Scotoma
This can happen through a head injury involoving hitting the back of the head at the bottom of the occiput
Lesions to the Primary Visual Cortex include: Upper and Lowe Banks of Calcarine
What are the 2 Main branches of the Retinal Artery? What may an occlusion of one of these arteries cause?
- Superior b.-this supplies superior half
- Inferior b.-this supplies the inferior half
An occulusion of causes an Altitudinal Scotoma in one eye
What is the Primary Visual Cortex supplied by?
PCA
Most of the time, when there are large infarcts of the PCA, they are going to whip out the entire one side of the primary visual cortex.
- This will cause a Contralateral Homonymous Hemianopsia; due to damage of both inferior and superior banks of calcarine fissure
