Visual System II- Pathways and Target Areas Flashcards
What do the pathways in the visual system consist of?
- Central Projections of Retinal Ganglion Cells
2. Subcortical Regions
What do the central projections of retinal ganglion cells consist of?
- Optic Nerve
- Optic Chiasm
- Optic Tract
How is the optic nerve formed?
Axons of the retinal ganglion cells conveying input from all areas converge on the optic disc–> Penetrate the choroid and sclera to form Optic Nerve.
What is the structure of the optic nerve?
Extends from the caudal aspect of the eye to the optic chiasm.
What is the function of the optic nerve?
Conveys fibers from the retina of one eye
What happens when there is a lesion in the optic nerve?
Interrupts visual input from that eye, which results in an blind spot or blindness of that eye.
What is impacted if a person can only see through 1 eye?
depth perception
Where is the optic chiasm located?
rostral to the infundibulum
What is the function of the optic chiasm?
- Fibers from the nasal hemi-retinas cross to enter the contralateral optic tract.
- Fibers from the temporal hemi-retinas enter the ipsilateral optic tract.
How are visual defects named?
Always according to the visual field loss, and not according to the area of the retina that is not functioning.
What happens if there is a lesion in the midline of the optic chiasm?
- Nasal Hemi-Retinal fibers are cut
- Interrupts Temporal Visual Hemifield
- Loss of peripheral vision, losing monocular crecents on each side of the field
What happens if there is a lesion to the lateral part of the optic chiasm?
- Temporal Hemi-Retinal fiber is cut
- Loss of Nasal Visual Hemifield on the ipsilateral side
- Right hemianopia of the left eye OR left hemianopia of the right eye
What is the structure of the optic tract?
Extends caudolaterally over the surface of the crus cerebri at the midbrain at its junction with the hemisphere and ends in the lateral geniculate nucleus of the thalamus.
What is the function of the optic tract?
Each 1/2 of the brain receives the fibers corresponding to the contralateral half of the visual world.
What would happen if there was a lesion of the optic tract?
- Ipsilateral Temporal Hemi-Retina Fiber and Contralateral Nasal-Hemi Retina Fibers are cut.
- Loss of Ipsilateral Nasal Visual Hemifield and Contralateral Temporal Visual Hemifield.
- Results in Contralateral Hemianopia, only see ipsilateral side of the visual field.
What do the subcortical regions consist of?
- Suprachiasmatic Nucleus
- Superior Colliculus
- Pretectum
- Lateral Geniculate Nucleus
Where do most of the retinal ganglion cells terminate?
Lateral geniculate nucleus
What is the structure of the Suprachiasmatic Nucleus?
Small collection of neuron cell bodies located above the optic chiasm in the hypothalamus
What is the function of the Suprachiasmatic Nucleus?
Controls diurnal rhythms, visceral functions that are influenced by light levels of day/night cycles.
What is the structure of the Superior Colliculus?
Alternating gray and white layers located in the tectal portion of the midbrain.
What is the function of the Superior Colliculus?
- Receives extensive cortical input, including auditory and somatosensory inputs
- coordinates head and eye movements to visual targets
What is the structure of the Pretectum?
A collection of neuron cell bodies located rostral to the superior colliculus where the midbrain fuses with thalamus.
-Where retinal ganglion cell projections synapse with pretectal neurons that project bilaterally to the Edinger-Westphal nucei.
How are the Edinger-Westphal nuclei and occulomotor nerve related?
The Edinger-Westphal nuclei contain pre-ganglionic parasympathetic neurons that send their axons via the oculomotor nerves to terminate on the ciliary ganglia.
What is the function of the ciliary ganglia?
The neurons innervate the sphincter pupillae of the irises of the eyes
What is the function of the Pretectum?
Coordinates the pupillary light reflex
Under normal conditions, how would the pupil of the right eye react if light was flashed into the left eye?
Both pupils would constrict
What would it mean if light was directed at the left eye but the right eye did not constrict?
Something would be wrong with motor outflow of the right eye
What would it mean if light was directed at the left eye and it did not constrict, but when light was directed at the right eye, both eyes constricted?
Something would be wrong with sensory input from the left eye
What would it mean if there was an absence of pupillary light reflexes in an unconscious patient?
Damage to the midbrain because the oculomotor nerve originates there.
Describe the structure of the Lateral Geniculate Nucleus
A collection of six layers of neuron cell bodies separated by intralaminar layers of axons and dendrites located in the dorsal thalamus.
Where does each layer of the Lateral Geniculate Nucelus receive projections from?
- Either the contralateral nasal hemiretina or ipsilateral temporal hemiretina.
- Either the M or P retinal ganglion cells.
How are receptive fields in the lateral geniculate nucleus similiar to the retinal ganglion cells?
- Circular
- Have a concentric center-surround organization
How are receptive fields in the lateral geniculate nucleus different from the retinal ganglion cells?
- Larger (due to convergence)
- Show a greater sensitivity to contrasts
After the Lateral Geniculate Nucleus, where is the next place the signals project to?
Go to the primary visual cortex via the optic radiations that go through the sublenticular and retolenticular parts of the internal capsule.
What is the pathway of the fibers representing the inferior retinal quadrants?
Inferior retinal quadrants= superior visual quadrants
-Loop into the temporal lobe before turning caudally towards the primary visual cortex
What is the pathway of the fibers representing the superior retinal quadrants?
Superior retinal quadrants= inferior visual quadrant
-Run caudally straight back toward the primary visual cortex.
What is the function of the lateral geniculate nucleus?
