Visual Pathways Flashcards
What is the cornea?
Clear protective outer layer of the eye
What is the iris?
This circular structure that controls the diameter of the pupil and defines eye color
What is the pupil?
The hole in the center of the iris that allows light to enter the eye
What is the lens?
Structure in the eye that refracts light passed through the cornea to form an image on the retina
What is the vitreous body?
clear, gel-like structure between the lens and the retina
What is the retina?
Layer of cells lining the back wall of the eye that senses light and sends signals to the brain for vision
What is the fovea?
Region of the retina with the highest visual acuity
What is the macula?
Oval region that surrounds the fovea that has relatively high visual acuity
What is the optic disc?
Region where the axons leaving the retina gather to form the optic nerve, no photoreceptors are in this area (blind spot)
How are images projected onto the retina?
- The lens inverts and reverses the the projection of the visual image on the retina
- Left and right swapped, up and down swapped
How is the blind spot formed?
- Central fixation point for each eye falls on the fovea
- Optic discs sits 15 degrees medial to the fovea which creates a blind spot 15 degrees lateral from center of visual field
True or False
You are aware of the blind spots created by the optic disc during daily life
False
- No functional deficit caused by blind spots
- Unaware of blind spot even with one eye closed
- Visual analysis pathways “fill in” blind spot
What are the two classes of photoreceptors?
Rods and cones
What are rods and what do they do?
- Most numerous type of photoreceptors
- Responsible for vision in low lighting conditions
- Does not detect color
- Poor spatial and temporal resolution
What are cones and what do they do?
- Less numerous type of photoreceptors
- Highly represented in the fovea for increased visual acuity
- Detects color: 3 types contain 3 forms of pigment and absorb light from different parts of the spectrum
- High spatial and temporal awareness
What are the layers of the retina?
- Ganglion cell layer
- Inner plexiform layer
- Inner nuclear layer
- Outer plexiform layer
- Outer nuclear layer
- Photoreceptor layer
Why are most layers of the retina absent at the fovea?
Allows light to reach receptors without distortion
How do images go from the retina to the optic nerve?
- Photoreceptors activated by light and synapse on bipolar cells
- Bipolar cells synapse n ganglion cells
- Axons of ganglion cells sent to the optic nerve
True or False
Photoreceptors communicate with bipolar cells via action potential
False
- Neither photoreceptors or bipolar cells fire action potentials
- Information is conveyed via passive electrical conduction
- Neurotransmitters are released in a graded fashion
What are the types of interneurons involved in the visual pathway?
Horizontal cells and amacrine cells
How do interneurons influence the visual pathway?
- Interneurons have lateral inhibitory and excitatory connections with nearby photoreceptors, bipolar cells, and ganglion cells
- Light on the retina can have one effect (excitatory or inhibitory) on bipolar and ganglion cells directly in it’s path
- The opposite effect (excitatory or inhibitory) will to surrounding bipolar and ganglion cells via interneurons
What are the two classes of center-surround bipolar and ganglion cells?
On-center cells and off-center cells
How are on-center cells influenced by light?
Cells in the center of receptive field: excited by light
Cells surrounding the area: inhibited by light
How are off-center cells influenced by light?
Cells in the center of receptive field: inhibited by light
Cells surrounding the area: excited by light
What are the two types of retinal ganglion cells?
Parasol cells and midget cells
What are the characteristics of parasol cells?
- Large cell bodies
- Large receptive field
- Responds best to gross stimulus features and movement
- Large diameter fibers
- Project to magnocellular layers of the lateral geniculate nucleus of the thalamus
What are the characteristics of midget cells?
- Small cell bodies
- Small receptive fields
- More numerous
- Sensitive to fine visual details and colors
- Smaller diameter fibers
- Project to parvocellular layers of the lateral geniculate nucleus of the thalamus
Describe the optic nerve
- Formed by retinal ganglion cells
- L and R optic nerves meet at the optic chiasm
Describe the optic chiasm
- Area where L and R optic nerves meet
- located on the ventral surface of the brain beneath the frontal lobes and anterior to the pituitary gland
- Medial retinal fibers for each eye cross, lateral retinal fibers for each eye remain ipsilateral
Describe the left optic tract
- Consists of fibers from the left hemiretina of each eye to convey images from right visual field
- Medial fibers from right eye, lateral fibers from left eye
Describe the right optic tract
- Consists of fibers from the right hemiretina of each eye to convey images from left visual field
- Medial fibers from left eye, lateral fibers from right eye
What two pathways is the optic tract involved in?
- Geniculate pathway
- Extrageniculate pathway
Describe the geniculate pathway
- Optic tract synapses in lateral geniculate nucleus of the thalamus
- Relays to the primary visual cortex via optic radiations
- Function: visual discrimination and perception
Describe the extrageniculate pathway
- Optic tract synapses in the superior colliculus and pretectal areas of the midbrain
- Projects to pulvinar and lateral posterior nucleus of the thalamus
- Projects to lateral parietal cortex and frontal eye fields
- Function: direct visual attention and eye movement towards stimuli
True or False
The lateral geniculate nucleus has 6 layers from ventral to dorsal that keeps information from the left and right eye segregated
True
- Dorsal parvocellular layers (3-6): input from retinal ganglion midget cells, relay information for form and color
- Ventral magnocellular layers (1-3): input from retinal ganglion parasol cells, relay information for motion and spatial analysis
What are optic radiations?
