Photoreceptors
- Rods and Cones of Retina
- Convert light waves into electrical impulses
- Create visual image on the retina
3 Neuron Pathway in Primary Visual Cortex
(1) First Order Neurons: Bipolar cells of retina
(2) Second Order Neurons: Ganglion cells of retina; their axons converge at the optic disc and form the Optic Nerve
(3) Third Order Neurons: Located in Lateral Geniculate Body; axons terminate in primary visual cortex
Light from the top of an object gets reflected to the ___ part of the retina
Lower
Light form the bottom of an object gets reflected to the ___ part of the retina
Upper
The optics of the eyes are the reason why an object is seen ___ at the retina
Upside-down
Superior visual field is deflected towards the ___ retina
Inferior
Inferior visual field is deflected towards the ___ retina
Superior
Binocular Vision
- Overlapping area in the center of visual fields
- Brought to both cerebral hemispheres
- Important for depth perception, stereoscopic vision
Monocular Blindness
- Blindness in one eye
Hemianopia
- ## Partial blindness/loss of sight in half of the visual field
Quadrantanopia
- Partial blindness in a quadrant of visual field
Homonymous
- Term used to describe lesions that occur after the optic chiasm
- Losses of the same side (right or left)
Heteronymous
- Losses at different sides of both eyes
- Can be bitemporal or binasal
Macular Sparing
- Loss of visual field with preserved center of the visual field (macula)
- Caused by lesion to to primary visual cortex, PCA
- Anastomosis between the MCA and PCA can result in homonymous hemianopia with preservation of macular function
Scotoma
Spot-like defect of visual field
What artery passes thru the Optic Canal with the Optic N (CN II)?
Ophthalamic A
Lesion of Optic N leads to:
Monocular blindness (Anopia)
Nasal fibers of the Optic N:
Cross at the Optic Chiasm
Temporal fibers of the Optic N:
Remain uncrossed (ipsilateral)
Lesion of the optic chiasm causes:
Heteronymous bitemporal hemianopia
What axons can be impacted by a tumor of the pituitary gland?
Nasal axons of the Optic N
Axons from the inferior retina cross ____ in the chiasm, where the superior nasal fibers cross ____ in the chiasm
- anteriorly
- posteriorly
Unilateral lesion impinging on the anterior chiasm can give rise to:
Superior Temporal Contralateral Quadrantanopia
Meningioma compressing the right optic N and anterior chiasm may result in a:
R central scotoma and a L superior temporal visual field defect (junctional scotoma) – due to involvement of the cross inferior nasal retinal fibers from the L eye
Axons from the LGB form the:
Optic Radiations
Optic Radiations
- Originate in the LGB
- Run along the temporal horn of the lateral ventricle approaching the anterior tip of the temporal lobe then run backwards thru the parietal lobe
- Pass thru retrolenticular part of the internal capsule
- LGB –> Temporal horn of lateral ventricle –> parietal lobe –> retrolenticular part of internal capsule –> occipital lobe
Axons from superior retinal quadrants (inferior visual field) run thru the ___ lobe
Parietal
Axons from the inferior retinal quadrants (superior visual field) run thru the ___ lobe
Temporal
Meyer’s Loop
Loop formed by axons from lower retinal quadrants (superior visual field); located in temporal lobe
Lesion of the Inferior optic radiations in the temporal lobe (Meyer’s Loop) result in:
Homonymous Superior Quadrantanopia
***Temporal produces Top quadrantanopia
Lesion of the Superior optic radiations in the parietal lobe result in:
Lower Homonymous Quadrantanopia
Disease affecting the posterior visual pathway (radiations or visual cortex) result in:
Scotomas that are extremely congrous (i.e same shape in each eye – this is b/c as nerve fibers travel back in the visual pathway they become more organized and perfectly aligned
Blood supply of the Primary Visual Cortex V1 (Brodmann’s Area 17)
Mostly from the Posterior Cerebral A – Middle Cerebral A also contributes
Primary Visual Cortex V1 (Brodmann’s Area 17)
- Basic visual images are created here (color, shape, size of object)
- Located predominantly on the medial surface of the hemisphere above and below the calcarine sulcus
Contralateral nasal axons synapse in layers ___, ___, and ___ of the LGB
1, 4, 6
Ipsilateral temporal axons synapse in layers ___, ___, and ___ of the LGB
2, 3, 5
V1: Clinical Anastomosis
- Anastomosis between PCA and MCA
- Occlusion PCA -> homonymous hemianopia w/ macular sparing d/t MCA contribution
Nerve fibers from macula (fovea) of retina occupy:
- Posterior occipital tip of striate cortex
- Larger area
Nerve fibers from periphery occupy:
- Anterior aspect of striate cortex
- Smaller area
Higher Order Visual Pathways: Ventral Stream
- “What” area
- Occipital lobe –> Parietal Lobe –> Frontal Lobe
- Responsible for object recognition and storage of long-term visual information
Higher Order Visual Pathways: Dorsal Stream
- “Where” area
- Occipital Lobe –> Temporal Lobe
- Associated with the spatial location of objects and perception of motion
Peripheral Rods, although not high in acuity, are particularly sensitive to:
Movement
Ability to detect movement is central to the function of the:
Superior Colliculus
Superior Colliculus
- Detects movement
- Receives visual information that has already been processed by the primary visual cortex
- Involved in the control of two types of eye movements
Two Types of Eye Movements that the Superior Colliculus controls:
(1) Saccades: rapid eye movements between fixation points
(2) Pursuit: slow movements that are used for tracking moving visual targets
Pupillary Light Reflex
(1) Light shines in one eye and stimulates CN II
(2) Signal sent to Pretectal Area in Midbrain
(3) Projections sent bilaterally in Posterior Commissure to Edinger-Westphal Nuclei
(4) EWN sends preganglionic PS to CN III
(5) Ciliary Ganglion
(6) Postganglionic PS in short ciliary
(7) Sphincter Pupillae stimulated
(8) Bilateral pupil constriction
Corneal (Blinking) Reflex
(1) Free nerve endings in Cornea detect touch
(2) CN V synapses in Trigeminal Sensory Nucleus and Spinal Trigeminal Nucleus
(3) Synapse bilaterally on CN VII motor nuclei
(4) Neuron in Facial N causes eye to blink
Lesion of Optic Tract:
Homonymous Hemianopia
Lesion of Optic Radiations from Superior Retinal Field:
Lower Homonymous Quadrantanopia
Lesion of Optic Radiations from Inferior Retinal Field:
Upper Homonymous Quadrantanopia
Blows to back of head or blockage occipital branches middle cerebral artery:
Loss of macular representation of visual fields
Bilateral visual cortex lesions:
Cortical blindness
Cortical Blindness
Patient can’t see but pupillary reflex is still intact