Lecture 6: Visual Pathways and Eye Movements Flashcards
What is a visual field vs. retinal field?
Visual field: area that a person is able to see when both eyes are fixed in one position
Retinal field: light passes from objects in the visual field, through the pupil to subtend an image upon the retina
In the visual field, the object of attention is focused and centered at which location?
Fovea centralis and macula lutea
What is found medial to the macula, and what leaves this region; what is absent from this region?
- Optic disc
- Region where retinal axons leave the eye as the optic nerve
- No photoreceptors here, which creates our blind spot
Visual fields are subdivided into what 2 zones?
1) Binocular zone, broad central region seen by both eye
2) Monocular zone (R/L), seen only by the corresponding eye
The location on the retina that an object in the visual field is the retinal field, how is each visual field divided within the retinal field?
- Each visual field is divided into retinal hemifields (nasal and temporal halves of retina)
- Each hemifield is divided into upper and lower, quadrants
The image formed on the retina is inverted how?
In both the lateral and vertical dimensions
Explain how the left and right half of the visual field forms an image on the temporal and nasal half of each retina.
- Left half forms image upon the nasal (right) half of left retina and the temporal (right) half of the right retina
- Right half forms image upon the nasal (left) half of right retina and the temporal (left) half of the left retina
Where do the optic nerves partially decussate; which parts cross?
- Optic chiasm
- Nasal half of each retina —> contralateral optic tract
- Temporal half of each retina —> ipsilateral optic tract
What is the optic tract composed of?
Fibers from temporal retina (ipsilateral) eye + fibers from nasal retina (contralateral eye)
*Only the nasal fibers decussate here
What is the purpose of partial decussation of the optic nerve at the optic chiasm?
- Necessary to delivery information from contralateral visual field to each optic tract
- Brings together information from comparable areas of both retinas —-> DEPTH PERCEPTION
Optic tract curves posteriorly around the cerebral peduncle and terminates where?
Lateral geniculate nucleus (LGN)
How many layers make up the lateral geniculate nucleus and how do the fibers of the optic tract terminate here?
- 6 layers w/ myelinated fibers sandwiched between them
- Fibers terminate in a precise retinotopic pattern
What forms the ventral base, and dorsal/lateral borders of the lateral geniculate nucleus (LGN)?
- Ventral base is formed by the incoming optic tract (retinogeniculate) fibers
- Dorsal and lateral borders formed by the outgoing optic radiations
What is the Magnocellular (M) layers of the lateral geniculate nucleus (LGN); contains what size cells; receives what inputs?
- Layers 1 and 2 (ventral)
- Contains large cells
- Receive ganglion cell inputs relaying from rods —> larger receptive fields and thick, rapidly conducting axons, sensitive to moving stimuli
The Magnocellular (M) layers of the lateral geniculate nucleus is sensitive to what stimuli?
Sensitive to moving stimuli
What are the Parvocellular (P) layers of the lateral geniculate nucleus; what size cells; what inputs does it receive?
- Layers 3-6 (dorsal)
- Contains small cells
- Receive ganglion cell inputs relaying from cone —> small receptive fields, slower conducting axons, tonically responsive to stationary stimuli, high-acuity vision
Ganglion cell axons that arise in the temporal retina remain uncrossed and terminate in which layers of the LGN on which side?
- Layers 2, 3, and 5
- Ipsilateral LGN
Ganglion cell axons that arise in the nasal retina cross and terminate in which layers of the LGN on which side?
- Layers 1, 4, and 6
- Contralateral side
Looking at the RIGHT visual field, the nasal retina of the right eye and temporal retina of the left eye terminate in which layers and which LGN?
- Nasal (R)–> Layers1, 4, 6ofleft LGN
- Temporal (L) –> Layers 2, 3, 5 of right LGN
Looking at the LEFT visual field, the nasal retina of the left eye and temporal retina of the right eye terminate in which layers and which LGN?
- Nasal (L) —> Layers 1, 4, 6 of right LGN
- Temporal (R) –> Layers 2, 3, 5 of right LGN
How is the same point in visual space able to be represented six times?
- Once in each layer of the LGN
- Optic tract axons branch in multiple layers even though they arise from same visual field
What forms Optic Radiations; what else is this pathway referred to as?
- Secondary neurons from the LGN extend a large bundle of myelinated fibers
- Also referred to as the geniculostriate or geniculocalcarine pathway
Where do the secondary neuron bundles (optic radiations) relay to, which is located where?
- Relay to the primay visual cortex (striate cortex)
- Located on the upper and lower banks of the calcarine sulcus
Fibers from the lower quadrant of the contralateral hemifields originate from what part of LGN and target where?
- Originate from dorsomedial portion of LGN
- Arch caudally to pass through the retrolenticular limb of internal capsule
- Target superior bank of the calcarine sulcus, on the cuneus
Fibers from the upper quadrant of the contralateral hemifields originate where in LGN and describe their route and target?
- Originate from ventrolateral portion of LGN
- Arch rostrally, passing into white matter of temporal lobe to form a broad U-turn, the Meyer loop
- Target inferior bank of the calcarine sulcus, on the lingual gyrus
What is the Meyer loop and why is it clinically significant?
- Formed by fibers from the upper quadrant of the contralateral hemifields
- These fibers arch into the white matter of Temporal lobe on way to inferior bank of calcarine sulcus, on lingual gyrus
- Strokes or lesions to temporal lobe may interrupt this pathway and cause visual deficits
Fibers conveying information from the macula and fovea originate from where in LGN and target what?