- Unclear
- Control the flow of information from the retina to the cortex
What would happen if there was a lesion of the lateral geniculate nucleus or the entire optic radiation?
Same deficit as seen with a lesion of the optic tract
What would happen if there was a lesion of the optic radiation?
May involve visual input from the quadrant of the ipsilateral nasal visual hemifield and the contralateral temporal visual hemifield, resulting in a right or left, superior or inferior quadrantanopia.
What are the target areas of the visual system?
- Primary Visual Cortex
2. Extrastriate Visual Areas ( Secondary Visual Cortex and Posterior Parietal Cortex)
Where does the Cuneus receive input from?
Superior Retinal Quadrant
Inferior visual field
Where does the Lingual Gyrus receive input from?
Inferior Retinal Quadrant
Superior visual field
Describe the structuer of the Primary visual cortex.
- Central part (fovea)= large portion of the primary visual cortex
- Peripheral Part= located rostrally, smaller part of visual cortex
What is the composition of the Primary Visual Cortex?
- Six layers of cell bodies
- Layer 4 receives input from the lateral geniculate nucleus
- Layer 2&3 receive input from neurons in the intralaminar layers of the lateral geniculate nucleus, where they innervate patches of cells called blobs
How is the primary visual cortex organized?
- 3 major vertically oriented systems:
1. Orientation Columns
2. Ocular Dominance Columns
3. Blobs
What are the Orientation columns in the Primary visual cortex?
Contain neurons that respond selectively to bars of light with specific axes of orientation.
What are the ocular dominance columns in the primary visual cortex?
- superimposed upon the orientation column organization
- contain neurons that respond selectively to inputs from one or the other eye
- critical for binocular vision
What are blobs?
- Peg shaped region only located within layers 2&3
- Contain neurons that respond to different color stimuli
How do the vertically oriented columns in the Primary visual cortex communicate with each other and what do they form collectively?
By means of horizontal connections that link cells within a layer
-Form hypercolumns which represent visual properties of one region of the visual field
What are the 4 different types of neurons seen in the Primary Visual Cortex?
- Concentric Center-Surround
- Simple
- Complex
- Hypercomplex
Describe the Concentric-Center surround neurons.
- Same as retinal ganglion and lateral geniculate neurons
- Associated with layer 4
Describe the simple neurons.
- Circles that are overlapping
- Represent the receptive fields of many concentric center-surround lateral geniculate neurons
- Respond best to a bar of light with a specific orientation in a specific position within the receptive field (excitatory and inhibitory zones)
- Associated with neurons in layer 4
Describe the complex neurons.
- Respond best to a bar of light with specific orientation, but the specific position within the receptive field is less crucial
- No excitatory or inhibitory zones
- Represent the receptive fields of several neurons with orientated simple fields
- Associated with neurons in layers 2, 3, 5, and 6.
Describe a hypercomplex neuron
- Respond to stimuli similar to neurons with complex fields, however lengthening the bar of light past a limit results in a weaker response
- Associated with neurons in layers 2, 3, 5, and 6.
What is macular sparing?
Occurs because the area of cortex representing the macula is large and has a collateral blood supply
What are the two major parallel processing pathways, and how are they organized?
- Dorsal Pathway
- Ventral Pathway
- Hierarchically organized
Where does the dorsal pathway receive input from?
The Magnocellular (M) pathway in the primary visual cortex
What kind of information does the dorsal pathway process?
Information related to motion (location, speed, and direction) and depth.
-“where” pathway
What region does the dorsal pathway go to?
Middle Temporal Area–>Posterior Parietal Cortex
How do we understand that something is moving?
The middle temporal area contains neurons with large receptive fields that are sensitive to spots or bars of light by detecting differences in luminance, texture, and color
What mechanism does depth perception require, and what are the two categories of depth perception?
- One that converts a 2D retinal image into 3D
1. Far field (>100’)
2. Near Field ()
How do we perceive depth in a far field?
With one eye by relying on monocular depth cues
What are some examples of monocular depth cues?
- size of an object
- parallel lines converging
- size perspective
- motion parallax
How do we perceive depth in a near field?
with both eyes, by utilizing the disparity in the retinal images to estimate distance.
-V1, V2, V3, V5 contain neurons that are sensitive to binocular disparity, they respond when the inputs from the two eyes are spatially disparate on the two retinas
A lesion in the middle temporal area could result in what two injuries?
- Cerebral Akinetopsia
2. Decreased depth perception
What is Cerebral Akinetopsia?
a deficit in detecting the speed of movement. An individual with such a deficit would have difficulty pouring a drink, following a dialogue, and crossing the street.
A lesion of the posterior parietal cortex would result in what?
Visual neglect, a deficiency in attentiveness to the left visual field.
Where does the ventral pathway receive input from?
Parvocellular and Magnocellular pathways in the Primary Visual Cortex.
What type of information does the ventral pathway process?
Info related to high acuity and color
-“what” pathway
What region does the ventral pathway go to?
Inferior Temporal Cortex
How do we process high acuity and color information?
The inferior temporal cortex has very large receptive fields that are sensitive to a variety of shapes and colors OR are selective only for shape or color.
-some neurons only respond to specific complex stimuli such as the hand or face
A lesion of the inferior temporal cortex would result in what type of injuries?
- Associative Agnosias
2. Cerebral Achromatopsia
What is associative agnosias?
Deficits where objects are perceived but devoid of meaning.
What is prosopagnosia?
The inability to recognize faces
What is cerebral achromatopsia?
Deficits in seeing color
Explain how visual attention works.
- Only a fraction of the information available is processed by the two retinas
- limits the info that reaches the highest centers of processing
- filters out features and sharpens the perception of others