- Axons that traveling from lateral geniculate nucleus to primary visual cortex
- Broken up into inferior optic radiations (meyer’s loop) and superior optic radiations
Describe the inferior optic radiations (meyer’s loop)
- Information from inferior retina (superior visual field)
- Arcs into temporal lobe
- Terminates inferior to calcarine fissure (lingula)
Describe the superior optic radiations
- Information from superior retina (inferior visual field)
- Passes under the parietal lobe
- Terminates superior to calcarine fissure (cuneus)
Where is the fovea and retina in terms of retinotopical organization of the brain
Fovea = medial and lateral occipital poles
Peripheral retina = along calcarine fissure
Describe visual processing in the neocortex
- Input to the primary visual cortex arrives in layer 4
- Layer 4 is divided into sublaminae 4A, 4B, 4C alpha, 4C beta
Describe layer 4B of the neocortex in the primary visual cortex
- Contains myelinated axon collaterals that results in a pale appearance called the stria of gennari
- gives the name of striate cortex
Describe layer 4C alpha of the neocortex in the primary visual cortex
- Receives input from the magnocellular layers of lateral geniculate nucleus
- Information about movement and gross spatial features
Describe layer 4C beta of the neocortex in the primary visual cortex
- Receives input from the parvocellular layers of lateral geniculate nucleus
- Information about fine spatial information
True or False
The channels for color, form, and motion/spatial analysis cross as they travel to the higher order visual association cortex
False
These channels are parallel and never cross
Describe the pathway for motion/spatial analysis
Parasol cells –> magnocellular layers of LGN –> layer 4C alpha to 4B of the neocortex –> thick strip of visual association cortex –> dorsolateral parieto-occipital cortex
Describe the pathway for form
Midget cells –> parvocellular layers of LGN –> layer 4C beta to layer 2, interblobs of neocortex –> pale stripe of visual association cortex –> inferior occipitotemporal cortex
Describe the pathway for color
Midget cells –> parvocellular and interlaminar regions of LGN –> layer 4C beta to layer 2,3 blobs of neocortex –> thin stripe of visual association cortex –> inferior occipitotemporal cortex
What are ocular dominance columns?
Inputs from each eye terminate in different alternating bands of the cortex
What are orientation columns?
- Vertical columns selectively that collectively respond to lines of a specific orientation
- Detects orientation of light to help detect shape
What are hypercolumns?
A region that contains a complete sequence of both ocular dominance and orientation columns
What are blobs and interblobs sensitive to?
Blobs: color
Interblobs: form
What is the difference between the dorsolateral parieto-occipital cortex and inferior occipitotemporal cortex?
Dorsolateral parieto-occipital cortex: where things are, motion and spatial relationships
Inferior occipitotemporal cortex: what things are, form and color
What is monocular scotoma?
- Small region of visual loss in one eye
- Caused by lesion to retina
What is monocular vision loss?
- Loss of vision in one eye
- Caused by lesion of optic nerve
- Glaucoma, optic neuritis, elevated ICP, tumor, trauma
What is bitemporal hemianopia?
- Loss of vision of the temporal fields (lateral fields) of both eyes
- Caused by lesion to optic chiasm
- Tumor located near pituitary gland
What are the clinical features of optic neuritis?
- Eye pain, especially with eye movement
- Monocular vision problems (central scotoma, decreased visual acuity, impaired color vision, complete loss of vision in one eye)
-50% of individuals will eventually develop MS - Recovery begins at 2 weeks, ends at 6 to 8 weeks
- Some residual loss
What is a homonymous defect?
- Visual field defect in the same region for both eyes (left side or right side of both eyes)
- Caused by retrochiasmal lesion (Structures in the pathway AFTER the chiasm: optic tracts, LGN, optic radiations, visual cortex)
- Usually contralateral presentation
What is contralateral superior quadrantanopia?
- Pie in the sky!
- Visual defect that effects the same superior quarter region in both eyes
- Caused by lesion to meyer’s loop or primary visual cortex below calcarine fissure
- Contralateral presentation
What is contralateral inferior quadrantanopia?
- Pie on the floor!
- Visual defect that effects the same inferior quarter region in both eyes
- Caused by lesion to superior optic radiations or primary visual cortex above calcarine fissure
- Contralateral presentation
What is macular sparing?
- Visual loss sparing the fovea
- Can be caused by: chronically elevated ICP, R PCA infarct sparing occipital lobe, or lesion to inferior bank of R calcarine fissure preserving the occipital pole
What is monocular altitudinal scotoma?
Occlusion to either upper or lower branch of ophthalmic artery causing deficit in upper or lower have of visual field in one eye
What is binocular altitudinal scotoma?
Bilateral occlusion of the PCA branches supplying the lingular gyri causing deficit in upper or lower visual field in both eyes
What is cortical blindness?
- Syndrome of the primary visual cortex
- Leads to loss of blink to threat, loss of closure to bright light, loss of optokinetic nystagmus, and blind sight