- Central regions of LGN
- Pass to caudal portions of the visual cortex
The macula is represented by disproportionally large volumes (relative to size) of the _____ and _______.
LGN and visual cortex
The Macula is represented most _________, in the region of the _________ pole.
The Macula is represented most posteriorly, in the region of the occipital pole
Visual pathways end retinotopically in the primary visual cortex/striate cortex above and below the calcarine sulcus, where do the inferior, superior, macular and peripheral fields project in this cortex?
- Inferior visual fields project to the cortex above the calcarine sulcus
- Superior fields project to the cortex below the sulcus
- Macula is represented more posteriorly and peripheral fields more anteriorly
What Brodmann area is the primary visual cortex/striate cortex?
Brodmann’s area 17
The striate cortex is surrounded by which Brodmann areas?
Areas 18 and 19 (comprise rest of occipital lobe)
Brodmann’s areas 18, 19, and related parts of the temporal and parietal lobes comprise what; what components of vision does this cortex help us with?
- Visual Association Cortex (extrastriate cortex)
- Parieto-occipital-temporal area
- Helps interpret: location, motion, form, and color
Which visual cortical area is responsible for spatially directing head movements and visual reflexes; what inputs does it receive to help with this?
- Superior Colliculus
- Retinal and Cortical input
- Spinotectal (somatosensory) and auditory inputs
*Important in directing eye movements
How does the Superior Colliculus receive retinal input?
- Select fibers from each optic tract bypass the LGN
- Pass over the medial geniculate nucleus in a bundle, the brachium of the superior colliculus
The pretectal/pretectum area is a bilateral group of interconnected nuclei which respond to and mediate what; related to what reflex?
- Respond to varying intensities of illuminance
- Mediate non-conscious behavioral responses to acute changes in light
- Important in the pupillary light reflex, illumination of 1 eye results in bilateral pupillary constriction
Voluntary eye movements are controlled by which frontal eye field and where is it located?
- Frontal eye fields (area 8)
- Posterior portion of the middle frontal gyrus
Which fibers from the frontal eye field will descend to the superior colliculus and control the lower motor neurons for voluntary eye movements; when testing these movements what state does the patient need to be in?
- Corticotectal fibers
- Control LMN’s of CN III, IV, and VI
- Patient must be concious (aroused, alert and attentive)
Which eye fields control Nonvolitional movements and where do the fibers project from; what type of eye movements fit this category?
- Occipital eye fields in the visual association cortex proejct corticotectal fibers to the superior colliculus
- Controls unconcious (nonvolitional) eye movements, may be associated w/ following a target
What is Hemianopia?
Blindness (-anopia or -anopsia) in one-half of the visual field
What is Quadrantanopia?
Blindness of a quadrant of the visual field
What is Homonymous vs Heteronymous visual fields?
Homonymous: conditions in visual field losses are similar in both eyes
Heteronymous: conditions in which the two eyes have non-overlapping field losses
How does damage anterior to chiasm, at the chiasm, and behind the chiasm affect visual field?
- Anterior to chiasm affects only the ipsilateral eye
- At the chiasm causes heteronymous deficits
- Behind the chiasm causes homonymous deficits
A lesion where would cause this deficit; what is the name of the deficit?
Lesion of right optic nerve = Monocular blindness
A lesion where would cause this deficit; what is the name of this deficit?
Lesion of optic chiasm = non-homonymous bitemporal hemianopia
A lesion where would cause this deficit; what is the name of this deficit?
- Lesion of right optic tract or right Geniculocalcarine tract = contralateral homonymous hemianopia
- Indicated by #3 and #6 on the figure
A lesion where would cause this deficit; what is the name of this deficit?
- Lesion temporal lobe (Meyer’s loop) = Superior left homonymous quadrantanopia
- Indicated by #4 on figure
A lesion where would cause this deficit; what is the name of this deficit?
- Parietal lobe lesion = Inferior left homonymous quadrantanopia
- Indicated by #5 on figure
A lesion where would cause this deficit; what is the name of this deficit?
- Lesion of both banks of calcarine fissure on the right = Contralateral (left) homonymous hemianopia (with macular sparing)
- Indicated by #9 on the figure
- Damage to one occipital lobe or occlusion of PCA
A lesion where would cause this deficit; what is the name of this deficit?
Lesion of Inferior bank of calcarine fissure = Superior left homonymous quadrantanopia (w/ macular sparing)
Which artery is associated with Associative Visual Agnosia, what is affected, and what is the presentation of this disorder?
- Occlusion of posterior cerebral artery leads to secondary infarction of the left occipital love and posterior corpus callosum
- Disconnects the language area from the visual association cortex
- Patient cannot name or describe an object in the visual field, BUT he/she can recognize and demonstrate its use
- Patient may also be alexic (unable to read) and writing ability may be affected (agraphia)
A lesion to the left temporal lobe affecting Meyer’s loop would cause what type of visual deficit?
Superior right homonymous quadrantanopia
What is Associative visual agnosia and infarction of which artery to which artery causes this?
- Infarction of left occipital lobe secondary to occlusion of PCA
- May disconnect the language area from the visual association cortex
- Pt cannot name or describe an object in visual field, he can recognize and demonstate its use
